Cardiac hypertrophy ensues as a response to multiple stimuli, such as mechanical stress, neurohumoral activation, growth factors and cytokines. Connective Tissue Growth Factor (CTGF), a potent fibrogenic cytokine, regulates a wide range of biological functions including ECM deposition, wound repair, angiogenesis, migration, differentiation, survival and proliferation. While CTGF overexpression in fibroblasts has been shown to be responsible for fibrosis in various organs, controversy exists about the source of CTGF. Since interstitial fibrosis contributes to ventricular wall stiffness and impairs diastolic function, understating how key factors such as CTGF are expressed and released for the genesis of fibrosis in the hypertrophying heart is important to develop new treatment options. To this end, we explored the signaling pathway(s) involved in the phenylephrine (PE), a hypertrophic agonist, induced expression of CTGF by cardiomyocytes (CMs). Since mammalian target of rapamycin (mTOR) is reported to regulate PE-induced hypertrophic signaling, we hypothesize that mTOR plays a role in PE induced CTGF expression in CMs.
To test if CMs produce CTGF, we treated adult feline CMs with phenylephrine. PE stimulated CTGF mRNA expression in a dose and time dependent manner. mTOR forms two distinct complexes, mTORC1 and mTORC2. Whereas both complexes are sensitive to a pharmacological inhibitor Torin1, only mTORC1 is sensitive to Rapamycin inhibition. Our results indicate that PE stimulated CTGF expression could be substantially enhanced by torin1 pretreatment of CMs. Moreover, shRNA mediated silencing of Rictor in CMs, one of the components of mTORC2, significantly augmented the PE induced CTGF mRNA expression. But mTORC1 inhibition using Rapamycin or activation of its downstream target S6K1 using Rapamycin resistant S6K1 adenovirus had no impact in PE -stimulated CTGF expression. The same trend was also observed in the level of secreted CTGF. In conclusion, these results strongly indicate that mTORC2 plays a repressive role in CTGF mRNA expression in adult CMs, and that the loss of such repression in PO myocardium might be a potential mechanism for the onset of cardiac fibrosis in hypertrophying myocardium.
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