Direct activation of protein kinase C by 1α,25-dihydroxyvitamin D3

Abstract

The key metabolite of vitamin D3, 1α,25-dihydroxyvitamin D3 (1,25- D3), induces rapid cellular responses that constitute a so-called 'non- genomic' response. This effect is distinguished from its 'classic' genomic role in calcium homeostasis involving the nuclear 1,25-D3 receptor. Evidence is presented that protein kinase C (PKC) is directly activated by 1,25-D3 at physiological concentrations (EC50 = 16 ± 1 nM). The effect was demonstrable with single PKC-α, -γ, and -ε isoform preparations, assayed in a system containing only purified enzyme, substrate, co-factors, and lipid vesicles, from which it is inferred that a direct interaction with the enzyme is involved. The finding that calcium-independent isoform PKC-ε was also activated by 1,25-D3 shows that the calcium binding C2 domain is not required. The level of 1,25-D3-induced activation, paired with either diacylglycerol or 4β-12-O-tetradecanoylphorbol-13-acetate, was greater than that achievable by any individual activator alone, each at a saturating concentration, a result that implies two distinct activator sites on the PKC molecule. Phosphatidylethanolamine present in the lipid vesicles potentiated 4β-12-O-tetradecanoylphorbol-13-acetate- and diacylglycerol-induced PKC activities, whereas 1,25-D3-induced activity decreased, consistent with 1,25-D3-activated PKC possessing a distinct conformation. The results suggest that PKC is a 'membrane-bound receptor' for 1,25-D3 and that it could be important in the control of non-genomic cellular responses to the hormone.