The roles of Bcl-2,p53, and cysteine proteases in the regulation of adenovirus E1A-induced apoptosis

Expression of the adenovirus E1A oncoprotein induces apoptosis. The E1B 19K and the Bcl-2 oncoproteins both inhibit apoptosis. Bcl-2 and E1B 19K proteins have limited sequence homology in their central functional regions. Bcl-2 expression complemented the requirement for E1B 19K expression during productive adenovirus infection, and expression of Bcl-2 in HeLa cells conferred resistence to tumor necrosis factor {dollar}alpha{dollar} and Fas antigen-induced apoptosis, which are established functions of E1B 19K. Thus Bcl-2 and E1B 19K may function by similar mechanisms to inhibit apoptosis. E1A-induced apoptosis is mediated by the p53 tumor supressor. E1B 19K inhibits E1A-induced apoptosis by blocking p53 function. Expression of Bcl-2 also inhibited p53-induced apoptosis. p53 levels in Bcl-2 expressing cell lines remain unaltered, suggesting that Bcl-2 does not regulate p53 at the protein half-life level. Previous studies show that Bcl-2 expression does not alter p53 localization, and that E1B 19K and Bcl-2 inhibit the apoptosis but not the growth arrest function of p53. Thus both Bcl-2 and E1B 19K function downstream of p53.; The mechanism of apoptosis induction by E1A was also investigated. Expression of E1A induced accumulation of the p53 protein in adenovirus infected HeLa cells. It is likely that this p53 induction by E1A is related to apoptosis induction. Genetic analysis showed that p53 induction by E1A cosegregated with the binding of p300 protein. Surprisingly, neither function was required for induction of apoptosis by E1A. Thus E1A may induce redundant p53-dependent and independent apoptosis pathways during productive adenovirus infection. Recently the interleukin-1{dollar}beta{dollar} converting enzyme (ICE) related cysteine proteases have been implicated in E1A-induced, p53-dependent apoptosis in BRK cells and in apoptosis during viral infections. The Z-VAD-FMK cysteine protease inhibitor blocked premature cell death or apoptosis in E1B 19K mutant adenovirus infected HeLa cells, suggesting that ICE related proteases are involved in this apoptosis pathway. Inhibition of apoptosis by Z-VAD-FMK increased viral production and attenuated the release of both wild-type and E1B 19K mutant viral particles into the environment. Thus apoptosis is a mechanism by which host cells combat viral infection, and the adenovirus may utilize for release of its progeny from the host cell at the end of a normal infection cycle.