Noncanonical control of C. elegans germline apoptosis by the insulin/IGF-1 and Ras/MAPK signaling pathways

Cell Death Differ. 2013 Jan;20(1):97-107. doi: 10.1038/cdd.2012.101. Epub 2012 Aug 31.

Abstract

The insulin/IGF-1 pathway controls a number of physiological processes in the nematode worm Caenorhabditis elegans, including development, aging and stress response. We previously found that the Akt/PKB ortholog AKT-1 dampens the apoptotic response to genotoxic stress in the germline by negatively regulating the p53-like transcription factor CEP-1. Here, we report unexpected rearrangements to the insulin/IGF-1 pathway, whereby the insulin-like receptor DAF-2 and 3-phosphoinositide-dependent protein kinase PDK-1 oppose AKT-1 to promote DNA damage-induced apoptosis. While DNA damage does not affect phosphorylation at the PDK-1 site Thr350/Thr308 of AKT-1, it increased phosphorylation at Ser517/Ser473. Although ablation of daf-2 or pdk-1 completely suppressed akt-1-dependent apoptosis, the transcriptional activation of CEP-1 was unaffected, suggesting that daf-2 and pdk-1 act independently or downstream of cep-1 and akt-1. Ablation of the akt-1 paralog akt-2 or the downstream target of the insulin/IGF-1 pathway daf-16 (a FOXO transcription factor) restored sensitivity to damage-induced apoptosis in daf-2 and pdk-1 mutants. In addition, daf-2 and pdk-1 mutants have reduced levels of phospho-MPK-1/ERK in their germ cells, indicating that the insulin/IGF-1 pathway promotes Ras signaling in the germline. Ablation of the Ras effector gla-3, a negative regulator of mpk-1, restored sensitivity to apoptosis in daf-2 mutants, suggesting that gla-3 acts downstream of daf-2. In addition, the hypersensitivity of let-60/Ras gain-of-function mutants to damage-induced apoptosis was suppressed to wild-type levels by ablation of daf-2. Thus, insulin/IGF-1 signaling selectively engages AKT-2/DAF-16 to promote DNA damage-induced germ cell apoptosis downstream of CEP-1 through the Ras pathway.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis / genetics
  • Apoptosis / physiology*
  • Caenorhabditis elegans / cytology*
  • Caenorhabditis elegans / genetics
  • Caenorhabditis elegans / metabolism
  • DNA Damage
  • Insulin / genetics
  • Insulin / metabolism*
  • Insulin-Like Growth Factor I / genetics
  • Insulin-Like Growth Factor I / metabolism*
  • MAP Kinase Signaling System
  • Mitogen-Activated Protein Kinase 3 / genetics
  • Mitogen-Activated Protein Kinase 3 / metabolism*
  • Phosphatidylinositol 3-Kinases / genetics
  • Phosphatidylinositol 3-Kinases / metabolism
  • Proto-Oncogene Proteins c-akt / genetics
  • Proto-Oncogene Proteins c-akt / metabolism
  • ras Proteins / genetics
  • ras Proteins / metabolism*

Substances

  • Insulin
  • Insulin-Like Growth Factor I
  • Phosphatidylinositol 3-Kinases
  • Proto-Oncogene Proteins c-akt
  • Mitogen-Activated Protein Kinase 3
  • ras Proteins