Role of p90 Ribosomal S6 Kinase (p90RSK) in Reactive Oxygen Species and Protein Kinase C β (PKC-β)-mediated Cardiac Troponin I Phosphorylation.

Protein kinase C (PKC)-induced phosphorylation of cardiac troponin I (cTnI) depresses the acto-myosin interaction and may be important during the progression of heart failure. Although both PKCβII and PKC∊ can phosphorylate cTnI, only PKCβ expression and activity are elevated in failing human myocardium during end-stage heart failure. Furthermore, although increased cTnI phosphorylation was observed in mice with cardiac-specific PKCβ II overexpression, no differences were observed in cTnI phosphorylation status between wild type and cardiac-specific PKC∊ overexpression mice. A potentially important downstream effector of PKCs is p90 ribosomal S6 kinase (p90RSK), which plays an important role in cell growth by activating several transcription factors as well as Na+/H+ exchanger. Since both Ser23 and Ser24 of cTnI are contained in putative consensus sequences of p90RSK phosphorylation sites, we hypothesized that p90RSK is downstream from PKCβ II and can be a cTnI (Ser23/24) kinase, p90RSK, but not ERK½ activation, was increased in PKCβII overexpression mice but not in PKC∊ overexpression mice. p90RSK could phosphorylate cTnI in vitro with high substrate affinity but not cardiac troponin T (cTnT). To confirm the role of p90RSK in cTnI phosphorylation in vivo, we generated adenovirus containing a dominant negative form of p90RSK (Ad-DN-p90RSK). We found that the inhibition of p90RSK prevented H2O2-mediated cTnI (Ser23/24) phosphorylation but not ERK½ and PKCα/βII activation. Next, we generated cardiac-specific p90RSK transgenic mice and observed that cTnI (Ser23/24) phosphorylation was significantly increased. LY333,531, a specific PKCβ inhibitor, inhibited both p90RSK and cTnI (Ser23/24) phosphorylation by H2O2. Taken together, our data support a new redox-sensitive mechanism regulating cTnI phosphorylation in cardiomyocytes. [ABSTRACT FROM AUTHOR]