Cells can die via apoptosis, necrosis and autophagy 8
. The signaling pathways involved in all three of these pathways are extremely complex and beyond the scope of this review. Moreover, a significant amount of crosstalk exists between these pathways, producing a complex and interconnected system of signals. Nevertheless, several key nodes in these pathways have been identified including PI3K, AKT and caspase-3 (Figure ).
Schematic of principal cell death/survival pathways.
The PI3-kinases are a family of lipid kinases that are able to phosphorylate the 3-hydroxyl group of the phosphoinositidyl ring 9
. Three classes of PI3K have been described. Class 1A PI3K is the focus of this paper and plays a major role in cell death and survival, as well as autophagy via mTOR signaling. Class 1A PI3K has a regulatory subunit (p85) and a catalytic subunit (p110). The activation of PI3K is produced by the binding of growth factors such as HER2, IGF and EGRF to their receptor tyrosine kinases. Activation of the receptor tyrosine kinases results in an interaction with the p85 regulatory subunit of PI3K and removes its inhibitory effect on the p110 catalytic subunit. Negative feedback from downstream molecules, and the activity of the tumor suppressor PTEN, keep PI3K activation in balance 10
. Class 2 PI3K are catalytic enzymes that function downstream to activated EGF receptors, play a role in cell death/survival, and have no regulatory/adapter subunit. Class 3 PI3Ks, like class 1, contain a regulatory (p150) and catalytic (Vps34, 100 kDa) subunit and play an important role in cellular autophagy.
Activation of PI3K produces a robust pro-growth and survival effect, principally by activating the serine/threonine kinase AKT 11
. The activation of PI3K converts phosphatidylinositol-bisphosphate (PIP2) into phosphatidylinositol-triphosphate (PIP3). This in turn recruits AKT and the kinase PDK-1 (phosphoinositidyl dependent kinase 1) to the cell membrane, where AKT is phosphorylated and activated. Activated AKT exerts a myriad of effects on the cell: AKT stimulates protein synthesis and cell growth by activating mTOR (mammalian target of rapamycin). (It should be noted that mTOR plays a major role in autophagy signaling and thus plays a highly multi-faceted role in cell death). AKT promotes cellular proliferation by promoting the cell cycle proteins (c-Myc and cyclin D1) and inactivating the cell cycle inhibitors (p27 and p21). Functioning indirectly through the mTOR complex (mTORC1), AKT also regulates cell proliferation and hypertrophy. AKT inhibits the pro-apoptotic genes (BAD and BIM) thus promoting cell survival 10-12
The pro-survival role of AKT in the cell is opposed by a family of cysteine-aspartate proteases, or caspases, which play a pivotal role in cell death by apoptosis 8
. Apoptosis can be initiated via an intrinsic or extrinsic pathway, with different initiator caspases involved. The intrinsic pathway typically is initiated by the interaction of cytochrome c with caspase-9. This pathway plays a major role in cardiomyocyte death in ischemic injury. The extrinsic or death receptor pathway involves caspase-8 and plays an important role in inflammatory conditions and chemotherapy. Both the intrinsic and extrinsic apoptotic pathways lead to the activation of caspase-3, which is the principal effector or executioner caspase in the cell 8
. In addition to executing the apoptotic program, caspase-3 is able to cleave sarcomeric proteins in the myocardium, leading to a loss of function independent of cell death 13
The pivotal role played by PI3K, AKT and caspase-3 in cell death/survival has been shown in transgenic animal models, by using chemical inhibitors and by insights provided from genomic analysis of human cancer. Reduced PTEN expression and mutations in PIK3A (gene on chromosome 3 encoding for the p110 subunit) are found in numerous human cancers 10
. These tumor cells are resistant to apoptosis due to hyperactivity of the PI3K-AKT pathway. The activation of AKT, frequently observed in human tumor cells, can promote growth factor-mediated cell survival directly by inactivating several proapoptotic factors (BAD, procaspase-9 and Forkhead/FKHR transcription factors), and indirectly by activating transcription factors that upregulate anti-apoptotic genes (cyclic-AMP response element-binding protein (CREB), Nuclear Factor-κB).
In the heart, the activation of the PI3K/AKT cascade by IGF-1 has a profound cardioprotective effect in acute ischemia 14
. Conversely, the inhibition of PI3K by wortmannin (Wm) promotes apoptosis and exacerbates cell death in the heart during ischemia 12
. Wm has been extensively studied as a chemotherapeutic agent in vitro, and recent data show that it also exerts a robust anti-inflammatory effect that may prove useful in rheumatological syndromes and other inflammatory conditions 10, 15, 16
. The inhibition of caspase-3 can be achieved with small molecules and has been shown to be extremely cardioprotective in ischemia and heart failure 17-19
. Transient overexpression of AKT in the myocardium during ischemic injury is also highly protective 20
. Chronic overexpression of AKT, however, leads to excessive negative feedback of PI3K and is harmful 21
. This underscores the complexity of the signaling involved in cell death/survival and the need for novel imaging tools to better understand how to modulate these processes.