p70 S6 kinase regulation of Mdm2 and p53 in ovarian cancer cells during stress conditions

Abstract

Ovarian cancer is a leading cause of death among of gynecological cancers. Current

therapies are ineffective with a poor 5-year survival of only ~25%. p70 S6 kinase (p70

S6K) is a downstream target of the phosphatidylinositol 3-kinase pathway and is

frequently activated in human ovarian cancer. However, the molecular targets and

signaling pathways by which p70 S6K may affect tumor development and progression

are poorly understood. Interestingly, in the laboratory, Mdm2, an important negative

regulator of the p53 tumor suppressor, was identified in a yeast two hybrid screening of

potential interacting partners for p70 S6K. In this study, I aimed to investigate the

specific interaction of p70 S6K and Mdm2 and determine how this may contribute to

ovarian tumorigenesis. Using a co-immunoprecipitation assay, the in vivo interaction of

p70 S6K and Mdm2 in human ovarian cancer cells was confirmed. Upon UV-induced

genotoxic stress, p70 S6K activation was associated with Mdm2 phosphorylation on

S166 and subsequent p53 accumulation. This could be reversed by the use of rapamycin

and p70 S6K siRNA to inhibit its kinase activity and expression respectively, confirming

that the effect was p70 S6K specific. Conversely, ectopic expression of wildtype p70

S6K or a constitutively active mutant of p70 S6K, D3E-E389 (D3E) was sufficient to

induce phosphorylation of Mdm2. Moreover, the p70 S6K mediated activation of Mdm2

was independent of p53 mutations. Similar results were observed upon other stress

challenges such as hypoxia using hypoxia mimicking agent desferrioxamine (DFX).

These findings identify Mdm2 as a new target of p70 S6K and reveal that p70 S6K

intervenes the Mdm2-p53 regulatory loop in ovarian cancer, which may provide a

survival advantage to cancer cells under stress conditions.