Introduction:
Heart disease and stroke are the primary causes of death in adults with type-2 diabetes thus strict metabolic control is essential in these patients. Here, we show that tristetraprolin (TTP), a protein involved in regulation of inflammation and iron homeostasis via mRNA degradation, may alleviate diabetic phenotype through modulation of key enzymes involved in gluconeogenesis and fatty acid (FA) metabolism in insulin-sensitive tissues.
Results:
TTP protein content was significantly reduced in livers of diabetic mice fed high-fat diet compared to control, suggesting that mRNA targets of TTP may be stabilized in diabetes. We then assessed the effects of TTP knockdown using siRNA in HepG2 liver cells on the expression of enzymes in four major metabolic pathways: glycolysis, gluconeogenesis, FA oxidation and synthesis. Consistent with reduced TTP, we found increased expression of a key gluconeogenic regulator - pyruvate dehydrogenase kinase 4 (PDK4). In silico analysis of the 3’ untranslated region (UTR) of PDK4 revealed five putative TTP binding sites. Importantly, elevated PDK4 levels were previously reported in diabetic patients, and are thought to exacerbate the disease by increasing hepatic glucose output. Assessment of FA metabolism revealed increased levels of PPARα and two of its targets, carnitine palmitoyltransferase I (CPT1) and fatty acid translocase (FAT). Consistent with increased FA flux, triglyceride levels were elevated with TTP siRNA in hepatocytes. In silico analysis revealed multiple well-conserved putative TTP-binding sites in the 3’UTR of PPARα, consistent with regulation by TTP. We also observed significant upregulation of PPARα and its targets in HL-1 cardiac cell line, indicating an overlapping function for TTP in insulin-sensitive tissues. Notably, cardiac-specific overexpression of PPARα was previously shown to cause a phenotype resembling diabetic cardiomyopathy, further suggesting that reduction of TTP in diabetes may exacerbate this disease.
Conclusions:
Our studies show that TTP is downregulated in diabetic mouse livers, and knockdown of this protein in hepatic and cardiac cells increases the levels of key metabolic enzymes, PDK4 and PPARα, both of which are implicated in diabetes and heart disease.
Comparative in silico analysis identifies bona fide MyoD binding sites within the Myocyte Stress 1 gene promoter