mTOR signaling pathway in osteoarthritis: a narrative review

Journal Pre-proof The PI3K/AKT/mTOR signaling pathway in osteoarthritis: a narrative review Kai Sun, Jiahui Luo, Jiachao Guo, Xudong Yao, Xingzhi Jing, Fengjing Guo PII:

S1063-4584(20)30079-0

DOI:

https://doi.org/10.1016/j.joca.2020.02.027

Reference:

YJOCA 4603

To appear in:

Osteoarthritis and Cartilage

Received Date: 26 September 2019 Revised Date:

5 February 2020

Accepted Date: 6 February 2020

Please cite this article as: Sun K, Luo J, J, Guo c, Yao X, Jing X, Guo F, The PI3K/AKT/mTOR signaling pathway in osteoarthritis: a narrative review, Osteoarthritis and Cartilage, https://doi.org/10.1016/ j.joca.2020.02.027. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd on behalf of Osteoarthritis Research Society International.

1

2 3

The PI3K/AKT/mTOR signaling pathway in osteoarthritis: a narrative review.

4 5

Kai Sun1, Jiahui Luo2, Jiachao Guo1, Xudong Yao1, Xingzhi Jing1, Fengjing Guo1*

6 7

1

8

University of Science and Technology, Wuhan, Hubei 430030, China.

9

2

Department of Orthopedics, Tongji Hospital, Tongji Medical College, Huazhong

The Center for Biomedical Research, the Tongji Hospital Research Building, Tongji

10

Hospital, Tongji Medical College, Huazhong University of Science & Technology,

11

Wuhan, 430030, China.

12 13

*Corresponding author:

14

Fengjing Guo1*, Department of Orthopedics, Tongji Hospital, Tongji Medical

15

College, Huazhong University of Science and Technology, Wuhan, Hubei 430030,

16

China.

17

E-mail: [email protected]

18 19

Authors’ Information:

20

Kai Sun1: [email protected]

21

Jiahui Luo2: [email protected]

22

Jiachao Guo1: [email protected]

23

Xudong Yao1: [email protected]

24

Xingzhi Jing1: [email protected]

25

Fengjing Guo1*: [email protected]

26 27

Funding sources:

28

National Natural Science Foundation of China [no. 81874020].

29

Declarations of interest: none.

30 31

Abstract

32

Osteoarthritis (OA) is a complicated degenerative disease that affects whole joint

33

tissue. Currently, apart from surgical approaches to treat late stage OA, effective

34

treatments to reverse OA are not available. Thus, the mechanisms leading to OA, and

35

more effective approaches to treat OA should be investigated. According to available

36

evidence, the PI3K/AKT/mTOR signaling pathway is essential for normal metabolism

37

of joint tissues, but is also involved in development of OA. To provide a wide

38

viewpoint to roles of PI3K/AKT/mTOR signaling pathway in osteoarthritis, a

39

comprehensive literature search was performed using PubMed terms ‘PI3K OR AKT

40

OR mTOR’ and ‘osteoarthritis’. This review highlights the role of PI3K/AKT/mTOR

41

signaling in cartilage degradation, subchondral bone dysfunction, and synovial

42

inflammation, and discusses how this signaling pathway affects development of the

43

disease. We also summarize recent evidences of therapeutic approaches to treat OA

44

by targeting the PI3K/AKT/mTOR pathway, and discuss potential challenges in

45

developing these strategies for clinical treatment of OA.

46 47

Key words: osteoarthritis; PI3K; AKT; mTOR; cartilage.

48 49

Abbreviations: Osteoarthritis (OA), nonsteroidal anti-inflammatory drugs (NSAIDs),

50

extracellular matrix (ECM), receptor tyrosine kinases (RTK), G protein-coupled

51

receptors (GPCRs), phosphatidylinositol-4,5-bisphosphate (PIP2),

52

phosphatidylinositol-3,4,5-trisphosphate (PIP3), phosphatase and tensin homologue

53

(PTEN), phosphoinositide-dependent kinase-1 (PDK1), mammalian target of

54

rapamycin complex 2 (mTORC2), tuberous sclerosis complex 2 (TSC2), S6 kinase-1

55

(S6K-1), eukaryotic translation initiation factor 4E binding protein-1 (4EBP-1),

56

unc-51-like kinase 1 (ULK1), matrix metalloproteinases (MMPs), a disintegrin and

57

metalloprotease with thrombospondin motifs (ADAMTSs), interleukin -1β (IL-1β),

58

lipopolysaccharide (LPS), tumor necrosis factor α (TNF-α), tert-butyl hydroperoxide

59

(tBHP), magnetic resonance imaging (MRI), rheumatoid arthritis (RA), inducible

60

nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), extracellular regulated

61

kinase (ERK), fibroblast-like synoviocytes (FLS), prostaglandin-endoperoxide

62

synthase 2 (PTGS2), basic calcium phosphate (BCP), peroxiredoxin 4 (PRDX4).

63 64

Introduction

65

Osteoarthritis (OA), a common degenerative disease affecting more than 240 million

66

people, is a primary cause of disability . It is characterized by destructive alteration of

67

articular cartilage, synovial tissue, and subchondral bone which are responsible for

68

pain, joint dysfunction, and loss of tissue integrity . To date, non-surgical treatments

69

for OA, such as nonsteroidal anti-inflammatory drugs (NSAIDs) and physical therapy

70

have not reversed the course of the disease . Therefore, more specific therapeutic

71

approaches need to be researched and developed. Current evidence demonstrates that

72

OA causes progressive degeneration and dysfunction of whole joints, accompanied by

73

chondrocyte loss, inflammatory responses, and imbalance in extracellular matrix

74

(ECM) homeostasis . Complex signaling networks have been suggested to mediate

75

these cell fate events. In particular, the PI3K/AKT/mTOR signaling pathway is

76

essential for maintaining joint health, and is correlated with OA pathogenesis . Based

1

2

1

3

4

77

on these reports, we focused on the PI3K/AKT/mTOR signaling pathway and its

78

multiple functions in cartilage, synovial tissue, and subchondral bone, and its

79

interactions with other signaling pathways. We also present an overview of

80

therapeutic approaches for targeting PI3K/AKT/mTOR to comprehensively

81

understand the role of this signaling pathway in the pathological progression of OA.

82 83

The PI3K/AKT/mTOR signaling pathway

84

Various molecules, including insulin, glucose, and many growth factors and cytokines

85

can initiate PI3K/AKT/mTOR signaling . Generally, these molecules activate

86

receptor tyrosine kinases (RTK) and G protein-coupled receptors (GPCRs), which

87

subsequently activate PI3K to generate phospholipids . Downstream effectors of

88

PI3K, such as AKT and mammalian target of rapamycin complex 1(mTORC1), are

89

activated by these signals . PI3K includes three classes of molecule (I, II, III). Of

90

these, PI3K Class I, which contributes to various bioactivities, is most researched .

91

PI3K Class I are heterodimers containing a regulatory subunit (i.e. p85), and a

92

catalytic subunit (p110s). Binding between regulatory and catalytic subunits stabilizes

93

PI3K, and the interface provides the site at which RTKs and GPCRs activate PI3K .

94

Activated PI3K converts phosphatidylinositol-4,5-bisphosphate (PIP2) into

95

phosphatidylinositol-3,4,5-trisphosphate (PIP3), and further activates downstream

96

effectors such as AKT . In the process, phosphatase and tensin homologue (PTEN), a

97

negative regulator of PI3K, acts to limit signal strength by reversing PIP3 to PIP2 .

5

6

6

7

8

7

9

98 99

AKT is a vital messenger in PI3K signaling. In the canonical PI3K/AKT pathway,

100

phosphoinositide-dependent kinase-1 (PDK1) and AKT are recruited to the inner

101

surface of the cell membrane via PH domains, where PDK1 initiates AKT1

102

phosphorylation at Thr308 . Another vital AKT activating pathway is mediated by

103

the mTORC2, which interacts with the regulatory hydrophobic domain of AKT to

104

phosphorylate it at Ser473 . Activated AKT transfers into other cell compartments to

105

activate various downstream substrates such as protein kinases, E3 ubiquitin ligases,

106

regulators of small G proteins, metabolic enzymes, transcription factors, and cell

107

cycle regulators . A critical downstream branch of AKT is mTORC1.

108

Phosphorylated AKT can phosphorylate mTOR at Ser 2448 to directly activate

109

mTORC1, and phosphorylate tuberous sclerosis complex 2 (TSC2) to indirectly

110

activate mTORC1 . Specifically, tuberous sclerosis complex (TSC) contains two

111

subunits: TSC1 and TSC2. TSC2 is a negative regulator of mTOR. inactivation of

112

TSC2 by AKT can inhibit the function of the TSC1/TSC2 complex, resulting in

113

mTORC1 activation When the complex is activated, the downstream Rheb-GTP is

114

converted to Rheb-GDP, which stabilizes mTORC1 . Altered activity of mTORC1

115

subsequently affects its effectors including S6 kinase-1 (S6K-1), eukaryotic

116

translation initiation factor 4E binding protein-1 (4EBP-1), and unc-51-like kinase 1

117

(ULK1)

118

angiogenesis by enhancing translation of mRNAs encoding HIF-1α, cyclin D1, and

119

c-Myc . Moreover, mTORC1 is a master regulator of ULK1, which correlates with

120

initiation of autophagy. Treatment with rapamycin, an inhibitor of mTORC1,

121

improves ULK1 kinase activity . On the other hand, Rheb-mediated active mTORC1

122

potently represses ULK1 . ULK1 functions in a complex with ATG13 and FIP200,

123

which serves as a node that translates autophagic signals into autophagosome

10

11

12

12

13

14, 15

. S6K-1 and 4EBP-1 serve as regulators of cell cycle progression or

15

14

16

124

biogenesis

125

Figure1.

17

(Fig.1).

126 127

PI3K/AKT/mTOR is an important and quite complex signaling pathway in which

128

over 150 proteins have been identified to involve in this pathway . Through these

129

effectors, PI3K/AKT/mTOR fulfills functions in many cellular processes essential for

130

homeostasis, including the cell cycle, cell survival, inflammation, metabolism, and

131

apoptosis

132

progression of diseases-such as cancer, diabetes, and cardiovascular diseases

133

Moreover, increased evidence supports the involvement of PI3K/AKT/mTOR in OA

134

development . We here review the literature describing the close relationship

135

between the PI3K/AKT/mTOR axis and OA pathophysiology.

18

19, 20

. Misregulation of these pathways is related to initiation and 15, 21, 22

.

23

136 137

The PI3K/AKT/mTOR pathway in the pathogenesis of OA

138

OA is a complex disease with multiple underlying molecular mechanisms, that affects

139

whole-joints with cartilage degeneration, synovial inflammation, and subchondral

140

bone sclerosis (Fig.2). We will focus on the role of the PI3K/AKT/mTOR pathway in

141

these three aspects.

142 143

Cartilage

144

Cartilage homeostasis

145

Cartilage homeostasis, defined as the state in which synthesis of extracellular matrix

146

is balanced by its degradation, is vital to articular health. The destruction of cartilage

147

homeostasis, marked by elevated production of matrix metalloproteinases (MMPs),

148

and a disintegrin and metalloprotease with thrombospondin motifs (ADAMTSs) and

149

reduced collagen II and aggrecan, is the initiator and booster of OA pathogenesis .

150

The PI3K/AKT/mTOR pathway are required for cartilage homeostasis . Previous

151

analysis has revealed that the PI3K-AKT pathway is downregulated in human

152

cartilage tissues with OA, compared to normal cartilage . Similar decreased

153

PI3K-AKT pathway activity has been found in OA-like chondrocytes exposed to

154

interleukin -1β (IL-1β), tumor necrosis factor α (TNF-α), and tert-butyl hydroperoxide

155

(tBHP, an organic hydrogen peroxide far more stable than H2O2)

156

activator of AKT phosphorylation, can activate AKT to promote synthesis of collagen

157

II . Additionally, IGF-1 has an inhibitory effect on IL-1β-induced NF-κB activation

158

in human chondrocytes, which is closely involved in cartilage degradation. The

159

inhibitory effect could be abolished by treatment with inhibitors of PI3K (wortmannin)

160

or AKT (SH-5) . Oxidative stress is associated with osteoarthritic cartilage, causing

161

reduced matrix synthesis, while overexpression of AKT dramatically enhanced

162

proteoglycan synthesis in human chondrocytes under the stimulation of tBHP. Not

163

only that, the knockdown of PTEN promoted the expression of Col2a1 and aggrecan

164

by increasing AKT phosphorylation under oxidative stress . These findings indicate

165

PI3K/AKT pathway involves in the ECM synthesis.

24

25

25

27-29

. IGF-1, a strong

30

30

29

166 167

This pathway also plays a role in ECM catabolism in addition to stimulating ECM

168

anabolism. Venkatesan, J K et al. found recombinant adeno-associated virus (rAAV)-

169

mediated production of TGF-β reduced expression of MMP13 in human OA

170

chondrocytes and cartilage explants . In agreement with this evidence, activation of

31

171

PI3K/AKT by TGF-β can attenuate rat cartilage injury by decreasing expression of

172

MMP13 , and chemical inhibitors of PI3K/AKT can repress expression of TIMP-3, a

173

natural inhibitor of matrix metalloproteinases . Above data indicates PI3K/AKT

174

mediates inhibitory effect of TGF-β on ECM catabolism. In addition, another study

175

demonstrated IGF-1 can promote expression of Col2a1and inhibit expression of

176

MMP13 with activation of PI3K and extracellular signal-regulated kinase (ERK). The

177

PI3K inhibitor (Wortmannin) markedly reversed the IGF-1 effect on Col2a1

178

expression without affecting effect of IGF-1 on MMP-13 expression . It suggests

179

that PI3K may not be involved in IGF-1-induced changes in the expression of

180

MMP13. By contrast, leptin can induce increased MMPs expression in chondrocytes,

181

Selective inhibitors of PI3K and AKT significantly reduce MMPs expression .

182

Collectively, although activation of PI3K/AKT promotes ECM anabolism, the precise

183

effects of PI3K/AKT on ECM catabolism remain unclear. Thus, further studies are

184

needed to determine the function of PI3K/AKT in cartilage homeostasis.

32

33

34

35

185 186

Inflammatory response

187

Previous studies have shown inflammatory response always occurs with OA

188

pathogenesis and OA-related symptoms . During OA progression, affected

189

chondrocytes and synovial cells overproduce inflammatory mediators such as IL-1β

190

and nitric oxide to accelerate cartilage degradation . In particular, IL-1β could trigger

191

strong inflammatory responses by activating complex signaling pathway networks in

192

which PI3K/AKT signaling is closely intertwined with IL-1β-induced inflammation .

193

Numerous studies suggest PI3K and AKT are rapidly phosphorylated under the

36

37

38

39, 40

194

stimulation of IL-1β

195

both inhibitory effects on IL-1β-induced PI3K, AKT, and NF-κB phosphorylation,

196

and inflammatory responses , indicating that PI3K/AKT/NF-κB pathway might

197

mediate initiation of an inflammatory response. Mechanistically, NF-κB is a master

198

regulator of OA-related inflammatory mediators and activated mainly by IκB

199

kinases- mediated IκBα degradation and p65/RelA phosphorylation. PKA/AKT could

200

activate NF-κB by affecting its upstream IκB kinases . Thus, active PI3K and AKT

201

are involved in NF-κB p65 phosphorylation and nuclear translocation, promoting

202

production of inflammatory mediators.

. Moreover, some biological and chemical compounds exert

41

42

43

203 204

Chondrocyte proliferation and apoptosis

205

Chondrocytes (the only cell type in cartilage) are the dominant influence on the health

206

and function of cartilage . Excessive chondrocyte apoptosis and reduced

207

proliferation are frequent companions to cartilage degradation

208

signaling pathway is a vital regulator of chondrocyte survival and apoptosis .

209

17β-estradiol (E2) loss may be accompanied by increased incidence of knee and hip

210

OA . By contrast, E2 mediated PI3K/AKT activation significantly promotes cell

211

proliferation in rat OA model chondrocytes . Administration of 17β-estradiol also

212

elevates proliferation and viability of ATDC5 chondrocytes via this signaling . In

213

addition, a study by Huang et al. suggested that PTEN silencing increased AKT

214

phosphorylation, and promoted proliferation of OA chondrocytes . Rapid cellular

215

proliferation is also found during cartilage repair under exosomal CD73-mediated

216

adenosine activation of AKT and ERK signaling while inhibitors of AKT or ERK

44

45, 46

. The PI3K/AKT 20

47

48

49

50

51

217

phosphorylation repress exosome-mediated increases in cell proliferation . Apart

218

from regulating cell proliferation, PI3K/AKT could minimize cartilage degeneration

219

by preventing chondrocyte death. Overproduction of nitric oxide (NO) in articular

220

chondrocytes induces apoptosis by modulating multiple intracellular signaling

221

processes including PI3K/AKT signaling . Chun-Do et al. reported that NO

222

production in chondrocytes led to cell apoptosis, with downregulation of PI3K and

223

AKT activities, while IGF-1 treatment blocked the process through PI3K and AKT

224

activation . Notably, IL-1β is also a crucial inducer of cell apoptosis, which

225

simultaneously suppresses PI3K and AKT activity . Growing evidence indicates

226

multiple growth factors, such as FGF18 , IGF-1, and platelet-derived growth factor

227

55

228

activation of PI3K/AKT. Not only that, many bioactive compounds including

229

berberine

230

activation. Consistent with these results, other evidence reveals that activated PI3K

231

/AKT can block OA chondrocyte apoptosis induced by TNF-α and LPS

232

together, the PI3K /AKT signaling negatively modulates chondrocyte apoptosis under

233

multiple pathological conditions and the activated signaling can protect against OA by

234

reducing chondrocyte apoptosis.

52

53

54

28

, could rescue IL-1β-induced increase in mitochondrial-related apoptosis by the

56

57

and tormentic acid , exert similar anti-apoptotic effects via PI3K/AKT

58, 59

. Taken

235 236

Autophagy

237

Autophagy, an essential regulator of energy utilization and nutrient metabolism, is a

238

cellular homeostasis mechanism that removes dysfunctional and damaged

239

macromolecules and organelles . Failure to perform autophagy leads to elevated

60

240

production of reactive oxygen species and mitochondrial dysfunction, and can result

241

in death at the cellular level . A switch from autophagy to apoptosis is implicated in

242

progression of chondrocytes to OA . The mTOR pathway is a key suppressor of

243

autophagy, and is centrally regulated by upstream signaling pathways involving

244

PI3K/AKT, and AMPK . Upregulated mTOR in OA cartilage is linked to increased

245

chondrocyte apoptosis and decreased expression of autophagy-related genes.

246

Cartilage-knockdown of mTOR upregulates autophagy, and reduces apoptosis,

247

altering cartilage homeostasis in mice . Rapamycin, an mTOR inhibitor, also

248

attenuates the severity of experimental osteoarthritis by activating autophagy . By

249

contrast, mTORC1 activation by genetic deletion of TSC1 or by addition of the

250

activator MHY1485 could result in opposite effects .

251

In addition to its involvement in apoptosis, mTOR-regulated autophagy is correlated

252

with the inflammatory response. OA activity associated with synovitis is accompanied

253

by increased expression of mTOR in peripheral blood mononuclear cells in OA

254

patients . Pro-inflammatory cytokine IL-1β exposure also results in an obvious

255

increase in mTOR expression in human OA chondrocytes . Inhibiting autophagy has

256

been shown to enhance inflammasome activity, whereas promoting autophagy limits

257

production of IL-1β . In mouse chondrocytes, PPAR deficiency-induced

258

dysregulated expression of mTOR is associated with suppression of critical autophagy

259

markers and consequently induces abnormal expression of inflammatory markers

260

inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2), ultimately

261

exacerbating inflammatory activity in cartilage . In addition, Ansari MY, et.al found

262

hydromethanolic extract of Butea monosperma (BME) repressed IL-1β-induced

61

61

63

64

65

66

67

67

68

69

263

overexpression of IL-6, MMP3, 9, and 13. The effects were autophagy dependent

264

with upregulated protein expression of LC3-II and increased number of

265

autophagosomes, and could be abolished by inhibition of autophagy . In vivo,

266

rapamycin treatment of OA mice decreases IL-1β expression and maintains cartilage

267

cellularity . Thus, mTOR is an essential link between inflammation and autophagy in

268

OA pathogenesis.

70

71

269 270

Subchondral bone

271

Recently, a growing number of studies have indicated that subchondral bone-cartilage

272

crosstalk exerts a regulatory effect on OA development . Subchondral bone, best

273

known as the bony component lying under articular cartilage, supports articular

274

cartilage, and spreads mechanical loads across joint surfaces . Thus,

275

micro-architecture changes in subchondral bone might lead to cartilage destabilization,

276

and further contribute to cartilage degeneration. In turn, abnormal mechanical stresses

277

from cartilage may cause microfractures in the subchondral bone, or osteochondral

278

junction, which promotes subchondral bone remodeling and further accelerates

279

development of subchondral sclerosis as well as osteophyte formation .

280

PI3K/AKT/mTOR is a key metabolic pathway in subchondral bone in osteoarthritis .

281

Increased phosphorylation of AKT is seen in subchondral bone in a mouse model of

282

post-traumatic OA, promoting osteogenic differentiation and osteoblastic proliferation,

283

and resulting in aberrant bone formation. By contrast, PI3K/AKT inhibitor

284

(LY294002) treatment reduces subchondral bone sclerosis by decreasing osteogenesis

285

in OA mice, and simultaneously attenuates cartilage degeneration . A report by Lin

286

et.al. found that mTORC1 was activated in subchondral bone preosteoblasts in OA

72

72

73

74

75

287

patients and mice. Activated mTORC1 in preosteoblasts stimulated bone sclerosis and

288

secretion of CXCL12 which aggravate OA. Nevertheless, inhibiting mTORC1 by

289

disrupting Raptor (an mTORC1-specific component) delayed subchondral bone

290

formation and cartilage degeneration in OA mice . Consistent with these results,

291

activation of PI3K/AKT/mTOR also promoted osteoblastic differentiation in

292

pre-osteoblasts and bone mesenchymal stem cells, and targeted inhibition of

293

PI3K/AKT signaling reduced bone formation in vivo and in vitro

294

are two side to bone formation as reduced bone formation is also substantially

295

responsible for osteoporosis . To avoid the potential side effects induced by

296

pharmacological inhibition of PI3K/AKT, inhibitors that can specifically target

297

subchondral bone should be further researched.

76

77, 78

. However, there

79

298 299

Synovium

300

Synovial inflammation is a critical component of the pathophysiology of OA and it

301

has shown to predict the progression of structural damage. A clinical study has found

302

that synovitis occurred in ~50% of OA patients (n = 422) . Likewise, magnetic

303

resonance imaging (MRI)-estimated synovitis predicts the risk of incident knee OA .

304

Synovitis is mainly triggered by release of cytokines and catabolic products by

305

synovial cells including synoviocytes, fibroblasts, and macrophages . These

306

cytokines, such as IL-1β, IL-6, and TNF-α, diffuse into the cartilage, further

307

stimulating secretion of damage mediators in chondrocytes to amplify synovial

308

inflammation and cartilage destruction . PI3Kδ and PI3Kγ are highly expressed in

309

rheumatoid arthritis (RA) synovium and cultured synoviocytes, and closely related to

80

81

82

83

84, 85

310

modulation of synovial inflammation

311

proliferation and migration of fibroblast-like synoviocytes (FLS), which may

312

contribute to cartilage damage. Also, PI3Kδ expression is stimulated by inflammatory

313

cytokines IL-1β and TNF-α. Targeting PI3Kδ could decrease inflammatory levels in

314

synoviocytes . In addition, PI3Kγ deficiency reduces TNF-α-induced MMP

315

expression, and activation of AKT and ERK in synovial fibroblasts . IPI-145, a novel

316

inhibitor of PI3Kδ, and γ, represses inflammatory arthritis and cartilage damage .

317

The PI3K downstream effector mTOR also serves as a pro-inflammatory response

318

regulator in synovium. TNF-α stimulates mTOR activation in cultured FLSs, while

319

mTOR negatively modulates TNF-upregulated expression of multiple

320

pro-inflammatory cytokines or chemokines such as IL-6, IL-8, CCL20, CXCL11,

321

MMP1, MMP3, and prostaglandin-endoperoxide synthase 2 (PTGS2) by limiting

322

activation of NF-κB signaling to shift synovial FLS inflammation . In OA, the

323

mTOR inhibitor rapamycin alleviates synovitis and IL-1β levels in synovial tissue in

324

OA mouse knees . This finding suggests mTOR might positively modulates

325

OA-related inflammation response. The different roles of mTOR on synovial

326

inflammation between RA and OA may attribute to different pathogenesis of the

327

deceases and should be further clarified. In addition, accumulating evidence indicates

328

basic calcium phosphate (BCP) crystals are present in the majority of OA joints, and

329

are potent drivers of inflammation correlated with OA progression . Synovial

330

macrophages are reportedly involved in initiating and enhancing BCP-induced

331

damage . Activation of Syk and PI3 kinase, induced by BCP crystals in macrophages

332

might mediate production of damage-associated molecules. Administration of Syk and

. PI3Kδ is known to control the

86

85

87

88

89

90

90

333

PI3 kinase inhibitors reverses excess production of these molecule, identifying Syk

334

and PI3 kinase as regulators for treatment of BCP-related OA . Taken together, these

335

results suggest inhibition of PI3K/AKT/mTOR could alleviate synovial inflammation

336

in OA. Nevertheless, synovial microenvironment is complex and synovium contains

337

many kinds of cells. The precise mechanism of the regulatory role of

338

PI3K/AKT/mTOR signaling in these cells still not clear and needs more

339

investigations.

91

340 341

Figure2.

342 343

Therapeutic Prospects

344

To date, the key management strategies for osteoarthritis have included

345

non-pharmacological (e.g. education and self-management, exercises, weight loss if

346

overweight), pharmacological (e.g. NSAIDs and intra-articular injection of

347

corticosteroids), and surgical approaches . Traditional therapies are effective for

348

alleviating related clinical symptoms and improving quality of life to some extent.

349

Nevertheless, they fail to reverse cartilage degradation, and may cause adverse

350

events . The targeting of key molecules and signaling pathways involved in the

351

pathogenesis of OA has been extensively investigated. Considering the important role

352

of PI3K/AKT/mTOR signaling in OA, it might offer promising targets for treatment

353

of OA (table 1). Currently, PI3K/AKT/mTOR signaling-based intervention strategies

354

for OA can be divided into two main categories: (1) inhibition of PI3K/AKT/mTOR

355

signaling attenuates joint damage due to OA by restoring cartilage homeostasis,

356

enhancing autophagy, and suppressing inflammatory responses. (2) Activation of

357

PI3K/AKT/mTOR signaling may play an anti-arthritic role by promoting chondrocyte

92

93

358

proliferation, and reducing apoptosis.

359

Some approaches and agents that could block transduction of PI3K/AKT/mTOR

360

signaling might be beneficial to patients with the disease. For example,

361

small-molecule inhibitors of PI3K, AKT, and mTOR (LY294002, Casodex, and

362

rapamycin) are shown to promote autophagy of articular chondrocytes, and attenuate

363

the inflammation response in rats with OA . Additionally, blocking PI3K/AKT

364

signaling with LY294002 reduces bone sclerosis in subchondral bone, and delays

365

post-traumatic osteoarthritis . Apart from well characterized inhibitors, some

366

bioactive compounds isolated from herbs could protect joints from OA via inhibition

367

of this pathway. Leonurine, vanillic acid, and scoparone have been demonstrated to

368

ameliorate both chondrocyte and cartilage injury in mice by promoting autophagy

369

and/or repressing inflammatory responses

370

mTOR and NF-κB serve as master modulators responsible for initiation of autophagy

371

and inflammation.

372

In another way, activation of PI3K/AKT/mTOR signaling may be beneficial for

373

patients with the disease. Activated PI3K/AKT/mTOR signaling has been found to

374

promote chondrocyte proliferation and reduce apoptosis. Thus, development of some

375

therapeutic approach to activate signaling for its protective aspect is a worthy pursuit.

376

As expected, some agents, such as 17β-estradiol (E2), FGF18, and ghrelin, which

377

have been correlated with PI3K/AKT activation have a potential protective effect

378

against OA by increasing chondrocyte proliferation or reducing apoptosis

379

Furthermore, microRNAs play an essential role in modulating PI3K/AKT/mTOR

380

signaling and its effects on OA development. Cai et.al. found miR-27a is a regulator

381

of the PI3K-AKT-mTOR axis in human chondrocytes and participates in OA

94

75

39, 41, 95

. The key downstream effectors,

28, 54, 96

.

382

pathogenesis. Chondrocytes transfected with miR-27a inhibitor reduced

383

IL-1β-induced apoptosis via upregulation of PI3K activity . Similarly, miR-218-5p,

384

shown to target PIK3C2A mRNA, is a novel inducer of cartilage destruction. Its

385

expression substantially affected expression of matrix synthesis genes, chondrocyte

386

proliferation, and apoptosis. OA mice exposed to a miR-218-5p inhibitor were

387

protected from cartilage degradation . These evidences suggest microRNAs have

388

potential as therapeutic targets in osteoarthritis. In addition, some molecules that

389

affect PI3K/AKT/mTOR activity might also be considered as potential therapeutic

390

targets. For example, peroxiredoxin 4 (PRDX4) overexpression could activate AKT,

391

to reverse IL-1β-stimulated apoptosis mediated by increased BAX levels and

392

Caspase-3/9 activation . Consistent with the effects of PRDX4, activated

393

glucagon-like peptide-1 receptor exerts a similar influence on the PI3K/AKT axis and

394

downstream cell apoptosis

395

approaches to regulating the functions of PI3K/AKT/mTOR in OA development.

396

As PI3K/AKT/mTOR signaling undertakes multiple functions in normal and

397

abnormal cells, its effects are complex, making it difficult to verify the effects of this

398

axis in OA in general, and there remain many prominent issues to be addressed. First,

399

does activation or inhibition contribute most to OA pathogenesis, and which best

400

protects against OA progression. In other words, the PI3K/AKT/mTOR axis, at least

401

in part, mediates inflammation, autophagy, proliferation, apoptosis, ECM homeostasis,

402

and other cell processes in chondrocytes. Which are the dominant ones? What are the

403

cross talks among them? Cross talks between prominent cellular processes related to

404

the PI3K/AKT/mTOR pathway can occur also in joint cells in a timely manner during

405

the OA process. Thus, more detailed research into these cellular processes is needed

97

98

99

100

. Targeting these genes provide new insights and

406

to clarify their connection to PI3K/AKT/mTOR signaling. Moreover, its role in

407

mammals varies from tissue to tissue, particularly for whole joints, which include

408

cartilage, subchondral bone, and synovium. Hence, it is also important to target

409

tissues precisely and correctly to achieve axis-mediated protective effects during OA

410

treatment. As a consequence, it is not appropriate to simplistically link

411

PI3K/AKT/mTOR to the disease, and develop modifying agents. The challenges we

412

discussed above need to be solved (Fig.3).

413 414

Figure3.

415 416

Conclusion

417

As described in this review, PI3K/AKT/mTOR is a complex signaling pathway with

418

multiple regulators and effectors. Most importantly, this signaling is essential for

419

development of OA. Current research also provides evidence that targeting this axis

420

may be a viable therapeutic approach. However, simply activating or inhibiting

421

PI3K/AKT/mTOR signaling to protect against OA may be a double-edged sword,

422

since side effects seem to be unavoidable with this approach. Therefore, in the near

423

future, it is imperative to clarify the roles of PI3K/AKT/mTOR in OA during different

424

pathophysiological stages, and elucidate more specific molecular mechanisms, such

425

as how this axis interacts with other signaling pathways, and how to target its function

426

in OA without disrupting important physiological axis-regulated processes. If these

427

intractable points are addressed, PI3K/AKT/mTOR-based treatments for OA may

428

become safe and effective.

429 430

Authors' contributions

431

Fengjing Guo worked on design and conception of this review. Kai Sun drafted the

432

paper. Jiahui Luo, Jiachao Guo, Xudong Yao and Xingzhi Jing contributed to revise

433

the paper. Fengjing Guo gave final approval of the version to be submitted.

434 435

Declarations of interest

436

The authors confirm that there are no conflicts of interest.

437 438

Acknowledgments

439

This study was supported by the National Natural Science Foundation of China [no.

440

81874020].

441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459

.

References:

460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502

1. Nelson AE. Osteoarthritis year in review 2017: clinical. Osteoarthritis and Cartilage 2018; 26: 319-325. 2. Fu K, Robbins SR, McDougall JJ. Osteoarthritis: the genesis of pain. Rheumatology 2017; 57: v43-v50. 3. Rahmati M, Nalesso G, Mobasheri A, Mozafari M. Aging and osteoarthritis: Central role of the extracellular matrix. Ageing Research Reviews 2017; 40: 20-30. 4. Litherland GJ, Dixon C, Lakey RL, Robson T, Jones D, Young DA, et al. Synergistic Collagenase Expression and Cartilage Collagenolysis Are Phosphatidylinositol 3-Kinase/Akt Signaling-dependent. Journal of Biological Chemistry 2008; 283: 14221-14229. 5. Engelman JA, Luo J, Cantley LC. The evolution of phosphatidylinositol 3-kinases as regulators of growth and metabolism. Nature Reviews Genetics 2006; 7: 606-619. 6. De Santis MC, Gulluni F, Campa CC, Martini M, Hirsch E. Targeting PI3K signaling in cancer: Challenges and advances. Biochimica et Biophysica Acta (BBA) - Reviews on Cancer 2019; 1871: 361-366. 7. Guo H, German P, Bai S, Barnes S, Guo W, Qi X, et al. The PI3K/AKT Pathway and Renal Cell Carcinoma. Journal of Genetics and Genomics 2015; 42: 343-353. 8. Vanhaesebroeck B, Guillermet-Guibert J, Graupera M, Bilanges B. The emerging mechanisms of isoform-specific PI3K signalling. Nature Reviews Molecular Cell Biology 2010; 11: 329. 9. Lee Y, Chen M, Pandolfi PP. The functions and regulation of the PTEN tumour suppressor: new modes and prospects. Nature Reviews Molecular Cell Biology 2018; 19: 547-562. 10. Alessi DR, James SR, Downes CP, Holmes AB, Gaffney PR, Reese CB, et al. Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Balpha. Curr. Biol. 1997; 7: 261-269. 11. Huang J, Manning BD. A complex interplay between Akt, TSC2 and the two mTOR complexes. Biochemical Society Transactions 2009; 37: 217. 12. Manning BD, Toker A. AKT/PKB Signaling: Navigating the Network. Cell 2017; 169: 381-405. 13. Dibble CC, Cantley LC. Regulation of mTORC1 by PI3K signaling. Trends in Cell Biology 2015; 25: 545-555. 14. Park J, Jung CH, Seo M, Otto NM, Grunwald D, Kim KH, et al. The ULK1 complex mediates MTORC1 signaling to the autophagy initiation machinery via binding and phosphorylating ATG14. Autophagy 2016; 12: 547-564. 15. Laplante M, Sabatini DM. mTOR Signaling in Growth Control and Disease. Cell 2012; 149: 274-293. 16. Nnah IC, Wang B, Saqcena C, Weber GF, Bonder EM, Bagley D, et al. TFEB-driven endocytosis coordinates MTORC1 signaling and autophagy. Autophagy 2019; 15: 151-164. 17. Ganley IG, Lam DH, Wang J, Ding X, Chen S, Jiang X. ULK1·ATG13·FIP200 Complex Mediates mTOR Signaling and Is Essential for Autophagy. Journal of Biological Chemistry 2009; 284: 12297-12305. 18. Jafari M, Ghadami E, Dadkhah T, Akhavan-Niaki H. PI3k/AKT signaling pathway: Erythropoiesis and beyond. J. Cell. Physiol. 2019; 234: 2373-2385. 19. Tang F, Wang Y, Hemmings BA, Rüegg C, Xue G. PKB/Akt-dependent regulation of inflammation in cancer. Semin. Cancer Biol. 2018; 48: 62-69.

503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532

20. Cravero JD, Carlson CS, Im HJ, Yammani RR, Long D, Loeser RF. Increased expression of the Akt/PKB inhibitor TRB3 in osteoarthritic chondrocytes inhibits insulin-like growth factor 1-mediated cell survival and proteoglycan synthesis. Arthritis Rheum 2009; 60: 492-500. 21. Lin Z, Zhou P, von Gise A, Gu F, Ma Q, Chen J, et al. Pi3kcb links Hippo-YAP and PI3K-AKT signaling pathways to promote cardiomyocyte proliferation and survival. Circ. Res. 2015; 116: 35-45. 22. Shum M, Bellmann K, St-Pierre P, Marette A. Pharmacological inhibition of S6K1 increases glucose metabolism and Akt signalling in vitro and in diet-induced obese mice. Diabetologia 2016; 59: 592-603. 23. Fisch KM, Gamini R, Alvarez-Garcia O, Akagi R, Saito M, Muramatsu Y, et al. Identification of transcription factors responsible for dysregulated networks in human osteoarthritis cartilage by global gene expression analysis. Osteoarthr. Cartil. 2018; 26: 1531-1538. 24. Mobasheri A, Rayman MP, Gualillo O, Sellam J, van der Kraan P, Fearon U. The role of metabolism in the pathogenesis of osteoarthritis. Nat Rev Rheumatol 2017; 13: 302-311. 25. Rosa SC, Rufino AT, Judas F, Tenreiro C, Lopes MC, Mendes AF. Expression and function of the insulin receptor in normal and osteoarthritic human chondrocytes: modulation of anabolic gene expression, glucose transport and GLUT-1 content by insulin. Osteoarthr. Cartil. 2011; 19: 719-727. 26. Fisch KM, Gamini R, Alvarez-Garcia O, Akagi R, Saito M, Muramatsu Y, et al. Identification of transcription factors responsible for dysregulated networks in human osteoarthritis cartilage by global gene expression analysis. Osteoarthr. Cartil. 2018; 26: 1531-1538. 27. Stanic I, Facchini A, Borzi RM, Vitellozzi R, Stefanelli C, Goldring MB, et al. Polyamine depletion inhibits apoptosis following blocking of survival pathways in human chondrocytes stimulated by tumor necrosis factor-alpha. J Cell Physiol 2006; 206: 138-146. 28. Yao X, Zhang J, Jing X, Ye Y, Guo J, Sun K, et al. Fibroblast growth factor 18 exerts anti-osteoarthritic effects through PI3K-AKT signaling and mitochondrial fusion and fission. Pharmacological Research 2019; 139: 314-324. 29. Iwasa K, Hayashi S, Fujishiro T, Kanzaki N, Hashimoto S, Sakata S, et al. PTEN regulates matrix synthesis in adult human chondrocytes under oxidative stress. J. Orthop. Res. 2014; 32: 231-237. 30. Greene MA, Loeser RF. Function of the chondrocyte PI-3 kinase-Akt signaling pathway is stimulus dependent. Osteoarthr. Cartil. 2015; 23: 949-956. 31. Venkatesan JK, Rey-Rico A, Schmitt G, Wezel A, Madry H, Cucchiarini M. rAAV-mediated

533

overexpression of TGF- stably restructures human osteoarthritic articular cartilage in situ. J Transl Med

534 535 536 537 538 539 540 541 542 543 544 545

2013; 11: 211. 32. Zhang Q, Lai S, Hou X, Cao W, Zhang Y, Zhang Z. Protective effects of PI3K/Akt signal pathway induced cell autophagy in rat knee joint cartilage injury. Am J Transl Res 2018; 10: 762-770. 33. Qureshi HY, Ahmad R, Sylvester J, Zafarullah M. Requirement of phosphatidylinositol 3-kinase/Akt signaling pathway for regulation of tissue inhibitor of metalloproteinases-3 gene expression by TGF-beta in human chondrocytes. Cell. Signal. 2007; 19: 1643-1651. 34. Zhang M, Zhou Q, Liang QQ, Li CG, Holz JD, Tang D, et al. IGF-1 regulation of type II collagen and MMP-13 expression in rat endplate chondrocytes via distinct signaling pathways. Osteoarthr. Cartil. 2009; 17: 100-106. 35. Hui W, Litherland GJ, Elias MS, Kitson GI, Cawston TE, Rowan AD, et al. Leptin produced by joint white adipose tissue induces cartilage degradation via upregulation and activation of matrix metalloproteinases. Ann. Rheum. Dis. 2012; 71: 455-462.

546 547 548 549 550

36. Robinson WH, Lepus CM, Wang Q, Raghu H, Mao R, Lindstrom TM, et al. Low-grade inflammation as a key mediator of the pathogenesis of osteoarthritis. Nat Rev Rheumatol 2016; 12: 580-592. 37. Kapoor M, Martel-Pelletier J, Lajeunesse D, Pelletier J, Fahmi H. Role of proinflammatory cytokines in the pathophysiology of osteoarthritis. Nature Reviews Rheumatology 2010; 7: 33.

551

38. Park C, Jeong JW, Lee DS, Yim MJ, Lee JM, Han MH, et al. Extract Attenuates Interleukin-1

552

-Induced Oxidative Stress and Inflammatory Response in Chondrocytes by Suppressing the Activation

553

of NF- B, p38 MAPK, and PI3K/Akt. Int J Mol Sci 2018; 19.

554

39. Hu ZC, Gong LF, Li XB, Fu X, Xuan JW, Feng ZH, et al. Inhibition of PI3K/Akt/NF- B signaling

555 556 557 558 559

with leonurine for ameliorating the progression of osteoarthritis: In vitro and in vivo studies. J. Cell.

560

41. Lu C, Li Y, Hu S, Cai Y, Yang Z, Peng K. Scoparone prevents IL-1 -induced inflammatory

561

response in human osteoarthritis chondrocytes through the PI3K/Akt/NF- B pathway. Biomed.

562

Pharmacother. 2018; 106: 1169-1174.

563 564

Physiol. 2019; 234: 6940-6950. 40. Xue JF, Shi ZM, Zou J, Li XL. Inhibition of PI3K/AKT/mTOR signaling pathway promotes autophagy of articular chondrocytes and attenuates inflammatory response in rats with osteoarthritis. Biomed. Pharmacother. 2017; 89: 1252-1261.

42. Choi MC, Jo J, Park J, Kang HK, Park Y. NF- B Signaling Pathways in Osteoarthritic Cartilage Destruction. Cells 2019; 8.

565

43. Balwani S, Chaudhuri R, Nandi D, Jaisankar P, Agrawal A, Ghosh B. Regulation of NF- B

566 567 568 569 570 571 572 573 574

activation through a novel PI-3K-independent and PKA/Akt-dependent pathway in human umbilical

575

48. Huang JG, Xia C, Zheng XP, Yi TT, Wang XY, Song G, et al. 17 -Estradiol promotes cell

576 577

proliferation in rat osteoarthritis model chondrocytes via PI3K/Akt pathway. Cell. Mol. Biol. Lett. 2011;

578

49. Fan DX, Yang XH, Li YN, Guo L. 17 -Estradiol on the Expression of G-Protein Coupled Estrogen

579 580

Receptor (GPER/GPR30) Mitophagy, and the PI3K/Akt Signaling Pathway in ATDC5 Chondrocytes In

vein endothelial cells. PLoS ONE 2012; 7: e46528. 44. Hunter DJ, Bierma-Zeinstra S. Osteoarthritis. Lancet 2019; 393: 1745-1759. 45. Kühn K, D'Lima DD, Hashimoto S, Lotz M. Cell death in cartilage. Osteoarthr. Cartil. 2004; 12: 1-16. 46. Varela-Eirin M, Loureiro J, Fonseca E, Corrochano S, Caeiro JR, Collado M, et al. Cartilage regeneration and ageing: Targeting cellular plasticity in osteoarthritis. Ageing Res. Rev. 2018; 42: 56-71. 47. Sowers MR, McConnell D, Jannausch M, Buyuktur AG, Hochberg M, Jamadar DA. Estradiol and its metabolites and their association with knee osteoarthritis. Arthritis Rheum. 2006; 54: 2481-2487.

16: 564-575.

Vitro. Med. Sci. Monit. 2018; 24: 1936-1947.

581 582 583 584 585 586 587 588 589 590 591 592 593 594

50. Huang Z, Zhang N, Ma W, Dai X, Liu J. MiR-337-3p promotes chondrocytes proliferation and inhibits apoptosis by regulating PTEN/AKT axis in osteoarthritis. Biomedicine & Pharmacotherapy 2017; 95: 1194-1200. 51. Zhang S, Chuah SJ, Lai RC, Hui J, Lim SK, Toh WS. MSC exosomes mediate cartilage repair by enhancing proliferation, attenuating apoptosis and modulating immune reactivity. Biomaterials 2018; 156: 16-27. 52. Kim SJ, Hwang SG, Kim IC, Chun JS. Actin cytoskeletal architecture regulates nitric oxide-induced apoptosis, dedifferentiation, and cyclooxygenase-2 expression in articular chondrocytes via mitogen-activated protein kinase and protein kinase C pathways. J. Biol. Chem. 2003; 278: 42448-42456. 53. Oh CD, Chun JS. Signaling mechanisms leading to the regulation of differentiation and apoptosis of articular chondrocytes by insulin-like growth factor-1. J. Biol. Chem. 2003; 278: 36563-36571. 54. Qu R, Chen X, Wang W, Qiu C, Ban M, Guo L, et al. Ghrelin protects against osteoarthritis through interplay with Akt and NF- B signaling pathways. FASEB J. 2018; 32: 1044-1058.

595

55. Montaseri A, Busch F, Mobasheri A, Buhrmann C, Aldinger C, Rad JS, et al. IGF-1 and PDGF-bb

596

suppress IL-1 -induced cartilage degradation through down-regulation of NF- B signaling: involvement

597 598

of Src/PI-3K/AKT pathway. PLoS ONE 2011; 6: e28663.

599

degeneration in interleukin-1 -stimulated rat chondrocytes and in a rat model of osteoarthritis via Akt

600

signalling. J. Cell. Mol. Med. 2014; 18: 283-292.

601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620

56. Zhao H, Zhang T, Xia C, Shi L, Wang S, Zheng X, et al. Berberine ameliorates cartilage

57. Yang Y, Wang Y, Zhao M, Jia H, Li B, Xing D. Tormentic acid inhibits IL-1 -induced chondrocyte apoptosis by activating the PI3K/Akt signaling pathway. Mol Med Rep 2018; 17: 4753-4758. 58. Vasheghani F, Zhang Y, Li YH, Blati M, Fahmi H, Lussier B, et al. PPARgamma deficiency results in severe, accelerated osteoarthritis associated with aberrant mTOR signalling in the articular cartilage. Ann Rheum Dis 2015; 74: 569-578. 59. Stanic I, Facchini A, Borzì RM, Vitellozzi R, Stefanelli C, Goldring MB, et al. Polyamine depletion inhibits apoptosis following blocking of survival pathways in human chondrocytes stimulated by tumor necrosis factor-alpha. J. Cell. Physiol. 2006; 206: 138-146. 60. Levine B, Kroemer G. Autophagy in the pathogenesis of disease. Cell 2008; 132: 27-42. 61. Mathew R, Karp CM, Beaudoin B, Vuong N, Chen G, Chen HY, et al. Autophagy suppresses tumorigenesis through elimination of p62. Cell 2009; 137: 1062-1075. 62. Duarte JH. Osteoarthritis: Autophagy prevents age-related OA. Nat Rev Rheumatol 2015; 11: 683. 63. Dalle PP, Ruf S, Sonntag AG, Langelaar-Makkinje M, Hall P, Heberle AM, et al. A systems study reveals concurrent activation of AMPK and mTOR by amino acids. Nat Commun 2016; 7: 13254. 64. Zhang Y, Vasheghani F, Li YH, Blati M, Simeone K, Fahmi H, et al. Cartilage-specific deletion of mTOR upregulates autophagy and protects mice from osteoarthritis. Ann. Rheum. Dis. 2015; 74: 1432-1440. 65. Caramés B, Hasegawa A, Taniguchi N, Miyaki S, Blanco FJ, Lotz M. Autophagy activation by rapamycin reduces severity of experimental osteoarthritis. Ann. Rheum. Dis. 2012; 71: 575-581. 66. Yang H, Wen Y, Zhang M, Liu Q, Zhang H, Zhang J, et al. MTORC1 coordinates the autophagy and

621 622 623 624 625 626

apoptosis signaling in articular chondrocytes in osteoarthritic temporomandibular joint. Autophagy 2019:

627

autophagy by inflammatory signals limits IL-1 production by targeting ubiquitinated inflammasomes

628

for destruction. Nat. Immunol. 2012; 13: 255-263.

1-18. 67. Tchetina EV, Poole AR, Zaitseva EM, Sharapova EP, Kashevarova NG, Taskina EA, et al. Differences in Mammalian target of rapamycin gene expression in the peripheral blood and articular cartilages of osteoarthritic patients and disease activity. Arthritis 2013; 2013: 461486. 68. Shi CS, Shenderov K, Huang NN, Kabat J, Abu-Asab M, Fitzgerald KA, et al. Activation of

629

69. Vasheghani F, Zhang Y, Li YH, Blati M, Fahmi H, Lussier B, et al. PPAR deficiency results in

630 631 632

severe, accelerated osteoarthritis associated with aberrant mTOR signalling in the articular cartilage.

633

suppresses the IL-1 -induced expression of IL-6 and matrix-metalloproteases by activating autophagy in

634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661

human osteoarthritis chondrocytes. Biomed. Pharmacother. 2017; 96: 198-207.

Ann. Rheum. Dis. 2015; 74: 569-578. 70. Ansari MY, Khan NM, Haqqi TM. A standardized extract of Butea monosperma (Lam.) flowers

71. Caramés B, Hasegawa A, Taniguchi N, Miyaki S, Blanco FJ, Lotz M. Autophagy activation by rapamycin reduces severity of experimental osteoarthritis. Ann. Rheum. Dis. 2012; 71: 575-581. 72. Goldring SR, Goldring MB. Changes in the osteochondral unit during osteoarthritis: structure, function and cartilage-bone crosstalk. Nat Rev Rheumatol 2016; 12: 632-644. 73. Funck-Brentano T, Cohen-Solal M. Subchondral bone and osteoarthritis. Curr Opin Rheumatol 2015; 27: 420-426. 74. Guo SM, Wang JX, Li J, Xu FY, Wei Q, Wang HM, et al. Identification of gene expression profiles and key genes in subchondral bone of osteoarthritis using weighted gene coexpression network analysis. J. Cell. Biochem. 2018; 119: 7687-7695. 75. Lin C, Shao Y, Zeng C, Zhao C, Fang H, Wang L, et al. Blocking PI3K/AKT signaling inhibits bone sclerosis in subchondral bone and attenuates post-traumatic osteoarthritis. Journal of Cellular Physiology 2018; 233: 6135-6147. 76. Lin C, Liu L, Zeng C, Cui ZK, Chen Y, Lai P, et al. Activation of mTORC1 in subchondral bone preosteoblasts promotes osteoarthritis by stimulating bone sclerosis and secretion of CXCL12. Bone Res 2019; 7: 5. 77. Baker N, Sohn J, Tuan RS. Promotion of human mesenchymal stem cell osteogenesis by PI3-kinase/Akt signaling, and the influence of caveolin-1/cholesterol homeostasis. Stem Cell Res Ther 2015; 6: 238. 78. Lauzon MA, Drevelle O, Daviau A, Faucheux N. Effects of BMP-9 and BMP-2 on the PI3K/Akt Pathway in MC3T3-E1 Preosteoblasts. Tissue Eng Part A 2016; 22: 1075-1085. 79. Compston JE, McClung MR, Leslie WD. Osteoporosis. Lancet 2019; 393: 364-376. 80. Ayral X, Pickering EH, Woodworth TG, Mackillop N, Dougados M. Synovitis: a potential predictive factor of structural progression of medial tibiofemoral knee osteoarthritis -- results of a 1 year longitudinal arthroscopic study in 422 patients. Osteoarthr. Cartil. 2005; 13: 361-367. 81. Felson DT, Niu J, Neogi T, Goggins J, Nevitt MC, Roemer F, et al. Synovitis and the risk of knee osteoarthritis: the MOST Study. In, vol. 24 2016:458-464. 82. Marchev AS, Dimitrova PA, Burns AJ, Kostov RV, Dinkova-Kostova AT, Georgiev MI. Oxidative

662 663 664 665 666 667 668 669 670 671

stress and chronic inflammation in osteoarthritis: can NRF2 counteract these partners in crime? Ann. N. Y. Acad. Sci. 2017; 1401: 114-135. 83. Bhattaram P, Chandrasekharan U. The joint synovium: A critical determinant of articular cartilage fate in inflammatory joint diseases. Semin. Cell Dev. Biol. 2017; 62: 86-93. 84. Bartok B, Boyle DL, Liu Y, Ren P, Ball ST, Bugbee WD, et al. PI3 kinase

is a key regulator of

synoviocyte function in rheumatoid arthritis. Am. J. Pathol. 2012; 180: 1906-1916. 85. Hayer S, Pundt N, Peters MA, Wunrau C, Kühnel I, Neugebauer K, et al. PI3Kgamma regulates cartilage damage in chronic inflammatory arthritis. FASEB J. 2009; 23: 4288-4298. 86. Bartok B, Boyle DL, Liu Y, Ren P, Ball ST, Bugbee WD, et al. PI3 kinase

is a key regulator of

synoviocyte function in rheumatoid arthritis. Am. J. Pathol. 2012; 180: 1906-1916.

672

87. Boyle DL, Kim HR, Topolewski K, Bartok B, Firestein GS. Novel phosphoinositide 3-kinase ,

673 674 675 676 677 678 679 680

inhibitor: potent anti-inflammatory effects and joint protection in models of rheumatoid arthritis. J.

681

induce monocyte/macrophage IL-1 secretion through the NLRP3 inflammasome in vitro. J. Immunol.

682 683 684 685 686 687 688 689 690 691 692 693 694

2011; 186: 2495-2502.

695

by regulating the MAPK and PI3K/AKT/NF- B pathways. Eur. J. Pharmacol. 2019; 859: 172481.

696 697 698 699 700

Pharmacol. Exp. Ther. 2014; 348: 271-280. 88. Karonitsch T, Kandasamy RK, Kartnig F, Herdy B, Dalwigk K, Niederreiter B, et al. mTOR Senses Environmental Cues to Shape the Fibroblast-like Synoviocyte Response to Inflammation. Cell Rep 2018; 23: 2157-2167. 89. Caramés B, Hasegawa A, Taniguchi N, Miyaki S, Blanco FJ, Lotz M. Autophagy activation by rapamycin reduces severity of experimental osteoarthritis. Ann. Rheum. Dis. 2012; 71: 575-581. 90. Pazár B, Ea HK, Narayan S, Kolly L, Bagnoud N, Chobaz V, et al. Basic calcium phosphate crystals

91. Corr EM, Cunningham CC, Helbert L, McCarthy GM, Dunne A. Osteoarthritis-associated basic calcium phosphate crystals activate membrane proximal kinases in human innate immune cells. Arthritis Res. Ther. 2017; 19: 23. 92. Nelson AE, Allen KD, Golightly YM, Goode AP, Jordan JM. A systematic review of recommendations and guidelines for the management of osteoarthritis: The chronic osteoarthritis management initiative of the U.S. bone and joint initiative. Semin. Arthritis Rheum. 2014; 43: 701-712. 93. Ghouri A, Conaghan PG. Update on novel pharmacological therapies for osteoarthritis. Ther Adv Musculoskelet Dis 2019; 11: 1759720X-19864492X. 94. Xue JF, Shi ZM, Zou J, Li XL. Inhibition of PI3K/AKT/mTOR signaling pathway promotes autophagy of articular chondrocytes and attenuates inflammatory response in rats with osteoarthritis. Biomed. Pharmacother. 2017; 89: 1252-1261. 95. Huang X, Xi Y, Mao Z, Chu X, Zhang R, Ma X, et al. Vanillic acid attenuates cartilage degeneration

96. Huang JG, Xia C, Zheng XP, Yi TT, Wang XY, Song G, et al. 17beta-Estradiol promotes cell proliferation in rat osteoarthritis model chondrocytes via PI3K/Akt pathway. Cell Mol Biol Lett 2011; 16: 564-575. 97. Cai C, Min S, Yan B, Liu W, Yang X, Li L, et al. MiR-27a promotes the autophagy and apoptosis of IL-1

treated-articular chondrocytes in osteoarthritis through PI3K/AKT/mTOR signaling. Aging

701 702 703 704 705 706 707 708

(Albany NY) 2019; 11. 98. Lu J, Ji ML, Zhang XJ, Shi PL, Wu H, Wang C, et al. MicroRNA-218-5p as a Potential Target for the Treatment of Human Osteoarthritis. Mol. Ther. 2017; 25: 2676-2688. 99. Rao Z, Wang S, Wang J. Peroxiredoxin 4 inhibits IL-1 -induced chondrocyte apoptosis via PI3K/AKT signaling. Biomed. Pharmacother. 2017; 90: 414-420. 100. Chen J, Xie JJ, Shi KS, Gu YT, Wu CC, Xuan J, et al. Glucagon-like peptide-1 receptor regulates endoplasmic reticulum stress-induced apoptosis and the associated inflammatory response in chondrocytes and the progression of osteoarthritis in rat. Cell Death Dis 2018; 9: 212.

709 710 711 712 713 714 715 716 717

Figure legends

718

Fig. 1. Simplified scheme of PI3K/AKT/mTOR signaling pathway in chondrocyte.

719 720

Fig. 2. Phenotypes of osteoarthritis. OA is a whole-joint disease, charactercised by

721

cartilage degeneration, synovial inflammation, subchondral bone sclerosis and

722

osteophyte formation. The PI3K/AKT/mTOR signaling pathway is responsible for the

723

destructive alteration of these tissues. The depicts shows how PI3K/AKT/mTOR

724

signaling pathway regulates synovial inflammation, subchondral bone sclerosis, ECM

725

homeostasis, chondrocyte proliferation, apoptosis, autophagy and inflammation. An

726

imbalance in these cell processes contributes to OA progression.

727 728

729

Fig. 3. Challenges of the PI3K/AKT/mTOR-based treatment for OA. The

730

PI3K/AKT/mTOR signaling mediated synovial inflammation, subchondral bone

731

sclerosis, ECM homeostasis, chondrocyte proliferation, apoptosis, autophagy, and

732

inflammation greatly affect cell fate and OA pathophysiology. There will be an

733

imbalance among these cell processes if simply activating or inhibiting

734

PI3K/AKT/mTOR signaling. Thus, how this axis interacts with other signaling

735

pathways, and how to target its function in OA without disrupting important

736

physiological axis-regulated processes need to be clarfied.

Table 1. Potential PI3K/AKT/mTOR signaling-related inhibitors and regulators in OA treatment. Inhibitor/regulator Target cell/tissue IGF-1

Rat endplate chondrocytes

TGF-β

Rat cartilage

Leonurine

Target

Main findings

PI3K PI3K/AKT

Reference

Induces increased expression of col2a1 and reduced expression of MMP13

31

Induces decreased expression of MMP13

33

Mouse

PI3K/AKT/NF-κB

Reduces IL-1β-induced inflammatory response

40

Scoparone

Human chondrocytes

PI3K/AKT/NF-κB

Reduces IL-1β-induced inflammatory response

42

LY294002 Casodex Rapamycin

Rat chondrocytes

PI3K AKT mTOR

Promotes autophagy of articular chondrocytes and attenuates inflammatory response

94

Vanillic acid

Rat chondrocytes

MAPK and PI3K/AKT/NF-κB

Attenuates inflammatory response and cartilage degeneration

95

17beta-Estradiol

Rat OA chondrocytes

PI3K/AKT

Promotes cell proliferation

96

Rat chondrocytes

PI3K/AKT

Promotes chondrocyte proliferation and migration and attenuates IL-1β-induced apoptosis in vitro. attenuates cartilage degradation in vivo

29

FGF18

Ghrelin

Human chondrocyte and cartilage

miR-218-5p

SW1353 and C28/I2 cells

miR-27a

Human

Peroxiredoxin 4 (PRDX4)

Liraglutide

Platelet-derived growth factor

Tormentic acid

PI3K/AKT/mTOR

Promotes the autophagy and apoptosis of IL-1β treated-articular chondrocytes

97

PRDX4 overexpression reverses IL-1β-stimulated apoptosis

99

Protects chondrocytes against endoplasmic reticulum stress and apoptosis induced by interleukin (IL)-1β or triglycerides and attenuates rat cartilage degeneration in an OA model of knee joints in vivo Rescues IL-1β-induced increases in mitochondrial-related apoptosis

100

Ameliorates cartilage degeneration from OA by promoting cell survival and matrix production of chondrocytes

57

Inhibits IL-1β-induced cytotoxicity and apoptosis in chondrocytes

58

mTOR

PPARgamma maintains articular cartilage homeostasis, in part, by regulating mTOR pathway

59

mTOR

BME has strong potential to activate autophagy and suppress IL-1β induced expression of IL-6 and MMP-3, -9 and - via inhibition of mTOR

71

Induces autophagy activation and alleviates synovitis and IL-1β levels in synovial tissue in OA mouse knees.

89

Disruption of Raptor reduces subchondral bone formation and cartilage degeneration, and attenuated post-traumatic OA in mice

77

PI3K

Reduces BCP-induced production of the pro-inflammatory cytokines

91

PI3K

Reduces bone sclerosis in subchondral bone and delays post-traumatic osteoarthritis.

76

Reduces bone formation in vivo and in vitro

78

GLP-1R/PI3K/AKT

Src/PI-3K/AKT

AKT

PI3K/AKT

Murine cartilage

The hydromethanolic extract of Butea monosperma (BME)

98

Rat chondrocytes

Human

PPARgamma

miR-218-5p inhibitor alleviates mice cartilage degradation with reduced proteoglycan loss and reduced loss of articular chondrocyte cellularity

AKT

Rat chondrocytes

Human

Rapamycin

Mouse articular cartilage and synovium

mTOR

Raptor

Preosteoblasts

mTORC1

LY294002

LY294002

LY294002

Primary and dendritic cells

human

Mouse osteoblastic cells

Human bone marrow MSCs

Down-regulates the production of various inflammatory 55 cytokines, inhibits apoptosis of chondrocytes, attenuate ECM degradation

PI3K/AKT/mTO R

Rat chondrocytes

Human

Berberin

AKT

PI3K/AKT

56