Zinc ions negatively regulate proapoptotic signaling in cells expressing oncogenic mutant Ras

Abstract

Mutational activation of the Ras family of proto-oncogenes promotes cell survival and proliferation. Studies using cells cultured in vitro have shown that ectopic expression of constitutively active Ras suppresses apoptosis induced by serum deprivation. However, in some cellular contexts, constitutively active Ras exerts the opposite effects, including apoptosis of serum-starved embryonic fibroblasts. Such observations first came over two decades ago, but the molecular mechanisms by which mutant Ras increases the susceptibility of cells to serum deprivation leading to apoptosis are still not fully understood. To revisit this issue, I investigate the effects of serum depletion and mutant Ras expression on intracellular signaling and transcriptome of cells carrying an inducible allele of constitutively active mutant Hras (Hras^G12V). I identify zinc ions (Zn²⁺) as a serum factor that suppresses proapoptotic signaling in cells expressing Hras^G12V. Mechanistically, Hras^G12V expression along with Zn²⁺ deficiency activates c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), which are required for caspase-3 activation involved in the induction of cell death. Transcriptome analyses suggest that Hras^G12V induces the unfolded protein response (UPR). Further analyses of intracellular signaling biomolecules related to the UPR indicate that Hras^G12V activates inositol-requiring protein 1 (IRE1), which synergizes with Zn²⁺ deficiency to activate JNK and p38 MAPK signaling. These results provide insights into a role of Zn²⁺ that counteracts proapoptotic signaling activated by mutationally activated Ras.

Keywords: ER stress; MAP kinase; Ras; Transformation; UPR; Zinc deficiency.