Downregulation of p68 RNA helicase (DDX5) activates a survival pathway involving mTOR and MDM2 signals

Abstract

The DEAD box p68 RNA helicase (DDX5) is required to manipulate RNA structures implicated in mRNA/rRNA processing and transcript export, and acts as a co-activator for a range of transcription factors. Previous research has indicated that p68 RNA helicase may also be important in tumour development. Wild-type HeLa and stable HeLa (clone 13) cell cultures containing RNAi-mediated depletion of p68 RNA helicase induced by doxycycline (DOX) were used to study how the p68 RNA helicase affects the mTOR cell signalling pathway. Relevant results were repeated using transient transfection with pSuper/pSuper-p68 RNA helicase, containing RNAi-mediated depletion of p68 RNA helicase, to avoid DOX interference. Here we provide strong evidence for the participation of p68 RNA helicase in mTOR regulation. In detail, depletion of this helicase decreases cell growth and activates the mTOR/ MDM2 cell survival mechanism, which ultimately leads to inhibition of the pro-apoptotic activity. p68 RNA helicase downregulation strongly stimulates 4E-BP1 phosphorylation, thereby provoking activation of cap-dependent translation. In contrast, the IRES-dependent translation of c-myc is reduced when p68 RNA helicase is depleted, thus indicating that at least this specific translation requires p68 RNA helicase activity to manipulate the complex 5' end of this mRNA. Interestingly, p68 RNA helicase depletion decreases cell growth while activating the mTOR/MDM2 cell survival mechanism. As MDM2 is a known negative regulator of p53, we infer that the activation of the cell survival mechanism may result in inhibition of the pro-apoptotic factor p53. Finally, p68 RNA helicase depletion activates capdependent translation and inhibits c-MYC IRES-mediated translation.