Activation and function of cyclin T-Cdk9 (positive transcription elongation factor-b) in cardiac muscle-cell...

Area of Science:

• Cardiovascular Biology
• Molecular Cardiology
• Gene Transcription

Background:

• Cardiac hypertrophy, an increase in heart cell size, is a significant risk factor for mortality in heart diseases.
• The underlying mechanisms for the global increase in cellular RNA and protein content during hypertrophy remain incompletely understood.
• Hypertrophic signaling pathways are known to influence the phosphorylation of RNA polymerase II, a key step in gene transcription.

Purpose of the Study:

• To investigate the role of RNA polymerase II kinases, specifically cyclin-dependent kinases-7 (Cdk7) and Cdk9, in the development of cardiac hypertrophy.
• To elucidate the specific kinase responsible for RNA polymerase II phosphorylation and subsequent cellular growth under various hypertrophic stimuli.
• To identify the regulatory mechanisms controlling Cdk9 activity and its impact on cardiac mass.

Main Methods:

• Activation analysis of Cdk7 and Cdk9 in response to different hypertrophic triggers (signaling proteins, mechanical stress).
• Pharmacological and dominant-negative inhibition studies to assess the role of Cdk9 in endothelin-induced cardiac growth.
• Investigation of the interaction between 7SK small nuclear RNA, cyclin T-Cdk9 complex, and hypertrophic signals.
• Experimental manipulation of Cdk9 inhibitor (7SK) and activator (cyclin T1) levels in cell culture and mouse myocardium.

Main Results:

• Both Cdk7 and Cdk9 were activated in hypertrophy induced by Galphaq, calcineurin, or chronic mechanical stress.
• Cdk9, but not Cdk7, was specifically activated by acute load and endothelin stimulation.
• Cdk9 played a preferential role in endothelin-induced RNA polymerase II phosphorylation and cardiac cell growth.
• All tested hypertrophic signals led to the dissociation of 7SK small nuclear RNA from the cyclin T-Cdk9 complex.
• Cdk9 was identified as a limiting factor for cardiac growth, as evidenced by experiments involving 7SK suppression and cyclin T1 regulation.

Conclusions:

• Cdk9 activation is a critical mediator of cardiac hypertrophy, promoting increased RNA and protein synthesis.
• The dissociation of 7SK RNA from Cdk9 is a key event in the hypertrophic signaling pathway.
• Targeting Cdk9 or its regulatory partners represents a potential therapeutic strategy for mitigating cardiac hypertrophy.