- Email: [email protected]
Assisted Living in the Atheroma: Elderly Macrophages Promote Plaques
Cell Metabolism
Previews Gao, X., Reid, M.A., Kong, M., and Locasale, J.W. (2016). Mol. Aspects Med. Published on September 9, 2016. http://dx.doi.org/10.1016/j.mam.2016.09. 001. Jeon, S.M., Chandel, N.S., and Hay, N. (2012). Nature 485, 661–665. Kottakis, F., Nicolay, B.N., Roumane, A., Karnik, R., Gu, H., Nagle, J.M., Boukhali, M., Hayward,
M.C., Li, Y.Y., Chen, T., et al. (2016). Nature 539, 390–395. Locasale, J.W. (2013). Nat. Rev. Cancer 13, 572–583. Maddocks, O.D., Labuschagne, C.F., Adams, P.D., and Vousden, K.H. (2016). Mol. Cell 61, 210–221.
Mehrmohamadi, M., Liu, X., Shestov, A.A., and Locasale, J.W. (2014). Cell Rep. 9, 1507–1519. Pan, M., Reid, M.A., Lowman, X.H., Kulkarni, R.P., Tran, T.Q., Liu, X., Yang, Y., Hernandez-Davies, J.E., Rosales, K.K., Li, H., et al. (2016). Nat. Cell Biol. 18, 1090–1101. Shackelford, D.B., and Shaw, R.J. (2009). Nat. Rev. Cancer 9, 563–575.
Assisted Living in the Atheroma: Elderly Macrophages Promote Plaques Peter Libby1,* 1Division of Cardiovascular Medicine, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2016.11.013
Age dominates as a risk factor for human atherosclerosis, yet the underlying mechanisms remain elusive. A recent report described accumulation of senescent cells in atheromata of hypercholesterolemic mice and provides novel insights into how genes expressed by aging cells associate with the characteristics of human plaques implicated in cardiovascular events. Only a few decades ago, most viewed the atherosclerotic plaque as an amorphous deposit of lipid on the artery wall that grew progressively to clog arteries, ultimately obstructing blood flow and causing heart attacks, strokes, and the other dreaded complications of this disease. We have now gained increasingly sophisticated information regarding the complex cellular interactions and signaling pathways that contribute to atherogenesis (Libby et al., 2016). We understand details of lipid metabolism, the control of uptake of cholesterol-laden lipoprotein particles by vascular wall cells and invading leukocytes, and the manner in which atherosclerotic lesions progress and produce the thrombotic complications that provoke clinical events such as myocardial infarction and ischemic stroke. Most currently regard atherosclerosis not as a simple plumbing problem due to blockage of arteries by progressive lesion growth, but as a process orchestrated by the insidiously dysregulated behavior of cells in response to increasingly well-understood molecular signals. Nonetheless, we have much yet to learn about the mechanisms that link risk fac-
tors such as hypercholesterolemia and the altered behavior of cells that give rise to lesion formation and complication. Many invoke oxidized lipoproteins as an initiating stimulus, but scant evidence actually supports this view in humans. Although inflammatory pathways likely link many traditional risk factors for atherosclerosis with the pathogenesis of the disease, knowledge of the coupling mechanisms remains incomplete (Libby, 2012). A recent report by Childs and colleagues (2016) helps to address this gap. In humans, age dominates as a risk factor for the development of the complications of atherosclerosis. Accordingly, investigators have probed aging as a contributor to experimental atherosclerosis, most recently using hypercholesterolemic mice. For example, Rauscher and colleagues ‘‘rescued’’ mice rendered hypercholesterolemic due to deficiency in Apolipoprotein E and consumption of a cholesterol-rich diet from atherosclerosis development by transfer of bone marrow-derived progenitor cells from juvenile animals. Transfer of bone marrow cells from aged atherosclerotic mice proved much less effective in this regard
(Rauscher et al., 2003). Aging ultimately ends badly for cells as well as whole organisms: in the extreme, death of macrophages and impaired clearance of their remains have received considerable attention in the context of atheroma formation (Geng and Libby, 1995; Tabas and Bornfeldt, 2016). But senescence, a step short of cell death, has not garnered as much consideration. The recent work of Childs et al. describes accumulation of cells bearing markers of senescence in atherosclerotic lesions at various stages of development by using hypercholesterolemic ldlr / mice (Figure 1). The senescent cells produced pro-inflammatory cytokines and matrix metalloproteinases (MMPs) that can degrade the arterial extracellular matrix. Childs et al. systematically sought the effects of cellular senescence at various stages of experimental atherogenesis. Early lesions produced only 9–12 days after initiating a lipid-rich diet contained monocytes with markers of senescence (b-galactosidase) in association with proinflammatory mediator mRNA accumulation. Analysis of later lesions (after 88– 188 days of consuming an atherogenic
Cell Metabolism 24, December 13, 2016 ª 2016 Elsevier Inc. 779
Cell Metabolism
Previews
IL-1, TNF, MCP-1
v
v v v v v v v v v vv v vv v v v
v
MMP(-3, -12, -13)
Senescent macrophage foam cell (β-galactosidase+, p16Ink4a+)
Myocardial
Infarction Ischemic Stroke Abrupt limb
ischemia
Figure 1. Senescent Macrophages and the Progression of Atherogenesis Certain lipid-laden macrophages within atherosclerotic plaques bear markers of senescence. These cells can elaborate proinflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor (TNF), or chemoattractant protein 1 (MCP-1, CCL2) that can amplify inflammation in the plaque. The senescent cells can elaborate matrix metalloproteinases (MMPs) implicated in lesion remodeling and in weakening the fibrous skeleton of the plaque, rendering them susceptible to rupture and hence precipitating the thrombotic complications listed.
diet) provided evidence that senescent cells contribute to thinner fibrous caps in association with elevated mRNA that encodes MMPs. Atheromata with a weakened matrix skeleton in humans can rupture and provoke thrombotic events due to a mechanical failure that permits blood to access procoagulant material within the lesion (Libby, 2013). Childs’ elegant experiments involving depletion of senescent cells or loss of function of p16Inc4a, a protein implicated in senescence, yielded retarded atherogenesis and reduced expression of pro-inflammatory mediators in the arteries of atherosclerotic mice. The genetic manipulation used to deplete senescent cells (gancyclovir activation of a transgenic viral thymidine kinase driven by a p16Inc4a promoter) provoked their death. This artifice could complicate the interpretation of their experiments, given the defect in 780 Cell Metabolism 24, December 13, 2016
clearance of dead cells (efferocytosis) demonstrated in mouse atheromata and the contribution of the detritus of the dead cells thus produced to plaque evolution (Tabas and Bornfeldt, 2016). The conditions or triggers that guide cells toward senescence merit further investigation. These novel findings furnish an excellent illustration of harnessing the power of manipulations in mice to dissect mechanisms rigorously and test specific hypotheses regarding the pathogenesis of a disease. As with many reports of studies regarding experimental atherosclerosis, the paper ends with a statement regarding clinical therapeutics. The authors ‘‘raise the possibility that drug-like molecules that remove senescent cells from patients without toxic side effects could contribute to therapeutic management of the disease’’ (Childs et al., 2016). Yet we should remain mindful of
the limitations of ready extrapolations to human disease of studies on mice engineered to develop exaggerated hyperlipidemia (Libby, 2015). Experimental atherosclerosis in mice usually involves exposure to hypercholesterolemia for a matter of months. In contrast, human atheromata generally evolve over many decades, generating lesions considerably more complex than those produced by short-term interventions in rodents. The usual mouse preparations used for experimental atherosclerosis achieve levels of cholesterolemia vastly higher than that seen in humans in the present era. A secular trend toward lower LDL, in large part due to wider use of statin therapy, has begun to modify human atherosclerosis. Imaging studies show that statin therapy reduces the fat content and increases the proportion of fibrous tissue in human atheromata. Studies of
Cell Metabolism
Previews surgically removed human plaques contain progressively less lipid and fewer macrophages over the last dozen years, as statin treatment and other preventive measure have increased (Pasterkamp et al., 2016). Given the current clinical reality, extrapolations of results on mice with exaggerated hypercholesterolemia may not translate well to humans with atherosclerotic risk treated with prevailing standard of care measures. Contemporary clinical guidelines mandate concerted preventive therapies for the management of risk factors, including statin treatment for most individuals with established atherosclerosis. While experts argue about targets for blood pressure control, guidelines recommend pharmacologic treatment for many hypertensive individuals. Social stigma and legislation have promoted smoking cessation and reduction in secondhand smoke. Many high-risk individuals should receive low-dose aspirin therapy to limit platelet aggregation ability. Thus, most humans prone to atherosclerotic complications should receive intensive risk factor control by lifestyle and often by pharmacologic measures. Studies on experimental atherosclerosis
seldom, if ever, consider the background therapy that our human patients should receive before deploying novel therapeutics. Many mouse studies, including that of Childs et al., refer to plaque ‘‘instability’’ without measuring the physical properties or other indices of resistance to rupture of the plaque. Indeed, the plaques produced in hyperlipidemic mice seldom rupture or cause blood clot formation, rendering the concept of ‘‘instability’’ of mouse atheromata a mirage. We possess increasingly potent tools for manipulating mice and analyzing murine diseases such as atherosclerosis. Such experiments provide enormous opportunity for gaining insights into mechanisms of disease. Yet a yawning gap separates experimental atherosclerosis in mice from the contemporary human disease. Animal experiments routinely mislead regarding the results of clinical trials. We must remain mindful of the limitations of premature extrapolation of our laboratory results to humans, lest we misdirect resources for development of new therapeutic approaches. We should reap the abundant fruits of mouse experimentation to refine our mechanistic understanding of diseases, but beware of
jumping to facile conclusions regarding translation to humans. Senescent cells seem to drive murine atherogenesis, and the challenge remains to determine the degree to which these experimental results apply to the human disease. REFERENCES Childs, B.G., Baker, D.J., Wijshake, T., Conover, C.A., Campisi, J., and van Deursen, J.M. (2016). Science 354, 472–477. Geng, Y.-J., and Libby, P. (1995). Am. J. Pathol. 147, 251–266. Libby, P. (2012). Arterioscler. Thromb. Vasc. Biol. 32, 2045–2051. Libby, P. (2013). N. Engl. J. Med. 368, 2004–2013. Libby, P. (2015). Circ. Res. 117, 921–925. Libby, P., Bornfeldt, K.E., and Tall, A.R. (2016). Circ. Res. 118, 531–534. Pasterkamp, G., den Ruijter, H.M., and Libby, P. (2016). Nat. Rev. Cardiol. Published online October 20, 2016. http://dx.doi.org/10.1038/ nrcardio.2016.166. Rauscher, F.M., Goldschmidt-Clermont, P.J., Davis, B.H., Wang, T., Gregg, D., Ramaswami, P., Pippen, A.M., Annex, B.H., Dong, C., and Taylor, D.A. (2003). Circulation 108, 457–463. Tabas, I., and Bornfeldt, K.E. (2016). Circ. Res. 118, 653–667.
Cell Metabolism 24, December 13, 2016 781
Previews Gao, X., Reid, M.A., Kong, M., and Locasale, J.W. (2016). Mol. Aspects Med. Published on September 9, 2016. http://dx.doi.org/10.1016/j.mam.2016.09. 001. Jeon, S.M., Chandel, N.S., and Hay, N. (2012). Nature 485, 661–665. Kottakis, F., Nicolay, B.N., Roumane, A., Karnik, R., Gu, H., Nagle, J.M., Boukhali, M., Hayward,
M.C., Li, Y.Y., Chen, T., et al. (2016). Nature 539, 390–395. Locasale, J.W. (2013). Nat. Rev. Cancer 13, 572–583. Maddocks, O.D., Labuschagne, C.F., Adams, P.D., and Vousden, K.H. (2016). Mol. Cell 61, 210–221.
Mehrmohamadi, M., Liu, X., Shestov, A.A., and Locasale, J.W. (2014). Cell Rep. 9, 1507–1519. Pan, M., Reid, M.A., Lowman, X.H., Kulkarni, R.P., Tran, T.Q., Liu, X., Yang, Y., Hernandez-Davies, J.E., Rosales, K.K., Li, H., et al. (2016). Nat. Cell Biol. 18, 1090–1101. Shackelford, D.B., and Shaw, R.J. (2009). Nat. Rev. Cancer 9, 563–575.
Assisted Living in the Atheroma: Elderly Macrophages Promote Plaques Peter Libby1,* 1Division of Cardiovascular Medicine, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2016.11.013
Age dominates as a risk factor for human atherosclerosis, yet the underlying mechanisms remain elusive. A recent report described accumulation of senescent cells in atheromata of hypercholesterolemic mice and provides novel insights into how genes expressed by aging cells associate with the characteristics of human plaques implicated in cardiovascular events. Only a few decades ago, most viewed the atherosclerotic plaque as an amorphous deposit of lipid on the artery wall that grew progressively to clog arteries, ultimately obstructing blood flow and causing heart attacks, strokes, and the other dreaded complications of this disease. We have now gained increasingly sophisticated information regarding the complex cellular interactions and signaling pathways that contribute to atherogenesis (Libby et al., 2016). We understand details of lipid metabolism, the control of uptake of cholesterol-laden lipoprotein particles by vascular wall cells and invading leukocytes, and the manner in which atherosclerotic lesions progress and produce the thrombotic complications that provoke clinical events such as myocardial infarction and ischemic stroke. Most currently regard atherosclerosis not as a simple plumbing problem due to blockage of arteries by progressive lesion growth, but as a process orchestrated by the insidiously dysregulated behavior of cells in response to increasingly well-understood molecular signals. Nonetheless, we have much yet to learn about the mechanisms that link risk fac-
tors such as hypercholesterolemia and the altered behavior of cells that give rise to lesion formation and complication. Many invoke oxidized lipoproteins as an initiating stimulus, but scant evidence actually supports this view in humans. Although inflammatory pathways likely link many traditional risk factors for atherosclerosis with the pathogenesis of the disease, knowledge of the coupling mechanisms remains incomplete (Libby, 2012). A recent report by Childs and colleagues (2016) helps to address this gap. In humans, age dominates as a risk factor for the development of the complications of atherosclerosis. Accordingly, investigators have probed aging as a contributor to experimental atherosclerosis, most recently using hypercholesterolemic mice. For example, Rauscher and colleagues ‘‘rescued’’ mice rendered hypercholesterolemic due to deficiency in Apolipoprotein E and consumption of a cholesterol-rich diet from atherosclerosis development by transfer of bone marrow-derived progenitor cells from juvenile animals. Transfer of bone marrow cells from aged atherosclerotic mice proved much less effective in this regard
(Rauscher et al., 2003). Aging ultimately ends badly for cells as well as whole organisms: in the extreme, death of macrophages and impaired clearance of their remains have received considerable attention in the context of atheroma formation (Geng and Libby, 1995; Tabas and Bornfeldt, 2016). But senescence, a step short of cell death, has not garnered as much consideration. The recent work of Childs et al. describes accumulation of cells bearing markers of senescence in atherosclerotic lesions at various stages of development by using hypercholesterolemic ldlr / mice (Figure 1). The senescent cells produced pro-inflammatory cytokines and matrix metalloproteinases (MMPs) that can degrade the arterial extracellular matrix. Childs et al. systematically sought the effects of cellular senescence at various stages of experimental atherogenesis. Early lesions produced only 9–12 days after initiating a lipid-rich diet contained monocytes with markers of senescence (b-galactosidase) in association with proinflammatory mediator mRNA accumulation. Analysis of later lesions (after 88– 188 days of consuming an atherogenic
Cell Metabolism 24, December 13, 2016 ª 2016 Elsevier Inc. 779
Cell Metabolism
Previews
IL-1, TNF, MCP-1
v
v v v v v v v v v vv v vv v v v
v
MMP(-3, -12, -13)
Senescent macrophage foam cell (β-galactosidase+, p16Ink4a+)
Myocardial
Infarction Ischemic Stroke Abrupt limb
ischemia
Figure 1. Senescent Macrophages and the Progression of Atherogenesis Certain lipid-laden macrophages within atherosclerotic plaques bear markers of senescence. These cells can elaborate proinflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor (TNF), or chemoattractant protein 1 (MCP-1, CCL2) that can amplify inflammation in the plaque. The senescent cells can elaborate matrix metalloproteinases (MMPs) implicated in lesion remodeling and in weakening the fibrous skeleton of the plaque, rendering them susceptible to rupture and hence precipitating the thrombotic complications listed.
diet) provided evidence that senescent cells contribute to thinner fibrous caps in association with elevated mRNA that encodes MMPs. Atheromata with a weakened matrix skeleton in humans can rupture and provoke thrombotic events due to a mechanical failure that permits blood to access procoagulant material within the lesion (Libby, 2013). Childs’ elegant experiments involving depletion of senescent cells or loss of function of p16Inc4a, a protein implicated in senescence, yielded retarded atherogenesis and reduced expression of pro-inflammatory mediators in the arteries of atherosclerotic mice. The genetic manipulation used to deplete senescent cells (gancyclovir activation of a transgenic viral thymidine kinase driven by a p16Inc4a promoter) provoked their death. This artifice could complicate the interpretation of their experiments, given the defect in 780 Cell Metabolism 24, December 13, 2016
clearance of dead cells (efferocytosis) demonstrated in mouse atheromata and the contribution of the detritus of the dead cells thus produced to plaque evolution (Tabas and Bornfeldt, 2016). The conditions or triggers that guide cells toward senescence merit further investigation. These novel findings furnish an excellent illustration of harnessing the power of manipulations in mice to dissect mechanisms rigorously and test specific hypotheses regarding the pathogenesis of a disease. As with many reports of studies regarding experimental atherosclerosis, the paper ends with a statement regarding clinical therapeutics. The authors ‘‘raise the possibility that drug-like molecules that remove senescent cells from patients without toxic side effects could contribute to therapeutic management of the disease’’ (Childs et al., 2016). Yet we should remain mindful of
the limitations of ready extrapolations to human disease of studies on mice engineered to develop exaggerated hyperlipidemia (Libby, 2015). Experimental atherosclerosis in mice usually involves exposure to hypercholesterolemia for a matter of months. In contrast, human atheromata generally evolve over many decades, generating lesions considerably more complex than those produced by short-term interventions in rodents. The usual mouse preparations used for experimental atherosclerosis achieve levels of cholesterolemia vastly higher than that seen in humans in the present era. A secular trend toward lower LDL, in large part due to wider use of statin therapy, has begun to modify human atherosclerosis. Imaging studies show that statin therapy reduces the fat content and increases the proportion of fibrous tissue in human atheromata. Studies of
Cell Metabolism
Previews surgically removed human plaques contain progressively less lipid and fewer macrophages over the last dozen years, as statin treatment and other preventive measure have increased (Pasterkamp et al., 2016). Given the current clinical reality, extrapolations of results on mice with exaggerated hypercholesterolemia may not translate well to humans with atherosclerotic risk treated with prevailing standard of care measures. Contemporary clinical guidelines mandate concerted preventive therapies for the management of risk factors, including statin treatment for most individuals with established atherosclerosis. While experts argue about targets for blood pressure control, guidelines recommend pharmacologic treatment for many hypertensive individuals. Social stigma and legislation have promoted smoking cessation and reduction in secondhand smoke. Many high-risk individuals should receive low-dose aspirin therapy to limit platelet aggregation ability. Thus, most humans prone to atherosclerotic complications should receive intensive risk factor control by lifestyle and often by pharmacologic measures. Studies on experimental atherosclerosis
seldom, if ever, consider the background therapy that our human patients should receive before deploying novel therapeutics. Many mouse studies, including that of Childs et al., refer to plaque ‘‘instability’’ without measuring the physical properties or other indices of resistance to rupture of the plaque. Indeed, the plaques produced in hyperlipidemic mice seldom rupture or cause blood clot formation, rendering the concept of ‘‘instability’’ of mouse atheromata a mirage. We possess increasingly potent tools for manipulating mice and analyzing murine diseases such as atherosclerosis. Such experiments provide enormous opportunity for gaining insights into mechanisms of disease. Yet a yawning gap separates experimental atherosclerosis in mice from the contemporary human disease. Animal experiments routinely mislead regarding the results of clinical trials. We must remain mindful of the limitations of premature extrapolation of our laboratory results to humans, lest we misdirect resources for development of new therapeutic approaches. We should reap the abundant fruits of mouse experimentation to refine our mechanistic understanding of diseases, but beware of
jumping to facile conclusions regarding translation to humans. Senescent cells seem to drive murine atherogenesis, and the challenge remains to determine the degree to which these experimental results apply to the human disease. REFERENCES Childs, B.G., Baker, D.J., Wijshake, T., Conover, C.A., Campisi, J., and van Deursen, J.M. (2016). Science 354, 472–477. Geng, Y.-J., and Libby, P. (1995). Am. J. Pathol. 147, 251–266. Libby, P. (2012). Arterioscler. Thromb. Vasc. Biol. 32, 2045–2051. Libby, P. (2013). N. Engl. J. Med. 368, 2004–2013. Libby, P. (2015). Circ. Res. 117, 921–925. Libby, P., Bornfeldt, K.E., and Tall, A.R. (2016). Circ. Res. 118, 531–534. Pasterkamp, G., den Ruijter, H.M., and Libby, P. (2016). Nat. Rev. Cardiol. Published online October 20, 2016. http://dx.doi.org/10.1038/ nrcardio.2016.166. Rauscher, F.M., Goldschmidt-Clermont, P.J., Davis, B.H., Wang, T., Gregg, D., Ramaswami, P., Pippen, A.M., Annex, B.H., Dong, C., and Taylor, D.A. (2003). Circulation 108, 457–463. Tabas, I., and Bornfeldt, K.E. (2016). Circ. Res. 118, 653–667.
Cell Metabolism 24, December 13, 2016 781