HPV16 E7-dependent transformation activates NHE1 through a PKA-RhoA-induced inhibition of p38alpha

Abstract

Background: Neoplastic transformation originates from a large number of different genetic alterations. Despite this genetic variability, a common phenotype to transformed cells is cellular alkalinization. We have previously shown in human keratinocytes and a cell line in which transformation can be turned on and followed by the inducible expression of the E7 oncogene of human papillomavirus type 16 (HPV16), that intracellular alkalinization is an early and essential physiological event driven by the up-regulation of the Na/+H+ exchanger isoform 1 (NHE1) and is necessary for the development of other transformed phenotypes and the in vivo tumor formation in nude mice. Methodology: Here, we utilize these model systems to elucidate the dynamic sequence of alterations of the upstream signal transduction systems leading to the transformation-dependent activation of NHE1. Principal Findings: We observe that a down-regulation of p38 MAPK activity is a fundamental step in the ability of the oncogene to transform the cell. Further, using pharmacological agents and transient transfections with dominant interfering, constitutively active, phosphorylation negative mutants and siRNA strategy to modify specific upstream signal transduction components that link HPV16 E7 oncogenic signals to up-regulation of the NHE1, we demonstrate that the stimulation of NHE1 activity is driven by an early rise in cellular cAMP resulting in the down-stream inhibition of p38 MAPK via the PKA-dependent phosphorylation of the small G-protein, RhoA, and its subsequent inhibition. Conclusions: All together these data significantly improve our knowledge concerning the basic cellular alterations involved in oncogene-driven neoplastic transformation. © 2008 Cardone et al.