Differences in hormone-sensitive lipase expression in white adipose tissue from various anatomic locations of the rat

Regional differences in lipolytic activity of isolated fat cells have been observed in rats, with internal fat depots displaying greater rates of lipolysis than subcutaneous fat. These differences in lipolysis have been attributed to a variety of mechanisms, including regional differences in adrenergic receptors, in adenosine sensitivity, and in regional blood flow. In the present study, we have explored whether differences in hormone-sensitive lipase (HSL), the rate-limiting enzyme in lipolysis, might contribute to regional differences in lipolysis in the rat. Adipocytes were isolated from epididymal, retroperitoneal, and dorsal-subcutaneous fat depots, and HSL activity, HSL immunoreactive protein, the rate of HSL synthesis, and the steady-state levels of HSL mRNA were assessed. HSL activity was similar in epididymal and retroperitoneal adipocytes, but was twofold to 2.5-fold greater in these cells than in subcutaneous adipocytes (P < .01). The amount of immunoreactive HSL protein was also similar in epididymal and retroperitoneal adipocytes, but was twofold greater in these cells than in subcutaneous adipocytes (P < .001). The rate of incorporation of 35S-methionine into immunoprecipitable HSL was approximately 2.5-fold greater in epididymal than in subcutaneous adipocytes (P < .01). Finally, HSL mRNA levels were similar in epididymal and retroperitoneal fat depots, and were approximately 1.4-fold greater in these cells than in subcutaneous adipocytes (P < .001). These results demonstrate that site-related differences in the rate of basal lipolysis among various fat depots in the rat are in part due to variations in the expression of HSL.