Beige differentiation of adipose depots in mice lacking prolactin receptor protects against high fat diet-induced obesity − Rakyat Biologi

While PRL is known as the pituitary hormone of lactation, accumulating evidence shows that it exerts pleiotropic effects including growth and metabolic actions (2,3). Here, we demonstrate that PRLR is an important effector modulating adipocyte fate and differentiation with impact on energy homeostasis and adaptive thermogenesis.

Despite increased food intake in both SC and HFD, PRLR^-/- mice remained leaner than controls and were protected against HFD-induced obesity with a marked reduction in adiposity, notably in perirenal fat mass. Thus, we hypothesized that change in overall energy expenditure may be at least in part responsible for this phenotype. This was supported by increased O₂consumption and enhanced heat dissipation that were significantly higher in PRLR^-/- mice as compared to PRLR^+/+ mice. The relative resistance to HFD-induced obesity was accompanied by a more favorable carbohydrate homeostatic profile in PRLR^-/- mice, consistent with the major implication of PRL signaling in energy balance. However, these changes in whole body homeostasis were likely not sufficient to account for the robust protection against HFD-induced weight gain. Increased energy expenditure through BAT activation constitutes a powerful mechanism by which high calorie intake could be dissipated, thereby defending against obesity (27). Indeed, enhanced conversion of white to thermogenically active adipocytes observed in PRLR^-/- mice seems to be the key regulatory event involved in protection against HFD-induced obesity, consistent with a role of PRL signaling in adipocyte fate determination and/or reprogramming.

In accordance with PRL-mediated transcriptional control of proadipogenic factors (11), PRLR deficiency was associated with a down-regulation of key regulators of adipocyte differentiation, whose expressions were restored to similar extent than in WT mice under HFD. Beyond this HFD-induced rescue observed in PRLR^-/- mice, the most striking finding is the emergence of BAT within white adipose depots. The question of where beige cells originate remains unresolved. It has been proposed that the adaptive UCP1-expressing brown-like adipose cells that develop in WAT in response to cold exposure or beta-adrenergic stimulation can be derived either from a specialized compartment of committed brown precursors, direct differentiation from white preadipocytes, and/or transdifferentiation from mature white adipocytes (28). Regardless of how they arise and the origin of mesenchymatous precursors (often referred to as Myf5^- and Sca1⁺ progenitors) (29,30), the development of beige adipocytes (17) in perirenal fat tissue correlates well with protection against obesity in PRLR^-/- mice. New advances in identification of cellular lineage specification have highlighted several key regulatory factors in specifying brown fat cell fate including PRMD16, a master co-regulator critical for the commitment towards brown adipocyte lineage (31,32). Loss of PRDM16 from brown fat precursors caused a massive reduction in molecular and morphological features of brown adipocytes (33). Conversely, transgenic overexpression of PRDM16 strongly induced the development of brown-like adipocytes in adipose depots (22). Remarkably, under HFD, PRMD16 expression is dramatically induced in the perirenal fat depots as well as in the subcutaneous white fat, associated with a concomitant increase of BAT markers. This is supported by the emergence of UCP1-expressing adipocytes and metabolically active beige cells located in the perirenal and paraspinal areas accounting for the increased metabolic rate in absence of PRLR. Accordingly, PRLR^-/- mice displayed an increased of AdRb3 expression, suggesting that PRL signaling may affect sympathetic nervous system activity and/or nerve fibers infiltration in fat depots. Altogether, these findings demonstrate that PRLR inactivation could lead to the emergence of inducible-brown adipocytes and its associated thermogenesis. Remarkably, this beige reprogramming is genetically determined (markedly in PRLR^-/- mice), induced under HFD and more pronounced in specific fat depots. By contrast, the presence of an active PRL signaling could participate to the white adipocyte phenotype maintenance of these fat depots. One question concerns the signaling molecules involved in acquisition of a brown adipose-like phenotype in perirenal depots. pRb has been suggested to regulate adipocyte differentiation (34). In addition, p107, a member of Rb family, is also implicated in adipose development. P107^-/- mice are refractory to HFD-induced fat accumulation associated with an increase of PGC1a expression (34). Consistently, marked decrease of pRb expression was found in mice deficient in fsp27, a member of the cide family protein regulating adipose tissue differentiation (35). Gonadal and subcutaneous white fat pads of Fsp27^-/- mice are reduced in size compared to wild type mice, together with the acquisition of BAT-like phenotype (35). Collectively, these findings underscore the implication of pRb pathway in adipocyte fate, in accordance with the pRb defect observed in PRLR^-/- fat depots. It was shown that pRb-deficient mouse embryonic fibroblasts exhibited an increased expression of the Forkhead transcription factor Foxc2 that accompanied a white to brown adipocyte transdifferentiation (36). Similarly, overexpression of Foxc2 in adipose tissue leads to a lean and insulin-sensitive phenotype accompanied by an increased BAT-like function due to an enhanced mitochondrial biogenesis through activation of mitochondrial transcription factors (26,37). PRLR^-/- mice under HFD displayed a sharp increase in Foxc2 expression, in agreement with the involvement of pRb/Foxc2 pathway in the “beigeing” conversion.

The functional role of PRL signaling seems to be highly dependent upon the developmental stage. Indeed, we have demonstrated that PRLR is pivotal for early development of BAT to support neonatal thermogenesis (14). In contrast, during adulthood, the absence of PRL signaling is associated with beige conversion, most notably as an adaptive mechanism to facilitate high fat-induced thermogenesis. Thus, PRL signaling plays important but changing roles in energy homeostasis during the lifespan. This is reminiscent of the metabolic action of PRL reported in amphibians (38), fish and birds (1) but far extends the pleiotropic function of PRL in mammals. From a metabolic point of view, PRL is critical during gestation and lactation, favoring anabolic lipid storages indispensable for energy needs during these two major physiological periods. Several lines of evidence suggest that in humans, PRL signaling could be involved in energy homeostasis including the orexigenic property of PRL (39,40), the genetic association between PRL and obesity (41,42) and the relationship between hyperprolactinemia and obesity (43,44). Several recent reports confirmed the major role of BAT in the control of energy balance. Thermogenic UCP1-expressing cells are found interspersed in adult human subcutaneous fat yet their presence negatively correlates with weight gain and insulin resistance (45-49). It would be relevant to examine whether patients with hyperprolactinemia are less prone to develop a functional BAT in response to cold exposure or overfeeding using PET.

In sum, our work demonstrates that mice lacking PRLR are highly resistant to HFD-induced obesity, owing to the emergence of a brown adipose-like phenotype in peculiar white fat depots. This is associated with a concomitant increase of PRDM16, PGC1a, AdRb3 and Foxc2 that constitutes a molecular switching mechanism. This in turn converges towards an activation of thermogenic brown capacity with the final increase of UCP1 responsible for heat dissipation and resistance to high calorie weight gain. We thus propose that PRL signaling represents an additional determinant of energy homeostasis during physiological and pathophysiological conditions.