Protein Kinase C δ Inhibits the Proliferation of Vascular Smooth Muscle Cells by Suppressing G1 Cyclin Expression

To elucidate the physiological role of protein kinase C (PKC) δ, a ubiquitously expressed isoform in vascular smooth muscle cells (VSMC), PKC δ was stably overexpressed in A7r5 cells, rat clonal VSMC. The [ 3 H]thymidine incorporation in A7r5 overexpressed with PKC δ (DVs) was suppressed to 37.1 ± 16.3% (mean ± S.D.) of the level in control or A7r5 transfected with vector alone (EVs). The reduction of [ 3 H]thymidine incorporation was strongly correlated with overexpressed PKC levels. Moreover, transient transfection of a dominant negative mutant of PKC δ restored the reduced proliferation in DVs. Flow cytometry analysis demonstrated that DVs were arrested in the G 0 /G 1 phase of the cell cycle. Expression of cyclins D1 and E and retinoblastoma protein phosphorylation were reduced, while the protein levels of p27 were elevated in DVs as compared with EVs. There were no significant differences in the expression of c- fos , c- jun , c- myc , cyclin D2, D3, cyclin-dependent kinase 2, cyclin-dependent kinase 4, and p21 among the clones. We conclude that PKC δ inhibits the proliferation of VSMC by arresting cells in G 1 via mainly inhibiting the expression of cyclin D1 and cyclin E.