Activation of the coactivator four-and-a-half-LIM-only protein FHL2 and the c-fos promoter through inhibition of protein phosphatase 2A

Previous studies have demonstrated that the serine/threonine protein phosphatase 2A (PP2A) can modulate the transcriptional activity of several sequence-specific DNA-binding proteins. However, less is known about the effect of PP2A on the activities of general transcription factors and transcriptional coregulators. Here we describe that the activity of a general coactivator, the four-and-a-half-LIM-only protein 2 (FHL2), is regulated in a PP2A-dependent manner. Specific inhibition of PP2A by simian virus 40 (SV40) small t-antigen (st-ag) stimulated the intrinsic transcriptional activity of FHL2 more than 10-fold, while a st-ag mutant unable to bind PP2A had no effect. Overexpression of the B56 subunits α, β, and γ1 of PP2A impaired the induction of FHL2 by st-ag. FHL2 functioned as a coactivator for CREB-mediated transcription, and inactivation of PP2A further increased FHL2-induced CREB-directed transcription. Overexpression of FHL2 readily enhanced the transcription of the luciferase reporter gene driven by the c-fos promoter, and inhibition of PP2A further stimulated FHL2-induced transactivation of this promoter. These results suggest that dephosphorylation of the general coactivator FHL2 may represent a novel mechanism by which PP2A modulates the transcription of FHL2-responsive genes.