Previously, we have demonstrated that oxidative stress or Ras/ERK activation leads to the transcriptional repression of alpha -subunit of epithelial Na+ channel (ENaC in lung and salivary epithelial cells. Here, we further investigated the coordinated molecular mechanisms by which alpha -ENaC expression is regulated. Using both stable and transient transfection assays, we demonstrate that the overexpression of high mobility group protein I-C (HMGI-C), a Ras/ERK-inducible HMG-I family member, represses glucocorticoid receptor (GR)/dexamethasone (Dex)-stimulated alpha -ENaC/reporter activity in salivary epithelial cells. Northern analyses further confirm that the expression of endogenous a-ENaC gene in salivary Pa-4 cells is suppressed by an ectopic HMGI-C overexpression. Through yeast two-hybrid screening and co-immunoprecipitation assays from eukaryotic cells, we also discovered the interaction between HMGI-C and PIAS3 (protein inhibitor of activated STAT3 (signal transducer and activator of transcription 3)). A low level of ectopically expressed PIAS3 cooperatively inhibits GR/Dex-dependent alpha -ENaC transcription in the presence of HMGI-C. Reciprocally, HMGI-C expression also coordinately enhances PIAS3-mediated repression of STAT3-dependent transactivation. Moreover, overexpression of antisense HMGI-C construct is capable of reversing the repression mediated by Ras V12 on GR- and STAT3-dependent transcriptional activation. Together, our results demonstrate that Ras/ERK-mediated induction of HMGI-C is required to effectively repress GR/Dex-stimulated transcription of alpha -ENaC gene and STAT3-mediated transactivation. These findings delineate a network of inhibitory signaling pathways that converge on HMGI-C.PIAS3 complex, causally associating Ras/ERK activation with the repression of both GR and STAT3 signaling pathways in salivary epithelial cells.
