Rev-ERBa Regulates Mitochondrial Biogenesis, Adiposity, and Insulin Action

Reynolds, Thomas

Abstract

The prevalence of obesity and type 2 diabetes in the United States has increased dramatically over the last three decades. Identifying genes that regulate adiposity and insulin action is essential for the development of novel anti-obesity and insulin sensitizing compounds. By treating obese mice with the manganese tetrakis benzoic acid (MnTBA) porphyrin, we have identified heme's nuclear receptor, Rev-ERBa, as a novel anti- obesity target that also improves insulin action. Rev-ERBa is a transcriptional repressor whose activity is enhanced upon binding its ligand, heme. In a negative feedback loop, heme regulates the expression of Rev- ERBa. Our preliminary data shows that treating mice with MnTBA porphyrin increases Rev-ERBa expression indicating that the porphyrin mimics low levels of heme. Rev-ERBa represses the expression of peroxisome proliferator-activated receptor-gamma (PPAR?) coactivator 1 alpha (PGC1a), a master regulator of mitochondrial biogenesis. Our preliminary data demonstrates that PGC1a is increased in mice treated with MnTBA porphyrin. Consistent with an increase in mitochondrial biogenesis, uncoupling protein 2 and 3 (UCP2 and UCP3) are increased in mice treated with MnTBA porphyrin, a response that likely explains the large reductions in adiposity we observed in these mice. Because our preliminary studies show that MnTBA porphyrin does not compete with heme for Rev-ERBa's binding site, but increases heme oxygenase-1 (HO-1), our hypothesis is that the HO-1 product, carbon monoxide, inhibits Rev-ERBa.
Specific aim #1 will determine if treating wildtype and HO-1 knockout mice with heme-like porphyrins or carbon monoxide decreases the binding of Rev-ERBa to the PGC1a promoter, a process that would stimulate mitochondrial biogenesis and reduces adiposity (increased UCP2/3). Since we observed a reduction in adiposity in mice treated with MnTBA porphyrin, it is not surprising that we also show that MnTBA porphyrin reverses diet-induced insulin resistance. However, the MnTBA porphyrin-induced increase in insulin sensitivity is associated with an increase in Akt/PKB expression, a finding that is distinct from caloric restriction induced weight loss. Our hypothesis is that the increase in Akt/PKB levels in mice treated with MnTBA porphyrin is due to the compound's ability to decrease Rev-ERBa activity. This hypothesis is supported by the evidence that Rev-ERBa represses IL-6, a process that would restrain STAT3 dependent induction of the Akt/PKB promoter. Therefore, Specific Aim #2 will determine if treating mice fed a high fat diet with heme-like porphyrins or carbon monoxide reverses insulin resistance by increasing STAT3 mediated Akt/PKB expression. By completing the specific aims of this R15 proposal, we will establish a novel mechanism explaining how porphyrins or carbon monoxide interact with Rev-ERBa to alter metabolism, possibly leading to novel treatments for obesity and type 2 diabetes.

Public Health Relevance

The prevalence of obesity and type 2 diabetes in the United States has increased dramatically in the last three decades. We have identified the transcriptional repressor, Rev- ERBa, as a potential anti-obesity target that improves type 2 diabetes. The overall goal of this proposal is to learn more about how Rev-ERBa reduces adiposity and enhances insulin action.