Department of Physiology and Functional Genomics, University of Florida, Gainesville, FL 32610-0274, USA.
Circulation. 2012 Nov 27;126(22):2575-88. doi: 10.1161/CIRCULATIONAHA.112.116202. Epub 2012 Oct 24.
Cardiac hypertrophy is a common response to circulatory or neurohumoral stressors as a mechanism to augment contractility. When the heart is under sustained stress, the hypertrophic response can evolve into decompensated heart failure, although the mechanism(s) underlying this transition remain largely unknown. Because phosphorylation of cardiac myosin light chain 2 (MLC2v), bound to myosin at the head-rod junction, facilitates actin-myosin interactions and enhances contractility, we hypothesized that phosphorylation of MLC2v plays a role in the adaptation of the heart to stress. We previously identified an enzyme that predominantly phosphorylates MLC2v in cardiomyocytes, cardiac myosin light-chain kinase (cMLCK), yet the role(s) played by cMLCK in regulating cardiac function in health and disease remain to be determined.
We found that pressure overload induced by transaortic constriction in wild-type mice reduced phosphorylated MLC2v levels by ≈40% and cMLCK levels by ≈85%. To examine how a reduction in cMLCK and the corresponding reduction in phosphorylated MLC2v affect function, we generated Mylk3 gene-targeted mice and transgenic mice overexpressing cMLCK specifically in cardiomyocytes. Pressure overload led to severe heart failure in cMLCK knockout mice but not in mice with cMLCK overexpression in which cMLCK protein synthesis exceeded degradation. The reduction in cMLCK protein during pressure overload was attenuated by inhibition of ubiquitin-proteasome protein degradation systems.
Our results suggest the novel idea that accelerated cMLCK protein turnover by the ubiquitin-proteasome system underlies the transition from compensated hypertrophy to decompensated heart failure as a result of reduced phosphorylation of MLC2v.
心脏肥大是循环或神经激素应激的常见反应，是增强收缩力的机制。当心脏持续承受压力时，肥大反应可能会发展为代偿性心力衰竭，尽管这种转变的机制仍知之甚少。由于肌球蛋白轻链 2（MLC2v）的磷酸化，与肌球蛋白在头杆交界处结合，促进了肌动球蛋白相互作用并增强了收缩力，我们假设 MLC2v 的磷酸化在心脏对压力的适应中起作用。我们之前鉴定了一种主要在心肌细胞中磷酸化 MLC2v 的酶，即肌球蛋白轻链激酶（cMLCK），但 cMLCK 在调节心脏健康和疾病中的功能仍有待确定。
我们发现，主动脉缩窄引起的压力超负荷使野生型小鼠的磷酸化 MLC2v 水平降低了约 40%，cMLCK 水平降低了约 85%。为了研究 cMLCK 的减少和相应的磷酸化 MLC2v 的减少如何影响功能，我们生成了 Mylk3 基因靶向敲除小鼠和特异性在心肌细胞中过表达 cMLCK 的转基因小鼠。压力超负荷导致 cMLCK 敲除小鼠发生严重心力衰竭，但在 cMLCK 过表达小鼠中则没有，因为 cMLCK 蛋白合成超过了降解。在压力超负荷期间，cMLCK 蛋白的减少被抑制泛素-蛋白酶体蛋白降解系统所减弱。
我们的结果表明了一个新的观点，即泛素-蛋白酶体系统加速 cMLCK 蛋白周转是由于 MLC2v 磷酸化减少导致从代偿性肥大向失代偿性心力衰竭转变的基础。
