Browning of White Adipose Tissue Uncouples Glucose Uptake from Insulin Signaling

<div><p>Presence of thermogenically active adipose tissue in adult humans has been inversely associated with obesity and type 2 diabetes. While it had been shown that insulin is crucial for the development of classical brown fat, its role in development and function of inducible brown-in-white (brite) adipose tissue is less clear. Here we show that insulin deficiency impaired differentiation of brite adipocytes. However, adrenergic stimulation almost fully induced the thermogenic program under these settings. Although brite differentiation of adipocytes as well as browning of white adipose tissue entailed substantially elevated glucose uptake by adipose tissue, the capacity of insulin to stimulate glucose uptake surprisingly was not higher in the brite state. Notably, in line with the insulin-independent stimulation of glucose uptake, our data revealed that brite recruitment results in induction of solute carrier family 2 (GLUT-1) expression in adipocytes and inguinal WAT. These results for the first time demonstrate that insulin signaling is neither essential for brite recruitment, nor is it improved in cells or tissues upon browning.</p></div

Lack of insulin impairs differentiation but not browning capacity of primary pre-adipocytes.

<p>(<b>A</b>) Heatmap showing differential mRNA expression between confluent primary inguinal white adipose tissue (iWAT) precursor cells differentiated for 24 h with white (EtOH treated) or brite (cPGI₂ treated) differentiation cocktail and between absence or presence of insulin (Ins) in the medium. Higher and lower expression is displayed in red and blue, respectively. (n = 3). (<b>B</b>) mRNA expression of UCP-1 and CIDEA or (<b>C</b>) FABP4 and RETN in primary iWAT precursor cells differentiated into white (EtOH treated) or brite (cPGI₂ treated) adipocytes for 8 days with insulin present in the differentiation medium for the indicated timepoints (n = 3). All values in bar graphs are expressed as means ± SEM, #p<0.05, ##p<0.01, ###p<0.001 white (EtOH treated) vs. brite (cPGI₂ treated) cells, *p<0.05, **p<0.01, ***p<0.001 normal conditions vs. insulin deprived conditions.</p

Brite adipose cells and tissues exhibit elevated glucose uptake independent of insulin stimulation, thereby enhancing glucose clearance from blood.

<p>(<b>A, B</b>) ³H-2-deoxy-D-glucose (3H-2DOG) uptake by primary inguinal white adipose tissue (iWAT) precursor cells (<b>A</b>) absolute or (<b>B</b>) relative to unstimulated basal uptake. Cells were differentiated into white (EtOH treated) or brite (cPGI₂ treated) adipocytes for 8 days and stimulated with different doses of Insulin for 20 minutes. (<b>C, D</b>) Intraperitoneal insulin (Ins) tolerance test (0.5 U/kg body weight insulin) of mice treated with CL316,243 (CL, 1 µg/g/day) or NaCl via s.c. implanted osmotic pumps for 10 days. (<b>C</b>) Absolute blood glucose levels and (<b>D</b>) levels relative to non-insulin-stimulated are shown. (<b>E, F</b>) 3H-2DOG uptake rate into inguinal or abdominal white (iWAT, aWAT) or brown (BAT) adipose tissue and heart of the same mice as in B, C. (<b>E</b>) Absolute uptake and (<b>F</b>) uptake relative to non-insulin-stimulated conditions is shown. Uptake rates were measured 45 minutes after intraperitoneal injection of insulin or vehicle. All values are expressed as means ± SEM, n = 3–6, #p<0.05, ##p<0.01, ###p<0.001 white vs. brite, *p<0.05, **p<0.01, ***p<0.001 no insulin vs. insulin stimulated.</p