Prdm16 is a key transcriptional regulator of the cellular lineage that gives rise to the classic brown adipose depots in mice. We report here that Prdm16 was expressed at substantial levels in subcutaneous WAT, where it regulated a functional thermogenic gene program. Thus, the Prdm16 pathway is a conserved mechanism that controls brown adipocyte development and thermogenic gene expression in 2 separate cell lineages.
The cellular precursor cell types for brown fat–like adipocytes in WAT are unknown. Notably, Prdm16 was highly expressed by mature adipocytes in subcutaneous WAT. Moreover, Prdm16 drove brown fat–like cell development in WAT when ectopically expressed in adipocytes from a differentiation-selective promoter. Finally, Prdm16 was cell-autonomously required in mature fat cells for induction of a brown fat–selective gene program. Together, these results suggest that mature adipocytes can be stimulated to express a brown fat–like phenotype, at least in ingWAT. Specific markers for white adipocytes are needed to answer whether brown fat–like cells can arise by transdifferentiation. This does not exclude the existence of alternative or additional progenitor cell types for the brown fat–like cells in WAT, including fibroblasts or preadipocytes, as has been proposed previously by us and many others. Additional marker genes and fate-mapping experiments are needed to resolve this issue.
Transgenic expression of Prdm16 in fat tissues stimulated the development of Ucp1-expressing, brown-like fat cells in ingWAT, but not epidWAT. Notably, there was a significant increase in the levels of thermogenic genes in the absence of morphological brown-like adipose development in the epidWAT of transgenic mice, although the absolute level of thermogenic gene expression was far lower in the epididymal than in the inguinal depot. Moreover, haploinsufficiency of Prdm16 caused a pronounced reduction in the expression of brown fat–specific genes in WAT, particularly in the subcutaneous depot, after β3-adrenergic stimulation. Thus, the extent of thermogenic gene induction in WAT is correlated with Prdm16 levels. The higher density of sympathetic nerve endings in the inguinal relative to the epididymal depot (61
) presumably amplifies the browning effects of ectopic Prdm16 expression. Consistent with this, brown adipocytes can be induced in the epididymal depots of transgenic animals in response to a strong pharmacological β3-adrenergic stimulus (53
). Although ectopic Prdm16
mRNA was equally increased in ingWAT and epidWAT of transgenic mice, the protein preferentially accumulated in the ingWAT depot. It is likely that epidWAT expresses a factor that degrades Prdm16, or affects its translation, to limit conversion of this tissue into brown-like fat. Based on its relatively large mass and increased expression of a brown fat–like gene program, it is likely that subcutaneous depots account for a substantial portion of the thermogenic output from WAT. However, we do not exclude a role for visceral depots in the thermogenic response to cold or high-fat diet, especially the depots that have propensity to develop brown-like adipocytes, like the mesenteric WAT (30
). The relative contribution of WAT to total Ucp1-mediated energy dissipation, much of which likely occurs in classic BAT depots, remains to be determined.
Prdm16 induces the formation of brown fat–like cells in WAT under basal conditions, but the thermogenic function of the transgenic WAT was only revealed when another stimulus was given — in this case, a high-fat diet. The ability of overfeeding to elicit sympathetic outflow to brown fat and heart is well known (62
). Therefore, we speculate that Prdm16 acts as a determining factor in subcutaneous adipocytes to establish a gene program under basal conditions that can allow for activation of thermogenesis in response to cold or overfeeding. Although these stimuli did not increase Prdm16 levels, further studies are needed to assess whether catecholamines can directly or indirectly enhance Prdm16 activity. Importantly, a program of adaptive thermogenesis in WAT of Prdm16 transgenic animals suppressed obesity caused by a high-fat diet. These mice also displayed a dramatic enhancement in the clearance of blood glucose relative to WT mice after intraperitoneal glucose infusion. This effect reflects enhanced insulin action, much of which is likely secondary to the reduced obesity and fat mass in transgenic mice, but it may also involve non–insulin-dependent glucose disposal into the transgenic WAT. Additional metabolic analyses of these mice are needed to answer this question.
In classic brown fat depots, thermogenesis is tightly regulated by catecholamines secreted by sympathetic nerve endings in BAT. Moreover, the density of brown-like adipocytes in WAT depots is positively correlated with the number of sympathetic nerve fibers (61
). The clusters of brown fat–like cells that appear in Prdm16 transgenic WAT may develop at and/or near sympathetic nerve terminals. If so, it may be that all transgenic adipocytes are competent to induce a brown fat program, but require an additional sympathetic stimulus. Alternatively, distinct subtypes of subcutaneous adipocytes exist, with different propensities to acquire brown fat–like features in response to Prdm16 expression. Notably, we observed a dramatic increase in the number of sympathetic parenchymal nerve fibers infiltrating the ingWAT of aP2-Prdm16
transgenic mice compared with WT animals (Figure ). How Prdm16 acts to stimulate the recruitment of sympathetic nerves to WAT is unknown, but of considerable interest. The increase in Prdm16 expression in epidWAT of transgenic mice may have been insufficient to stimulate nerve recruitment. Alternatively, other factors unique to subcutaneous depots may act in parallel with Prdm16 to fully stimulate brown fat cell development. Whatever the mechanism, the enrichment of sympathetic nerves in the vicinity of brown fat–like cells in WAT ensures that the tissue can efficiently undergo thermogenesis in response to the activity of the sympathetic nervous system. The sympathetic nervous system undoubtedly provides a major control mechanism for the thermogenesis of subcutaneous WAT in vivo. However, cultured subcutaneous adipocytes from WT mice expressed a differentiation-linked brown fat–like gene program ex vivo; this gene profile was intrinsic to subcutaneous adipocytes and thus not obligatorily linked to the sympathetic nervous system. Importantly, Prdm16 was cell-autonomously required for the expression of this thermogenic gene program in subcutaneous adipocytes.
It will now be important to carefully examine whether subcutaneous human WAT not chronically stimulated by cold exposure also expresses Prdm16 at reasonable levels. Interestingly, biopsies of adult human BAT contain an admixture of unilocular and multilocular adipocytes that express enriched amounts of Prdm16
); the morphological appearance of this tissue is reminiscent of subcutaneous adipose (beige fat) from cold-exposed or β3 agonist–treated rodents. Therefore, a key question is whether human BAT is analogous to the classic BAT or adaptive BAT in rodents.
It is still debated whether the amount of activated BAT in humans is sufficient to impact energy balance in a meaningful way. On the other hand, subcutaneous WAT is very abundant in humans. A recent report suggests that thermogenic, Ucp1-expressing beige fat cells in human subcutaneous fat correlates with insulin sensitivity (70
). If subcutaneous human WAT expresses Prdm16, this tissue could be a promising target for stimulating energy expenditure pathways to counteract obesity and insulin resistance.