Abstract 486: Down-regulation of type 1 cGMP-dependent Protein Kinase by the ubiquitin/proteasome pathway
Type 1 cGMP-dependent protein kinase (PKG-I) is a widely expressed serine/threonine protein kinase, and is a major mediator of nitric oxide (NO) signaling in vascular smooth muscle cells (VSMC). PKG-I level is highly variable in VSMC and several studies have shown that atherogenic inflammatory cytokines lower the steady-statel levels of PKG-I. The mechanism of action of down-regulation is not well defined, but induction of type II NO synthase (iNOS) and subsequent persistent elevation of cGMP appear to contribute to PKG-I down regulation. In the present study, we examined the role of the ubiquitin/proteasome pathway in PKG-Iα down-regulation in response to elevated cGMP. Incubation of cultured VSMC with 8-Br-cGMP for 6–12 hr lowered PKG-I expression as assessed by western blotting. To further examine the mechanism, Cos7 cells, which do not express PKG-I mRNA or protein, were transfected with PKG-Iα/pcDNA vector and incubated with 8-Br-cGMP. 8-Br-cGMP suppressed PKG-Iα protein level in Cos7 cells (half-maximal concentration = 250 μM). Pretreatment of these cells with the proteasome inhibitor, MG132, followed by 8-Br-cGMP treatment prevented the decline suggesting the involvement of the ubiquitin/26S proteasome pathway. Immunoprecipitation of PKG-I followed by immunoblotting with anti-ubiquitin revealed multiple ubiquitinated PKG bands in the 8-Br-cGMP treated samples but not in untreated samples. Ubiquitination and down-regulation were also inhibited by the specific PKG-I catalytic inhibitor DT-2, suggesting the possible involvement of PKG autophosphorylation in the 8-Br-cGMP induced down-regulation. Mutation of the PKG-Iα autophosphorylation sites to alanines was performed to identify the phosphorylated site responsible for cGMP-dependent ubiquitination. In contrast to wild type PKG-Iα, PKG-Iα S64A, but not the S50A mutant, was not down-regulated by 8-Br-cGMP suggesting that autophosphorylation of serine-64 is required for the ubiquitination and down-regulation of PKG-I. Autophosphorylation and cGMP-mediated down-regulation of PKG-I may be an important mechanism to control excess cGMP signaling in VSMC.