Decreased protein and phosphorylation level of the protein phosphatase inhibitor-1 in failing human hearts

Objective: The protein phosphatase inhibitor-1 (I-1) is a highly specific and potent inhibitor of type 1 phosphatases (PP1) that is active only in its protein kinase A (PKA)-phosphorylated form. I-1 ablation decreases, I-1 overexpression sensitizes beta-adrenergic signaling in the heart. It is controversial whether I-1 expression is altered in human heart failure (HF), likely because its detection in heart is difficult due to its low abundance. Methods and results: I-1 was >500-fold enriched from left ventricular myocardium (LVM) from patients with terminal HF (n = 16) and non-failing controls (NF, n = 5) and quantified with an affinity-purified I-1 and a I-1 phosphospecific antiserum. In non-failing I-1 protein levels amounted to 126 fmol/mg protein. In failing hearts, I-1 protein levels were reduced by 58% and I-1 phosphorylation by 77% (P < 0.001 vs. NF). I-1 phosphorylation correlated well with serine-16 phosphorylation of phospholamban (PLB) in the same hearts (P < 0.001). In contrast, PLB, troponin I (TnI) and PP1 protein and TnI phosphorylation levels did not differ between HF and NF. Conclusions: The results suggest that the reduction in I-1 protein and phosphorylation in failing human hearts leads to increased phosphatase activity which in turn may result in reduced phosphorylation of cardiac proteins such as PLB. (C) 2003 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.