A Novel PKCβ Inhibitor More Potent than Ruboxistaurin: Potential Therapeutic Tool for Obesity and Fatty Liver Disease

Abstract

The ongoing epidemic of obesity and its associated comorbidities has increased the demand for novel therapeutics targeted toward modulating appetite and/or energy metabolism. The search for clinically useful drugs has thus far met with limited success, and therefore the identification of novel potential targets remains of great interest. We previously showed that mice with systemic PKCβ depletion displayed many of the features one would desire in individuals treated with an ideal antiobesity drug; increase in lean body mass and elevated metabolic rate contributing to prevention of diet‐induced adiposity, hepatic steatosis, and insulin resistance (Hepatology 49:1525, 2009; J. Lipid Res. 286:22795, 2011). Even depletion of hepatocyte‐specific PKCβ recapitulated many of the bebeficial features of whole‐body knockout mice (Mol. Metab. 44:e101133, 2021; JCI Insight 6:e149023, 2021). Our results provide compelling evidence that PKCβ is an important therapeutic target for treatment of obesity and related metabolic complications. An alternative strategy to genetic manipulation of PKCβ expression is the chronic use of selective inhibitors to resolve obesity, or reverse obesity, or both. About 25 years ago, it was reported that a small, orally available molecule, called ruboxistaurin, (also known as LY333,531) reported to potently and selectively inhibited PKCβ. This inhibitor displayed kinetics consistent with direct competition for ATP binding to PKCβ. This selectivity fueled several randomized clinical trials of ruboxstaurin to resolve chronic diabetic microvascular complications such as neuropathy, retinopathy, and macular oedema. Although the mechanism through which hyperglycemia results in vascular dysfunction has not been well established, but the in vivo and in vitro preclinical evidence suggested that hyperglycemia‐induced activation of PKCβ might underlie the development of these microvascular diseases into chronic diabetic complications. To the best of our knowledge, primary outcomes were never achieved in any of the trials and ruboxistaurin has not been approved from the U. S. Food and Drug Administration. Considering that targeting PKCβ outside of the conserved ATP site may be an effective strategy to achieve greater selectivity and potency, we hereby describe screening of already known and newly synthesized bisindolylmaleimides and their derivatives to evaluate whether these chemicals inhibit PKCβ with similar or higher potency than ruboxistaurin. These chemicals were also used to evaluate inhibition of PKCβ vs PKCα. We identified INST3399 as the most potent inhibitor of PKCβ in the basal state, being about 5‐times more potent than ruboxistaurin and more than 100‐fold selectivity for PKCβ versus PKCα. INST3399 is also several orders of magnitude more selective for inhibition of PKCβ in comparison to other ATP dependent kinases. INST3399 appears to inhibit PKCβ by a mechanism different from ruboxistaurin. As expected, INST3399 prevented diet‐induced obesity and hepatic steatosis. This has clear implications for the treatment of metabolic conditions with excessive PKCβ activity. Further studies are in progress to assess the therapeutic efficacy of this compound in reversing obesity.