Protein kinase C β enhances growth and expression of cyclin D1 in human breast cancer cells

Although alterations in the expressions of protein kinase C (PKC) have been implicated in breast carcinogenesis, the roles of specific isoforms in this process remain elusive. In the present study, we examined the specific roles of PKC and beta 2 in growth control in human breast cancer cell line. The PKC beta-specific inhibitor LY379196 significantly inhibited growth of the breast cancer cell lines MCF-7, MDA-MB-231, and BT474, but not the normal mammary epithelial cell line MCF-10F. Treatment of MCF-7 cells with LY379196 caused an increase in the fraction of cells in the G(1) phase of the cell cycle. To explore the roles of PKC I and beta 2, we used cDNA expression vectors that encode wild-type and constitutively activated or dominant negative mutants of these two proteins. When compared with vector controls, derivatives of MCF-7 cells that stably overexpress wild-type PKC beta 1 or PKC beta 2 displayed a slight increase in growth rate; derivatives that stably express the constitutively active mutants of PKC I or PKC beta 2 displayed a marked increase in growth rate; and derivatives that stably express a dominant negative mutant of PKC beta 1 or beta 2 displayed inhibition of growth. The derivatives of MCF-7 cells that stably express the constitutively activated mutants of PKC beta 1 or beta 2 were more resistant to growth inhibition by LY379196 than the vector control MCF-7 cells. Immunoblot analysis indicated that MCF-7 cells that stably overexpress wild-type or constitutively activated mutants of PKC beta 1 or beta 2 had higher cellular levels of cyclin D1 than vector control cells, whereas cells that express a dominant negative mutant had decreased levels of cyclin D1. The derivatives that stably express the constitutively activated mutants of PKC beta 1 or beta 2 also displayed increased cyclin beta 1 promoter activity in transient transfection luciferase reporter assays, and this induction of activity requires activator protein 1. Constitutively activated PKC beta 1 and beta 2 also enhanced the transcription of c-fos in transient transfection luciferase reporter assays. Thus, PKC beta 1 and beta 2 may play important positive roles in the growth of at least a subset of human breast cancers. Therefore, inhibitors of these isoforms may be useful in breast cancer chemoprevention or therapy.