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Mechanisms of apoptosis and autophagy in cardiomyocytes
thesisposted on 05.12.2016, 05:10 by Wong, Huang Chung
Cardiomyocytes are terminally differentiated cells that have limited abilities to divide and regenerate. Due to the finite number of cardiomyocytes in an individual heart, cell death is detrimental. In the heart, cardiomyocyte cell death is important under conditions of ischemia/reperfusion, as well as in terminal heart failure. Apoptosis and autophagy are two mechanisms regulating cell survival and cell death, both of which can have major influences on the fate of cardiomyocytes. At the commencement of my studies, it was well accepted that the “O” subset of the Forkhead family of transcription factors (FoxO) were involved in apoptotic signalling, especially in neurons. I demonstrated apoptosis in neonatal rat ventricular myocytes (NRVM) caused by expressing a constitutively active mutant form of FoxO1 (CA-FoxO1). CA-FoxO1 promoted DNA fragmentation, increased Caspase-3 activation and promoted cytochrome c release from the mitochondria. My studies also examined an upstream activator of FoxO, the mammalian sterile-20 like kinase 1 (Mst-1) and, in particular, whether its apoptotic actions were mediated by FoxO. Mst-1 mediated apoptosis was shown to be partially mediated by FoxO1, because co-infection of the dominant negative mutant form of FoxO1 (dnFoxO1) 1 in NRVM reduced the Mst-1 induced apoptosis only by about 20%. Cardiomyocyte apoptosis mediated by FoxO is a slow response (after 48h), and may be a secondary effect following other changes. Autophagy is a regulated process that promotes the degradation of damaged cytosolic proteins and organelles. Autophagy, in the heart, acts primarily as a survival mechanism under conditions such as nutrient deprival and is also important in limiting the damage caused by heart failure. I showed that NRVM expressing CA-FoxO1 undergo autophagy that is detectable after 24h, before apoptosis is observed. NRVM infected with adenovirus expressing Mst-1, however, showed the opposite result with decreased autophagic signalling. So while both FoxO1 and Mst-1 promote apoptosis, only FoxO is autophagic in cardiomyocytes. Our laboratory had previously shown that Mst-1 expression in mouse heart in vivo results in heightened generation of inositol (1,4,5)trisphosphate (IP3), in addition to increased expression of Ins(1,4,5)P3 receptors (IP3-R). In some cell types, IP3-R have been shown to be involved in autophagic responses but, to date, the role of IP3-R in autophagy in heart has not been described. I considered the possibility that heightened IP3-R activation may contribute to the anti-autophagic action of Mst-1 and proceeded to examine the role of inositides in autophagy. Either depletion of Ins(1,4,5)P3, or antagonism of the IP3-R promoted autophagy. Evidence was then obtained that IP3-R suppress autophagy by sequestering Beclin-1, an early initiator of autophagy. Autophagy initiated by IP3-R antagonists was inhibited by Mst-1, whereas FoxO1-induced autophagy was not. This is consistent with a contribution of inositide signalling to Mst-1-induced autophagy. Overall conclusions from my studies are that FoxO1 is primarily an activator of autophagy in heart and that apparently apoptotic responses are observed only at later stages. Mst-1, in contrast, promotes apoptosis while inhibiting autophagy. Evidence is provided that IP3-R suppresses autophagy in cardiomyocytes and may contribute to the inhibitory action of Mst-1.