Hormonal Regulation of Glycine Decarboxylase and its Impact on Cellular Physiology

Abstract

The amino acid glycine is involved in generation of multiple critical metabolites including glutathione, heme, and creatinine. Interestingly, in both humans and rodents, circulating glycine levels are significantly reduced in obesity, glucose intolerance, type II diabetes and non‐alcoholic fatty liver disease. The glycine cleavage system (GCS) is the predominant glycine degradation pathway in humans. The rate‐limiting enzyme of GCS is glycine decarboxylase (GLDC), and loss‐of‐function mutations of GLDC cause hyperglycinemia. Here, we show that GLDC gene expression is upregulated in livers of mouse models of diabetes and diet‐induced obesity as well as in the fasted state in normal animals. In exploring the hormonal signals that mediate these regulatory events we found that both glucagon and insulin stimulated GLDC gene expression. In primary rat hepatocytes, GLDC expression was strongly stimulated by glucagon and cAMP, and mildly with insulin while in a rat hepatoma cell line, insulin strongly stimulated GLDC expression as compared to cAMP. We identified the cAMP‐response element binding protein 1 (Creb1) and insulin responsive transcription factor Sterol regulatory element binding protein 1c (Srebp‐1c) as mediators of the glucagon and insulin stimulatory effect, respectively. By knocking down or overexpressing GLDC in cell lines we observed GLDC expression levels are strongly linked to intracellular glutathione and reactive oxygen species (ROS) levels. Our findings suggest that the hormonal regulation of GLDC may contribute to compensatory changes in glutathione production as a defense against metabolic disease‐associated oxidative stress.Support or Funding InformationThis work is supported by NIH R21AG050741, Grants Boost OVPR Wayne State University and the Michigan Diabetes Research and Training Center.