Rottlerin inhibits tonicity-dependent expression and action of TonEBP in a PKCδ-independent fashion

Novel protein kinase C (PKC) isoforms PKCδ and PKCε have recently been implicated in signaling by hypertonic stress. We investigated the role of the putative PKCδ inhibitor rottlerin on tonicity-dependent gene regulation. In the renal medullary mIMCD3 cell line, rottlerin blocked tonicity-dependent transcription of a tonicity enhancer (TonE)-driven luciferase reporter gene, as well as tonicity-dependent transcription of the physiological tonicity effector gene aldose reductase, but not urea-dependent transcription. Consistent with these data, rottlerin inhibited tonicity-dependent expression of TonE binding protein (TonEBP) at the mRNA and protein levels. Another inhibitor of both novel and conventional PKC isoforms, GF-109203X, suppressed TonEBP-dependent transcription but failed to influence tonicity-inducible TonEBP expression. Global PKC downregulation with protracted phorbol ester treatment, however, failed to influence tonicity-dependent signaling, arguing against a PKCδ-dependent mechanism of rottlerin action in this model. In addition, hypertonic stress failed to induce phosphorylation of PKCδ. Furthermore, in a PC-12 cell model with a comparable degree of tonicity-dependent transcription, constitutive overexpression of dominant negative-acting PKCδ or PKCε effectively decreased tonicity signaling to extracellular signal-regulated kinase activation, as expected, but failed to influence TonE-dependent transcription. TonE-dependent transcription, however, remained rottlerin sensitive in this PC-12 cell model. In the aggregate, these data indicate that rottlerin dramatically inhibits tonicity-dependent TonEBP expression and TonE-dependent transcription but, despite its reputed mode of action, does so through a PKCδ-independent pathway.