Identification and characterization of genes that promote growth factor-independent survival: Beyond Bcl-2 proteins.

摘要

Metazoan cells initiate programmed cell death, or apoptosis, in the absence of growth factors. The susceptibility of cells to apoptosis is regulated in a manner that integrates multiple aspects of cell physiology into the decision to initiate programmed cell death. In this thesis, the bioenergetics of established anti-apoptotic proteins, Bcl-x_L and Akt, were compared, and the activity of a regulator of protein synthesis in promoting cell survival was studied.; Mitochondrial homeostasis was studied in Akt- and Bcl-x_L-protected cells. Although both genes prevented apoptosis to similar magnitudes upon growth factor withdrawal, Akt expression rendered cells more metabolically active, while Bcl-x_L expression promoted viability in a more vegetative state. Akt expressing cells were larger, and had greater glycolytic flux and cellular ATP than their Bcl-x_L counterparts.; The contribution of both proto-oncogenes for survival and tumorigenesis was determined next. Akt-mediated survival was found to be dependent on the supply of extracellular glucose while Bcl-x_L protection was not. This demonstrated that the survival pathways regulated by Akt and Bcl-x _L were distinct. Consistent with this hypothesis, Akt and Bcl-x _L cooperated to enhance survival. In vivo, cells expressing Bcl-x_L or low levels of Akt alone did not form tumors, whereas mice injected with cells expressing high levels of Akt or Akt with Bcl-x _L developed leukemia.; Together the above data suggested that there might be multiple, distinguishable regulators of apoptosis. To identify novel genes and pathways that antagonize apoptosis, a cDNA library was screened in IL-3-dependent cells, resulting in the identification of elongation factor-1 alpha (EF-1α). Enforced EF-1α expression provided dose-dependent protection from growth factor withdrawal-induced death without transforming cells. EF-1α expression additionally prolonged survival from ER stress, but did not protect from nuclear damaging agents, whereas Bcl-x_L expression was protective. Thus, Akt and EF-1α promote cell survival through contribution to distinct biochemical pathways that enhance apoptotic resistance.