Brown adipocytes are novel sites of expression and regulation of adiponectin and resistin

Adiponectin and resistin, two recently identified adipocyte-specific secretory factors, are able to modulate insulin actions in target tissues. To investigate their expression and hormonal regulation in brown adipocytes, we used the brown adipocyte cell line T37i, which, beside uncoupling protein expression, secretes leptin. Adiponectin and resistin mRNA were detected as a function of cell differentiation. Both transcripts were expressed at relatively high levels in differentiated T37i cells, reaching maximal levels on day 7, while resistin expression drastically fell afterwards. These stable transcripts (t(1/2)>8 h) were differentially regulated by factors involved in insulin responsiveness. Insulin and thiazolidinedione, a peroxisome proliferator-activated receptor gamma agonist, stimulated resistin expression two- to four-fold in differentiated T37i cells, whereas adiponectin mRNA levels increased 1.5-2-fold. In contrast, dexamethasone and isoproterenol reduced by two-fold the level of adiponectin and resistin transcripts in differentiated T37i cells. This study provides the first direct evidence that differentiated brown adipocytes are endocrine cells capable of expressing adiponectin and resistin. The complex hormonal regulation of their expression in brown adipocytes clearly differs from that reported in white adipose tissue, pointing to differential physiological and pathophysiological implications of brown fat in energy homeostasis.