Nutritional, hormonal, and depot-dependent regulation of the expression of the small GTPase Rab18 in rodent adipose tissue

Abstract

There is increasing evidence that proteins associated with lipid droplets (LDs) play a key role in the coordination of lipid storage and mobilization in adipocytes. The small GTPase, RAB18, has been recently identified as a novel component of the protein coat of LDs and proposed to play a role in both β-adrenergic stimulation of lipolysis and insulin-induced lipogenesis in 3T3-L1 adipocytes. In order to better understand the role of Rab18 in the regulation of lipid metabolism in adipocytes, we evaluated the effects of age, fat location, metabolic status, and hormonal milieu onRab18expression in rodent white adipose tissue (WAT).Rab18mRNA was undetectable at postnatal day 15 (P15), but reached adult levels by P45, in both male and female rats. In adult rats,Rab18immunolocalized around LDs, as well as within the cytoplasm of mature adipocytes. A weakRab18signal was also detected in the stromal-vascular fraction of WAT. In mice, fasting significantly increased, though with a distinct time–course pattern,Rab18mRNA and protein levels in visceral and subcutaneous WAT. The expression ofRab18was also increased in visceral and subcutaneous WAT of obese mice (diet-induced,ob/ob, and New Zealand obese mice) compared with lean controls.Rab18expression in rats was unaltered by castration, adrenalectomy, or GH deficiency but was increased by hypophysectomy, as well as hypothyroidism. When viewed together, our results suggest the participation ofRab18in the regulation of lipid processing in adipose tissue under both normal and pathological conditions.