Rbck1, A Protein Kinase C Beta I (Pkc Beta I)-Interacting Protein, Regulates Pkc Beta-Dependent Function

RBCK1 ((RBC) under barC protein interacting with P (K) under barC (1) under bar) has originally been identified as a protein kinase C beta I (PKC beta I)-binding partner by a two-hybrid screen and as one of the gene transcripts that increases during adult cardiac hypertrophy. To address whether RBCK1 and PKC beta I functions are interconnected, we used cultured neonatal myocytes where we previously found that the activity of PKC beta I is required for an increase in cell size, also called hypertrophy. In this study, we showed that acute treatment of cardiac myocytes with phenylephrine, a prohypertrophic stimulant, transiently increased the association of RBCK1 with PKC beta I within 1 min. A prolonged phenylephrine treatment also resulted in an increase of the interaction of the two proteins. Endogenous RBCK1 protein levels increased upon phenylephrine-induced hypertrophy. Further, adenovirus-based RBCK1 overexpression in the absence of phenylephrine increased cardiac cell size. This RBCK1-mediated hypertrophy required PKC beta activity, since the increase in cell size was inhibited when the RBCK1-expressing cells were treated with PKC beta-selective antagonists, supporting our previous observation that both PKC beta I and PKC beta II are required for hypertrophy. Unexpectedly, RBCK1-induced increased cell size was inhibited by phenylephrine. This effect correlated with a decrease in the level of both PKC beta isoforms. Most importantly, RNA interference for RBCK1 significantly inhibited the increase in cell size of cardiac myocytes following phenylephrine treatment. Our results suggest that RBCK1 binds PKC beta I and is a key regulator of PKC beta I function in cells and that, together with PKC beta II, the three proteins are essential for developmental hypertrophy of cardiac myocytes.