We previously demonstrated that
akt-1 regulates DNA damage-induced apoptosis of germ cells by antagonizing the activity of CEP-1, the sole
p53 family member (Quevedo et al., Curr Biol, Vol. 17: 286-92. 2007). Because the anti-apoptotic activity of Akt is induced by receptor tyrosine kinases and PI3K in mammals, we hypothesized that the insulin-like/PI3K pathway would fulfill this function in C. elegans. When we examined the response of multiple
daf-2(lf) and
pdk-1(lf) mutants to IR, however, we were surprised to observe that they were resistant to damage-induced germline apoptosis. Furthermore, germ cells in
pdk-1(gf) mutants were hypersensitive to IR, confirming that
daf-2 and
pdk-1 functionally oppose
akt-1. To determine whether
akt-1 could still function downstream of the insulin-like receptor, we inhibited
daf-2 by RNAi and found that this suppressed apoptosis in
akt-1(lf) mutants. The CEP-1 pro-apoptotic target gene
egl-1 was induced by IR in
daf-2 and
pdk-1 mutants, suggesting that these components of the insulin-like pathway bypass
cep-1 to promote apoptosis. In addition, the
pdk-1(gf) mutation was not able to restore apoptosis sensitivity in
daf-2(lf) mutants, indicating that
akt-1 and
pdk-1 likely function independently of
daf-2 in the control of germline apoptosis. Consistent with these findings, DNA damage did not affect phosphorylation of the PDK-1 site in AKT-1 (Thr350/Thr308) but increased phosphorylation at Ser517/Ser473 by ~2-fold.
daf-16 does still does mediate some of the apoptotic effects of
daf-2, however, since loss of this transcription factor partially rescued apoptosis in
daf-2(lf) mutant worms. This structural fragmentation of the worm insulin-like pathway is reinforced by different tissue-specific requirements for
akt-1 and
daf-2. Whereas,
akt-1 functions autonomously in the germline to modulate the apoptotic activity of CEP-1,
daf-2 is required in both the soma and germline to promote apoptosis in parallel to, or downstream of,
cep-1. Our data implies that differing functional modules of the insulin-like/PI3K signaling pathway can be selected in a stimulus-specific manner in vivo.