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 PKC beta II and PKC epsilon can phosphorylate cTnI, only PKC beta 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 PKC beta II overexpression, no differences were observed in cTnI phosphorylation status between wild type and cardiac-specific PKC epsilon 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 Ser(23) and Ser(24) of cTnI are contained in putative consensus sequences of p90RSK phosphorylation sites, we hypothesized that p90RSK is downstream from PKC beta II and can be a cTnI (Ser(23/24)) kinase. p90RSK, but not ERK1/2 activation, was increased in PKC beta II overexpression mice but not in PKC epsilon 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 (Ser(23/24)) phosphorylation but not ERK1/2 and PKC alpha/beta II activation. Next, we generated cardiac-specific p90RSK transgenic mice and observed that cTnI (Ser(23/24)) phosphorylation was significantly increased. LY333,531, a specific PKC beta 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.