UBC Theses and Dissertations
The RhoA/Rho kinase pathway in diabetic cardiomyopathy Soliman, Hesham
Diabetes mellitus leads to a unique pathological entity termed diabetic cardiomyopathy, the mechanisms of which have not been fully defined. In diabetic rat hearts, RhoA expression is increased and the RhoA/ROCK pathway is activated, while ROCK inhibition acutely improves contractile function of diabetic hearts. The mechanisms underlying this improvement and those responsible for the detrimental activation of RhoA/ROCK were investigated here. Nitric oxide (NO) has been reported to upregulate RhoA expression in smooth muscle, and previous reports showed that iNOS expression is increased in diabetic rat hearts. In the first part of this thesis, the hypothesis that in diabetic cardiomyopathy, iNOS induction is responsible for increased RhoA expression was investigated. The results demonstrate that increased NO production from iNOS induction leads to RhoA upregulation in the diabetic heart and in isolated cardiomyocytes, contributing to the RhoA/ROCK mediated contractile dysfunction by increasing the total pool of RhoA available for activation. In diabetic rat hearts, PKCβ₂ activation induces iNOS expression, leading to increased nitrosative/oxidative stress. This suggests that PKCβ2 might positively regulate RhoA expression, although in preliminary experiments inhibition of ROCK itself reduced RhoA expression. Therefore, in the second part, the hypothesis that PKCβ₂/iNOS and RhoA/ROCK interact together to form a positive feedback loop was tested. The results show that RhoA/ROCK overactivation is sustained by a positive feedback loop that involves PKCβ₂ activation and iNOS induction. This feedback loop requires an intact actin cytoskeleton and plays a key role in elevating superoxide production in diabetic rat hearts. In the third part, the hypothesis that ROCK inhibition augments contraction by improving Ca²⁺ signaling was tested. Inhibition of ROCK improved contractile function and abolished the diabetes-induced delayed aftercontractions in isolated cardiomyocytes, in association with an improvement in Ca²⁺ transients. Overall, the results show that in diabetic cardiomyopathy, overactivation of RhoA/ROCK contributes to contractile dysfunction by sustaining PKCβ2 activation, iNOS induction and superoxide production via a positive feedback loop that leads to impaired intracellular Ca²⁺ homeostasis. Inhibition of ROCK disrupts the loop, resulting in decreased oxidative stress, and improved Ca²⁺ handling and cardiomyocyte contraction, suggesting that ROCK inhibition might be a novel approach in treating diabetic cardiomyopathy.
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