Activation of Cdk9 represses PGC-1 and predisposes to heart failure

Abstract

Hypertrophic growth allows the heart to adapt to workload but typically culminates in eventual pump failure. It remains unsettled how mechanical signals are coupled to either the initial growth or the resultant decompensation. Cardiac hypertrophy is accompanied by the activation of the atypical cyclin-dependent kinase cyclin T/Cdk9, which phosphorylates the C-terminal domain (CTD) of the large subunit of RNA polymerase II and stimulates transcription elongation and pre-mRNA processing. Previously, we showed that Cdk9 activity was required for hypertrophic growth in culture and heart specific activation of Cdk9 by cyclin T1 overexpression (alphaMHCcyclin T1) sufficed to induce cardiac hypertrophy in mice. Here, we show that Cdk9 activities are increased in both mice and human failing myocardium. The alphaMHC-cyclin T1 mice appear functionally normal at baseline, but suffer fulminant apoptotic cardiomyopathy when challenged to pathological or genetic stress, such as pressure-overload or the small G-protein Gq. Excess Cdk9 activity globally suppresses genes for mitochondrial function along with downregulation of two master regulators of mitochondrial biogenesis: peroxisome-proliferatoractivated receptor-gamma co-activator-1 (PGC-1), a transcriptional co-activator, and nuclear respiratory factor-1 (NRF-1), transcription factor. The alphaMHC-cyclin T1 mice showed impaired mitochondrial respiratory chain enzyme activities and ultrastructure. In cultured cells, expression of cyclin T1/Cdk9 suppresses PGC-1 expression. Downregulation of genes for mitochondrial function caused by cyclin T1/Cdk9 can be reversed by exogenous PGC-1. Cyclin T1/Cdk9 reduces both baseline and MEF2-induced PGC-1 promoter activities in a dose-dependent manner. Chromatin immunoprecipitation assay demonstrates that cyclin T1/Cdk9 particularly perturbates PGC-1 expression at the level of preinitiation complex assembly. Cyclin T1/Cdk9 decreases mitochondrial membrane potential and sensitizes cardiomyocytes to apoptosis, especially in the context of Gq stimulation, both of which can be reversed by either PGC-1 or Bcl-2. Analogously, direct dysregulation of PGC-1/NRF-1-mediated mitochondrial transcription by dominant negative NRF-1 suppresses the genes for mitochondrial biogenesis and increases the susceptibility to apoptosis. Thus, chronic activation of Cdk9 at the levels found in pathophysiological settings causes not only cardiomyocyte enlargement, but also defective mitochondrial function via downregulation of PGC-1 and a resulting susceptibility to heart failure by stimulating apoptosis. 136