Sci. Aging Knowl. Environ., 25 February 2004
Vol. 2004, Issue 8, p. pe8
[DOI: 10.1126/sageke.2004.8.pe8]

PERSPECTIVES

Is Akt the Mastermind Behind Age-Related Heart Disease?

Patrick Kaminker

The author is at the Buck Institute for Age Research, Novato, CA 94947, USA. E-mail: pkaminker{at}buckinstitute.org

http://sageke.sciencemag.org/cgi/content/full/2004/8/pe8

Key Words: Akt • endothelial cells • kinase • cardiovascular disease • insulin • insulin-like growth factor

Introduction

Roughly 40% of all patients diagnosed with cardiovascular disease are over the age of 65, making age a major risk factor for developing this disease (1). Aging hearts are much more susceptible than their youthful counterparts to injury by a transient loss of blood flow (also known as ischemia-reperfusion injury). The post-ischemic reperfusion period is associated with a burst of reactive oxygen species (ROS), and although ROS can play an integral role in the proliferation, migration, and vasodilatory response of endothelial cells, elevated ROS concentrations can lead to lipid peroxidation, mitochondrial dysfunction, and cell death (2). A decrease in the production of nitric oxide--a multitalented signaling molecule that helps to open blood vessels and inhibit clot formation--and an increase in the generation of superoxide radicals during aging have been implicated in the pathogenesis of vascular diseases, including hypertension and atherosclerosis (3). Taken together, these findings indicate that aged heart tissue has increased exposure to oxidative damage and a decreased ability to mount a defense against ROS (see "The Two Faces of Oxygen").

Reports dating back to 1993 implicate cellular senescence (cell aging) in the development of atherosclerotic lesions (4). However, it is unclear whether the accumulation of senescent cells is a cause or an effect of atherosclerosis. Do atherosclerotic plaques induce local cell proliferation and thus exhaust the proliferative potential of cells around the lesion? Or does the accumulation of senescent cells, which occurs as a function of normal aging, facilitate the formation of these plaques by altering the microenvironment? Cellular senescence is defined as a state of irreversible growth arrest and can be induced by numerous stimuli, including telomere shortening (replicative senescence), DNA damage, mitogenic stimuli, or certain oncogenes (for example, activated Ras). Senescent cells accumulate with age, and many believe that this contributes to organismal aging or at least to some of the pathologies associated with aging (5). A recent report by Miyauchi and colleagues in The EMBO Journal (4) sheds light on the role of cell senescence in atherosclerosis. The new data indicate that cellular senescence directly affects the ability of endothelial cells to control ROS concentrations in a manner that is dependent on the phosphatidylinositol 3-kinase (PI3 kinase) Akt.

Akt is a component of the insulin/insulin-like growth factor (IGF) signaling pathway. Mutations in genes that encode elements of the insulin/IGF pathway can enhance life span in a variety of organisms (see Johnson Review and "One for All"). Because enhanced Akt function has been implicated in cell proliferation and tumorigenesis (6), the link between the activation of Akt by phosphorylation, the induction of cellular senescence, and atherogenesis may lend additional support to the idea that the senescence response is antagonistically pleiotropic (5). Antagonistic pleiotropy refers to the evolutionary theory that genes or processes that promote the survival of an organism to reproductive maturity can have unselected deleterious effects late in life. Therefore, the senescence of endothelial cells, which line organs of the circulatory system, and the resulting cardiovascular pathologies may be an evolutionary tradeoff for protection against tumor formation before reproductive maturity. The programmed mechanisms of senescence and apoptosis clearly allow potential cancer cells to either commit suicide (apoptosis) or irreversibly arrest growth (senescence) rather than replicate. p53, a major target for mutations in cancer cells, plays a crucial role in the decision of the cell to undergo apoptosis or senescence, or to attempt to survive and repair itself (see Campisi Perspective). In addition, recent findings suggest that senescent cells can create a tissue environment that facilitates the proliferation, invasion, and migration of transformed cells (7) (see "More Than a Sum of Our Cells"). DNA damage-induced activation of PI3 kinases such as ATM, CHK1, and CHK2 are known to trigger a p53-dependent apoptotic or senescence response, but more subtle environmental or extracellular signals that can trigger these responses are less well defined. In their recent report, Miyauchi et al. identify a role for insulin signaling in the senescence response of human endothelial cells. This study begins to address the mechanisms by which endothelial cell senescence can contribute to the cardiovascular pathologies associated with aging (see Heist Perspective) and, more importantly, highlights some potential mechanisms for prolonging the replicative life span of vascular endothelial cells.

DAF-16/FOXO3a

Mutations in the Caenorhabditis elegans gene daf-2, which encodes an insulin/IGF-like tyrosine kinase receptor, provided early evidence that the insulin/IGF-like signaling pathway can have a major effect on life span (see Johnson Review). Subsequent evidence showed that the components of this pathway are evolutionarily conserved in yeast, fruit flies, and mice (8). This pathway is known to transmit signals through a PI3 kinase (Akt) and a forkhead-like transcription factor (DAF-16).

In C. elegans, the forkhead transcription factor is encoded by daf-16 (see Larsen Perspective). DAF-16 is excluded from the nucleus under conditions of optimal growth and nutrition, mainly through Akt-mediated phosphorylation (9, 10). Under stressful conditions, such as exposure to ultraviolet light, heat, or increased concentrations of ROS, signaling through the insulin/IGF pathway is decreased, so that Akt exists in an unphosphorylated state; DAF-16 is then dephosphorylated, enters the nucleus, and transcribes genes that allow the organism to enter a nonreproducing alternative larval stage characterized by stress resistance and longevity (dauer formation).

In mammalian cells, the DAF-16 homolog FOXO3a (or FKHR-L1) can induce certain cells to undergo apoptosis (10). At the center of both the mammalian and the C. elegans insulin/IGF signaling pathways is the serine-threonine kinase Akt. Because of the multitude of pathways in which Akt participates (Fig. 1), it appears to be nearly impossible to delineate a specific downstream effect of Akt that is responsible for cell survival and longevity. After all, stress can enhance the activity of Akt, which increases the phosphorylation (and thus activation) of nitric oxide synthetase and mitochondrial biogenesis (11) in numerous cell types. However, increased concentrations of activated Akt also can prevent nuclear translocation of p27, which binds to cyclin-cyclin-dependent kinase complexes and inhibits their kinase activity. This causes cells to bypass the G1 cell cycle checkpoint, facilitating tumorigenesis (12).



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Fig. 1. Schematic diagram of the signaling cascade involving Akt. Multiple cell surface signals can trigger activation of the phosphatidylinositol 3-kinase (PI3K) family and subsequent phosphorylation of phosphoinositide-dependent kinases (for example, PDK-1 and Akt). Downstream stream substrates of Akt include the BCL-2-associated death promoter (BAD), nitric oxide synthase (NOS), the phosphatase and tensin homolog (PTEN), glycogen-synthase kinase 3 (GSK-3) and the 14-3-3 proteins. Activation of one or more of these substrates can lead to cell death, proliferation, or tumorigenesis. Specific targeting of forkhead transcription factors by phosphorylated Akt in endothelial cells can result in the induction of cellular senescence. [From (16), reproduced by permission]

 
Contribution of Akt to ROS Production

Miyauchi et al. report several important findings with regard to Akt and the replicative life span of endothelial cells that may parallel results seen in nematodes. Loss of oxidative stress tolerance with aging has previously been associated with decreased Akt activity, at least in rat hepatocytes (13). The evidence presented by Miyauchi et al. is the first indication that life span regulation by Akt in endothelial cells may work in a different manner. They expressed in primary human endothelial cells a kinase-defective (and hence dominant negative) Akt mutant, which extended the life span of these cells in culture. They further showed that this life span extension is the result of a FOXO3a-dependent decrease in ROS, presumably owing to transcriptional up-regulation of the gene encoding manganese superoxide dismutase (MnSOD or Sod2). The hypothesis, with respect to this cell type, is that inactivation of Akt allows FOXO3a nuclear translocation, which leads to an increase in Sod2 gene expression, a subsequent decrease in ROS production, and the resulting increase in the cells' replicative life span.

In the converse situation, when Akt is activated by phosphorylation, there is an increase in ROS production, and subsequent growth arrest induced p53 and p21, a cyclin-dependent kinase inhibitor that regulates p53-dependent inhibition of cell proliferation (Fig. 2). In support of this hypothesis, the authors showed that cells that express constitutively active Akt (caAkt) have lower amounts of MnSOD, have increased ROS production, and complete fewer population doublings prior to replicative senescence, relative to cells with wild-type Akt. These effects can be reversed by inactivating p21 or p53, indicating a need for an intact p53 response. Furthermore, expression of an Akt-insensitive version of the FOXO3a protein ablated the growth arrest seen in cells that express caAkt.



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Fig. 2. Schematic representation of the effects of Akt signaling on human and C. elegans life span. Elevated amounts of activated Akt have similar effects in two very different systems: They reduce the replicative life span of human endothelial cells and speed aging in C. elegans. Both pathways induce their effects on life span through a decrease in stress resistance.

 
Caveats and Conclusions

Cellular senescence is very likely an antioncogenic response (5). The authors have clearly shown that Akt can influence the in vitro life span of endothelial cells and that the reduced life span (early senescence) observed in cells that express caAkt is likely due to diminished antioxidant capabilities. In apparent conflict with this study are previous reports that senescent endothelial cells have a decreased production of nitric oxide (14, 15). Activated Akt has been reported to phosphorylate nitric oxide synthetase and induce increased concentrations of nitric oxide in endothelial cells (14-16). Recent studies reveal that nitric oxide has a protective effect against oxidative insults (17). The authors of this study show that senescent endothelial cells have increased amounts of activated Akt. We would therefore predict that they would also produce greater amounts of nitric oxide and be able to mount an even greater defense against ROS than do young endothelial cells. This apparent conflict indicates that the induction of Akt can result in multiple downstream effects with varying results.

An important caveat to the findings of Miyauchi et al. is that these experiments were performed in primary cell cultures, which do not mimic the balance of extracellular signals that endothelial cells are subjected to in vivo. The activation of a cellular senescence phenotype in endothelial cells likely involves a concerted effort by multiple pathways. However, this new work does show that Akt plays a major role in the senescence response. This study also shows an up-regulation of Akt activity within endothelial cells on the surface of coronary atherosclerotic lesions and implicates the insulin/IGF pathway in the development of atherosclerotic plaques. The authors further show that continuous exposure to elevated amounts of insulin induces phosphorylation of FOXO3a and early induction of senescence. Because the risk of cardiovascular disease is significantly higher in patients with type 2 diabetes (18), this study hints at a mechanism that links elevated excess insulin signaling, increased oxidative damage, and senescence of endothelial cells. However, whether increased concentrations of phosphorylated Akt trigger cellular senescence in vivo or arise as a consequence of exposing endothelial cells to an ischemic environment remains to be seen. Nonetheless, this work is an important contribution to our understanding of Akt function in insulin signaling, endothelial dysfunction, and cardiovascular disease.


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Citation: P. Kaminker, Is Akt the Mastermind Behind Age-Related Heart Disease? Sci. Aging Knowl. Environ. 2004 (8), pe8 (2004).








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