- Email: [email protected]
Translational Research: Not Just Bench to Bedside
NEWS AND VIEWS
Translational Research: Not Just Bench to Bedside Joel McCauley, MD,* and Jonathan D’Cunha, MD, PhD† The translation of proteins within the cell has been increasingly implicated in multiple malignancies as an important regulatory checkpoint. Although evidence continues to mount regarding the role of translation in cancer, questions persist as to how translation is activated and the overall significance in thoracic malignancies. Two recent articles are reviewed here that explore the role of enhanced translation in tumorigenesis and prognosis. Semin Thoracic Surg 22:197-199 © 2010 Elsevier Inc. All rights reserved. Keywords: thoracic oncology, translation, eIF4E, tumorigenesis, lung cancer Lung cancer continues to be the leading cause of cancer-related deaths. There will be an estimated 160,000 United States deaths from lung cancer this year.1 Although this number is significant, worldwide deaths attributed to lung cancer are expected to increase because the prevalence of smoking has not yet peaked in developing countries. Significant resources have been invested to elucidate molecular mechanisms and develop new therapies, but the overall 5-year survival remains abysmal at 15%. This clearly indicates that new molecular targets need to be identified for therapeutic intent. Most investigations of cancer genetics and epigenetics have focused on oncogenic mutations, transcriptional activation, or silencing of growth regulator genes. During the last decade, there has been mounting evidence that the translation of proteins within the cell is an important checkpoint in the control of cell growth and proliferation (Fig. 1). Although the translation of mRNA into protein is regulated at multiple levels, the rate-limiting factor is the binding of eukaryotic initiation factor 4E (eIF4E) to the 5= cap structure of mRNA. Enhanced eIF4E activity disproportionately alters the translation of mRNA encoding growth and survival factors associated with malignancy.2 eIF4E activity is primarily regulated by the eIF4E-binding protein (4E-BP) family of proteins. In the presence of favorable growth
*Division of Pulmonary, Allergy, Critical Care, and Sleep Medicine, Department of Medicine, University of Minnesota, Minneapolis, Minnesota. †Division of Thoracic and Foregut Surgery, Department of Surgery, University of Minnesota, Minneapolis, Minnesota. Address reprint requests to Jonathan D’Cunha, MD, PhD, Division of Thoracic and Foregut Surgery, Department of Surgery, University of Minnesota, MMC 207, 420 Delaware St SE, Minneapolis, MN 55455. E-mail: [email protected]
1043-0679/$-see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1053/j.semtcvs.2010.10.008
conditions, 4E-BPs are phosphorylated via the mammalian target of rapamycin (mTOR). Consequently, 4E-BPs dissociate from eIF4E, leaving eIF4E free to bind the 5= mRNA cap and initiate translation. In times of stress, 4E-BPs are dephosphorylated and associated with eIF4E, thereby inhibiting cap-mediated translation. In addition, eIF4E can be phosphorylated via the RAS/mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase pathway. Here we review 2 recent contributions that further solidify the role of translational activation in malignancy and a potential therapeutic target, but also suggest that the phosphorylation state of eIF4E plays an important role in tumor progression and survival.3,4 A series of well-designed experiments was published earlier this year (2010) by Furic et al3 investigating the role of phosphorylated eIF4E in tumorigenesis. The authors generated a transgenic mouse model with altered eI4FE, thus prohibiting its phosphorylation. From this transgenic mouse model cells were isolated for a series of in vitro experiments assessing cellular transformation and proliferation. Wild-type cells (exposed to oncogenic KRAS and c-myc) exhibited proficient colony formation in soft agar, a standard assay to establish cellular transformation. In the same setting, cells from the altered eIF4E mice formed 10fold fewer colonies, demonstrating the importance of phosphorylated eIF4E in transformation. To further dissect this point, the authors conducted a similar experiment comparing wild-type cells with those specifically engineered to overexpress eIF4E. As the downstream effector of RAS, mitogen-activated protein kinase-interacting kinase 1/2 (MNK1/2) phosphorylates eIF4E; thus, MNK1/2 double knockouts represent eIF4E overexpression. Wild-type cells overexpressing 197
TRANSLATIONAL RESEARCH
Figure 1. Schematic of cap-dependent translation. The phosphoinositide 3-kinase/AKT/ mTOR pathway is activated by growth factors, mitogens, and hormones. Hyperphosphorylated 4E-BPs then release eIF4E, thereby allowing it to be phosphorylated and cap-dependent translation may occur within the cytoplasm of the cell. This, in turn, potentially promotes the initiation of translation, mitogenesis, and oncogenesis.
eIF4E formed soft agar foci after 10 days. However, cells from the mice overexpressing eIF4E (in these mice this form of eIF4E is not able to be activated by phosphorylation) did not form any foci, confirming that eIF4E-transforming activity is dependent on its phosphorylation state. Although the in vitro data clearly implicate phosphorylated eIF4E in tumorigenesis, the authors proceeded to evaluate the in vivo effects in a prostate cancer mouse model and demonstrated that that phosphorylation of eIF4E plays a role in tumor progression in vivo. Although Furic et al clearly established the role of translational activation via eIF4E phosphorylation in cellular transformation and tumor progression, the question remains whether this is pertinent to thoracic malignancies. There are several studies conducted with human lung cancer cell lines and mouse lung cancer models that show enhancement of translation promotes tumorigenesis and that inhibition of eIF4E represses tumorigenesis. Yoshizawa et al4 reported earlier this year that the phosphorylation status of eIF4E can be used as a prognostic indicator in non-small cell lung 198
cancer (NSCLC). The authors wanted to elucidate whether activation of the canonical phosphoinositide 3-kinase/AKT/mTOR pathway has prognostic significance in NSCLC. In their study, 300 surgically resected lung cancers (150 adenocarcinoma, 150 squamous cell carcinomas) were used for microarray analysis. Survival time and outcome data were available for 252 of the samples using immunohistochemistry. Although upstream activators of mTOR were clearly activated, only overexpression of phosphorylated-eIF4E and phosphorylated-AKT correlated with a significantly worse 5-year prognosis (P ⬍ 0.0001; Fig. 2). In addition, a multivariate analysis using sex and stage identified only phosphorylated-eIF4E (P ⫽ 0.006) and cancer stage (P ⫽ 0.005) as independent prognostic factors. On the basis of this study, translational activation signified by phosphorylated-eIF4E can be used as a prognostic indicator and likely plays a significant role in NSCLC cancer progression. Enhanced translation within the cancer cell continues to be implicated as an important mechanism involved in tumorigenesis and cancer progression. As evidence continues to mount, efforts are underway to develop novel therapeutics capable of modulating this checkpoint.5,6 Although the 2 reports reviewed here further improve our understanding of how translation affects tumorigenesis and its role as a prognostic indicator, additional investigations are needed. Further dissection of translational activation will be insightful in clarifying mechanistic details as well as identifying further points that might be targeted for therapeutic intent.
Figure 2. Kaplan–Meier curve comparing overall survival for those patients with NSCLC and phosphorylated-eIF4E. There was worse survival for phosphorylated-eIF4E–positive patients (n ⫽ 91; blue line) compared with phosphorylatedeIF4E–negative patients (n ⫽ 143; (red line; A). (Reprinted by permission of the American Association for Cancer Research from Yoshizawa et al.4) (Color version of figure is available online at http://www.semthorcardiovascsurg.com.)
Seminars in Thoracic and Cardiovascular Surgery ● Volume 22, Number 3
TRANSLATIONAL RESEARCH
1. Jemal A, Siegel R, Xu J, et al: Cancer statistics. CA Cancer J Clin 60:277-300, 2010 2. Mamane Y, Petroulakis E, Rong L, et al: EIF4E—from translation to transformation. Oncogene 23:3172-3179, 2004 3. Furic L, Rong L, Larsson O, et al: EIF4E phosphorylation promotes tumorigenesis and is as-
sociated with prostate cancer progression. Proc Natl Acad Sci U S A 107:14134-14139, 2010 4. Yoshizawa A, Fukuoka J, Shimizu S, et al: Overexpression of phospho-eIF4E is associated with survival through AKT pathway in non-small cell lung cancer. Clin Cancer Res 16:240-248, 2010 5. Graff JR, Konicek BW, Carter JH, et al: Targeting
Seminars in Thoracic and Cardiovascular Surgery ● Volume 22, Number 3
the eukaryotic translation initiation factor 4E for cancer therapy. Cancer Res 68:631-634, 2008 6. Ghosh B, Benyumov AO, Ghosh P, et al: Nontoxic chemical interdiction of the epithelialto-mesenchymal transition by targeting capdependent translation. ACS Chem Biol 4: 367-377, 2009
199
Translational Research: Not Just Bench to Bedside Joel McCauley, MD,* and Jonathan D’Cunha, MD, PhD† The translation of proteins within the cell has been increasingly implicated in multiple malignancies as an important regulatory checkpoint. Although evidence continues to mount regarding the role of translation in cancer, questions persist as to how translation is activated and the overall significance in thoracic malignancies. Two recent articles are reviewed here that explore the role of enhanced translation in tumorigenesis and prognosis. Semin Thoracic Surg 22:197-199 © 2010 Elsevier Inc. All rights reserved. Keywords: thoracic oncology, translation, eIF4E, tumorigenesis, lung cancer Lung cancer continues to be the leading cause of cancer-related deaths. There will be an estimated 160,000 United States deaths from lung cancer this year.1 Although this number is significant, worldwide deaths attributed to lung cancer are expected to increase because the prevalence of smoking has not yet peaked in developing countries. Significant resources have been invested to elucidate molecular mechanisms and develop new therapies, but the overall 5-year survival remains abysmal at 15%. This clearly indicates that new molecular targets need to be identified for therapeutic intent. Most investigations of cancer genetics and epigenetics have focused on oncogenic mutations, transcriptional activation, or silencing of growth regulator genes. During the last decade, there has been mounting evidence that the translation of proteins within the cell is an important checkpoint in the control of cell growth and proliferation (Fig. 1). Although the translation of mRNA into protein is regulated at multiple levels, the rate-limiting factor is the binding of eukaryotic initiation factor 4E (eIF4E) to the 5= cap structure of mRNA. Enhanced eIF4E activity disproportionately alters the translation of mRNA encoding growth and survival factors associated with malignancy.2 eIF4E activity is primarily regulated by the eIF4E-binding protein (4E-BP) family of proteins. In the presence of favorable growth
*Division of Pulmonary, Allergy, Critical Care, and Sleep Medicine, Department of Medicine, University of Minnesota, Minneapolis, Minnesota. †Division of Thoracic and Foregut Surgery, Department of Surgery, University of Minnesota, Minneapolis, Minnesota. Address reprint requests to Jonathan D’Cunha, MD, PhD, Division of Thoracic and Foregut Surgery, Department of Surgery, University of Minnesota, MMC 207, 420 Delaware St SE, Minneapolis, MN 55455. E-mail: [email protected]
1043-0679/$-see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1053/j.semtcvs.2010.10.008
conditions, 4E-BPs are phosphorylated via the mammalian target of rapamycin (mTOR). Consequently, 4E-BPs dissociate from eIF4E, leaving eIF4E free to bind the 5= mRNA cap and initiate translation. In times of stress, 4E-BPs are dephosphorylated and associated with eIF4E, thereby inhibiting cap-mediated translation. In addition, eIF4E can be phosphorylated via the RAS/mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase pathway. Here we review 2 recent contributions that further solidify the role of translational activation in malignancy and a potential therapeutic target, but also suggest that the phosphorylation state of eIF4E plays an important role in tumor progression and survival.3,4 A series of well-designed experiments was published earlier this year (2010) by Furic et al3 investigating the role of phosphorylated eIF4E in tumorigenesis. The authors generated a transgenic mouse model with altered eI4FE, thus prohibiting its phosphorylation. From this transgenic mouse model cells were isolated for a series of in vitro experiments assessing cellular transformation and proliferation. Wild-type cells (exposed to oncogenic KRAS and c-myc) exhibited proficient colony formation in soft agar, a standard assay to establish cellular transformation. In the same setting, cells from the altered eIF4E mice formed 10fold fewer colonies, demonstrating the importance of phosphorylated eIF4E in transformation. To further dissect this point, the authors conducted a similar experiment comparing wild-type cells with those specifically engineered to overexpress eIF4E. As the downstream effector of RAS, mitogen-activated protein kinase-interacting kinase 1/2 (MNK1/2) phosphorylates eIF4E; thus, MNK1/2 double knockouts represent eIF4E overexpression. Wild-type cells overexpressing 197
TRANSLATIONAL RESEARCH
Figure 1. Schematic of cap-dependent translation. The phosphoinositide 3-kinase/AKT/ mTOR pathway is activated by growth factors, mitogens, and hormones. Hyperphosphorylated 4E-BPs then release eIF4E, thereby allowing it to be phosphorylated and cap-dependent translation may occur within the cytoplasm of the cell. This, in turn, potentially promotes the initiation of translation, mitogenesis, and oncogenesis.
eIF4E formed soft agar foci after 10 days. However, cells from the mice overexpressing eIF4E (in these mice this form of eIF4E is not able to be activated by phosphorylation) did not form any foci, confirming that eIF4E-transforming activity is dependent on its phosphorylation state. Although the in vitro data clearly implicate phosphorylated eIF4E in tumorigenesis, the authors proceeded to evaluate the in vivo effects in a prostate cancer mouse model and demonstrated that that phosphorylation of eIF4E plays a role in tumor progression in vivo. Although Furic et al clearly established the role of translational activation via eIF4E phosphorylation in cellular transformation and tumor progression, the question remains whether this is pertinent to thoracic malignancies. There are several studies conducted with human lung cancer cell lines and mouse lung cancer models that show enhancement of translation promotes tumorigenesis and that inhibition of eIF4E represses tumorigenesis. Yoshizawa et al4 reported earlier this year that the phosphorylation status of eIF4E can be used as a prognostic indicator in non-small cell lung 198
cancer (NSCLC). The authors wanted to elucidate whether activation of the canonical phosphoinositide 3-kinase/AKT/mTOR pathway has prognostic significance in NSCLC. In their study, 300 surgically resected lung cancers (150 adenocarcinoma, 150 squamous cell carcinomas) were used for microarray analysis. Survival time and outcome data were available for 252 of the samples using immunohistochemistry. Although upstream activators of mTOR were clearly activated, only overexpression of phosphorylated-eIF4E and phosphorylated-AKT correlated with a significantly worse 5-year prognosis (P ⬍ 0.0001; Fig. 2). In addition, a multivariate analysis using sex and stage identified only phosphorylated-eIF4E (P ⫽ 0.006) and cancer stage (P ⫽ 0.005) as independent prognostic factors. On the basis of this study, translational activation signified by phosphorylated-eIF4E can be used as a prognostic indicator and likely plays a significant role in NSCLC cancer progression. Enhanced translation within the cancer cell continues to be implicated as an important mechanism involved in tumorigenesis and cancer progression. As evidence continues to mount, efforts are underway to develop novel therapeutics capable of modulating this checkpoint.5,6 Although the 2 reports reviewed here further improve our understanding of how translation affects tumorigenesis and its role as a prognostic indicator, additional investigations are needed. Further dissection of translational activation will be insightful in clarifying mechanistic details as well as identifying further points that might be targeted for therapeutic intent.
Figure 2. Kaplan–Meier curve comparing overall survival for those patients with NSCLC and phosphorylated-eIF4E. There was worse survival for phosphorylated-eIF4E–positive patients (n ⫽ 91; blue line) compared with phosphorylatedeIF4E–negative patients (n ⫽ 143; (red line; A). (Reprinted by permission of the American Association for Cancer Research from Yoshizawa et al.4) (Color version of figure is available online at http://www.semthorcardiovascsurg.com.)
Seminars in Thoracic and Cardiovascular Surgery ● Volume 22, Number 3
TRANSLATIONAL RESEARCH
1. Jemal A, Siegel R, Xu J, et al: Cancer statistics. CA Cancer J Clin 60:277-300, 2010 2. Mamane Y, Petroulakis E, Rong L, et al: EIF4E—from translation to transformation. Oncogene 23:3172-3179, 2004 3. Furic L, Rong L, Larsson O, et al: EIF4E phosphorylation promotes tumorigenesis and is as-
sociated with prostate cancer progression. Proc Natl Acad Sci U S A 107:14134-14139, 2010 4. Yoshizawa A, Fukuoka J, Shimizu S, et al: Overexpression of phospho-eIF4E is associated with survival through AKT pathway in non-small cell lung cancer. Clin Cancer Res 16:240-248, 2010 5. Graff JR, Konicek BW, Carter JH, et al: Targeting
Seminars in Thoracic and Cardiovascular Surgery ● Volume 22, Number 3
the eukaryotic translation initiation factor 4E for cancer therapy. Cancer Res 68:631-634, 2008 6. Ghosh B, Benyumov AO, Ghosh P, et al: Nontoxic chemical interdiction of the epithelialto-mesenchymal transition by targeting capdependent translation. ACS Chem Biol 4: 367-377, 2009
199