Protein kinase CK2 mediates TGF-β1-stimulated type IV collagen gene transcription and its reversal by estradiol

Abstract

Background. We have previously shown that the transcription factor Sp1 mediates the stimulatory effects of transforming growth factor-β1 (TGF-β1) on type IV collagen gene transcription and protein synthesis, and that estradiol reverses these effects by down-regulating Sp1 activity. Protein kinase casein kinase II (CK2) phosphorylates Egr-1 and prevents its binding to Sp1. We hypothesized that TGF-β1 stimulates CK2 activity, which in turn activates type IV collagen gene transcription via increased availability of free Sp1. Methods. The effects of TGF-β1 and of estradiol on murine mesangial cell type IV collagen gene transcription were measured using a reporter mini gene construct and on collagen IV protein synthesis by Western blotting. Nuclear Egr-1, phosphorylated Egr-1, Sp1, Egr-1/Sp1 complexes and unbound Sp1 were measured using co-immunoprecipitation and Western blotting techniques. Results. TGF-β1 stimulated CK2 activity in murine mesangial cells. Although TGF-β1 failed to alter total Egr-1 protein, it increased phosphorylated Egr-1. This led to decreased Egr-1/Sp1 complex formation, increased unbound Sp1, increased binding of nuclear extracts to the collagen IV promoter, and increased type IV collagen gene transcription and protein synthesis. Physiologic concentrations of estradiol reversed these effects. Conclusions. These studies suggest that activation of CK2 mediates the stimulatory effect of TGF-β1 on type IV collagen gene transcription. Moreover, the ability of estradiol to reverse TGF-β1-stimulated type IV collagen synthesis is mediated by down-regulating CK2 activity, which ultimately limits the availability of unbound Sp1 to activate gene transcription.