Comparison of insulin signaling gene expression in insulin sensitive tissues between cats and dogs

Diabetes mellitus (DM) is a common endocrine disease in cats and dogs with increasing prevalence. Type 1 DM appears to be the most common form of diabetes in dogs whereas Type 2 DM prevails for cats. Since insulin resistance is more frequently encountered in cats than dogs, our laboratory was interested in determining whether differences at the insulin signaling pathway level and differences in glucose and lipid metabolism could be observed between cats and dogs. Insulin resistance has been positively correlated to insulin signaling pathway abnormalities. As such, this study measured insulin receptor substrate-1 (IRS-1), insulin receptor substrate-2 (IRS-2), and phosphatidylinositol 3-kinase (PI3-K) P-85 alpha mRNA expression levels in classical insulin-responsive sensitive tissues (liver, skeletal muscle, and abdominal fat) and peripheral leukocytes between cats and dogs by qRT-PCR. Different tissues were sampled because it is currently unknown where insulin-resistance arises from. In addition, enzymes involved in glucose and lipid metabolism, malate dehydrogenase (MDH), glucose-6-phosphate dehydrogenase (G6PDH) and fatty acid synthase (FAS) were also assessed since glucose and lipid metabolism differs between cats and dogs. Overall, IRS-1, IRS-2, PI3-K, MDH, G6DPH, and FAS mRNA tissue expression profiles demonstrated different levels of expression, in various tissues for both canines and felines, which was expected. No distinct expression pattern emerged; however, differences were noted between canines and felines. In addition, IRS-1, IRS-2, PI3-K, MDH, G6DPH, and FAS mRNA expression was significantly higher in canine versus feline tissues, including peripheral leukocytes. Remarkable differences in insulin signaling gene expression between felines and canines indicate that cats may have an underlying low insulin sensitivity level due to low IRS-1, IRS-2, and PI3-K P-85 alpha mRNA expression levels which would predispose cats to develop insulin resistance. Moreover, differences in glucose and lipid metabolism related gene expression (MDH, G6DPH, and FAS) demonstrate that felines have an overall lower metabolic rate in various tissues which may be attributed to overall lower insulin signaling gene expression and a lack of physical activity as compared to canines. Therefore, a combination of genetic and environmental factors appears to make felines more prone to suffer from insulin resistance and type 2 DM than canines.