Role of oxidative modifications of LKB1 in promoting myocardial hypertrophy
Calamaras, Timothy Dean
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The pathogenesis of heart failure (HF) involves compensatory left ventricular hypertrophy. Reactive oxygen species (ROS) are elevated in HF and mediate myocardial hypertrophy. ROS also mediate formation of lipid peroxidation byproducts, yet little is known about their role in promoting hypertrophy. One lipid peroxidation byproduct, 4-hydroxy-trans-2-nonenal (HNE) is a reactive aldehyde that forms covalent adducts on proteins. HNE levels are also elevated in HF and may mediate hypertrophy via HNE-adduct formation. LKB1 - a tumor suppressor protein - regulates cellular growth through activation of the downstream kinase AMPK. Activation of AMPK suppresses functions that consume ATP and simultaneously activates processes to generate energy. The LKB1 protein is inhibited by oxidants, but whether this results in myocardial hypertrophy is unclear. I hypothesized that HNE can directly promote cardiac hypertrophy via the modification of LKB1. In HEK293T cells I observed that HNE adducts inhibit activity of LKB1 through direct oxidative modification. Mutation of LKB1 Lys-96 or Lys-97 resulted in less HNE-LKB1 adduct formation. Mutation of LKB1 Lys-97 prevented the inhibitory effect of HNE, suggesting that HNE-adduction at this residue is sufficient to inhibit LKB1. In cardiomyocytes HNE inhibited both LKB1 and AMPK, increased phosphorylation of mTOR, p70S6K, and S6K, and increased protein synthesis. HNE also activated Erk1/2, which contributed to S6K activation but was not required for cellular growth. Hypertrophic S6K activation was dependent on mTOR. Mice fed a high-fat high-sucrose (HFHS) diet have myocardial hypertrophy that can be prevented by antioxidants. Hearts of HFHS mice have HNE-LKB1 adducts, inhibited LKB1 activity, yet no change in AMPK activation. Mice lacking aldehyde dehydrogenase 2 (ALDH2), an enzyme involved in HNE detoxification, have increased myocardial hypertrophy when fed HFHS diet yet have increased LKB1 activity. In summary HNE directly causes hypertrophy in cardiomyocytes. This occurs through inhibition of LKB1 and in part through Erk1/2 activation. In HFHS-fed mice HNE-LKB1 adduct formation is associated with decreased LKB1 activity. Impairing detoxification of reactive aldehydes in the ALDH2-KO mice is sufficient to increase myocardial hypertrophy, but this appears to be independent of LKB1. This study demonstrates a novel mechanism of cardiac hypertrophy caused by reactive aldehydes.