Branched-Chain Amino Acid Catabolism: Regulation and Effect on Insulin Resistance

Insulin resistance is the reduced responsiveness of a target tissue to insulin. Insulin resistance is an underlying cause of Type 2 diabetes mellitus (T2DM) and cardiovascular diseases, two debilitating diseases. Therefore, targeting insulin resistance can help prevent the development of T2DM and cardiovascular diseases.

Dietary protein, and in particular branched-chain amino acids (BCAAs; leucine, valine and isoleucine) stimulate muscle protein synthesis, and regulate body weight and glucose homeostasis. However, despite these benefits, circulating levels of BCAAs and BCAA metabolites, branched-chain a-ketoacids (BCKAs) are upregulated in insulin-resistant states like T2DM. This raises the question if upregulated levels of BCAAs and BCKAs cause insulin resistance or are a symptom of insulin resistance. Numerous studies have also shown how insulin resistant states can regulate the BCAA catabolic pathway and the enzymes involved, and how targeting this pathway can provide benefits in regards to insulin resistance and its sequalae. Thus, the purpose of this dissertation is to examine the link between BCAAs and their metabolism, and insulin resistance.

Previous in vitro studies have showed that depletion of branched-chain ketoacid dehydrogenase (BCKD), the enzyme responsible for the catabolism of BCKAs, suppressed insulin-stimulated glucose uptake in L6 myotubes and the metabolite of leucine, ketoisocaproic acid (KIC) worsens it. Thus, I demonstrated that interventions that increased BCKD activity improved insulin sensitivity and attenuated the suppressive effect of KIC on insulin sensitivity in L6 myotubes.

We have previously shown that KIC reduces insulin-stimulated glucose transport in muscle cells. However, contributions from other tissues aside from skeletal muscle in BCAA metabolism emphasize the importance of studying the effect of KIC gavage in vivo as well. Whereas KIC alters insulin signaling in the liver, it did not affect whole-body insulin tolerance.

Finally, since BCAA catabolism is implicated in many chronic conditions like insulin resistance, and age is a major risk factor in insulin resistance, we assessed how aging affects BCAA catabolism in both sexes. There was an increase in plasma BCAAs of old male mice, but changes in BCAA levels in plasma/tissues were not largely consistent with any changes in metabolic enzyme abundance/activity and did not correlate with changes in insulin sensitivity with sex or aging. Taken together, this thesis shows that although KIC suppresses glucose transport in vitro and its effect is attenuated by increasing BCAA oxidation, it does not affect insulin sensitivity in vivo likely due to contributions of the liver to catabolize KIC. Also, increases in plasma BCAAs seen in older male mice does not correlate with increased insulin resistance, suggesting that greater BCAAs in plasma may only be present in disease states. Together these studies suggest that altered BCAA levels do not cause insulin resistance but are a result of insulin resistance.