Exploring the metabolic role of GPR30 in mice

Abstract

Recent studies showed that GPR30, a seven-transmembrane G protein-coupled receptor, is a novel estrogen receptor (ER) that mediates some biological events elicited by estrogen in several types of cancer cells. However, its physiological or pathological role in vivo is unclear. For the first project of my dissertation, I investigated the physiological role(s) of GPR30 in energy metabolism by using transgenic mouse model as well as immortalized cell lines and primary stromal cells. We discovered for the first time that GPR30 knockout (GPRKO) female mice were protected from high-fat diet (HFD)-induced obesity, glucose intolerance, and insulin resistance. The decreased body weight gain in GPRKO female mice is due to the reduction in body fat mass. These effects occurred in the absence of significant changes in food intake, intestinal fat absorption, or triglyceride metabolism. However, GPR30 had no significant metabolic effects in male mice fed the HFD and both sexes of mice fed a chow diet. Further, GPR30 expression levels in fat tissues of WT obese female mice greatly increased, whereas ERα/β expression was not altered. Deletion of GPR30 reduced adipogenic differentiation of adipose tissue-derived stromal cells. Conversely, activation of GPR30 enhanced adipogenic differentiation of 3T3-L1 preadipocytes. For the second project, I explored whether estrogen acts through GPR30 to affect adiposity in female mice. For this study, I generated and examined three independent transgenic mouse models, aromatase (Ar) knockout (ArKO) mice, GPRKO, and GPR30 and Ar double knockout (DKO) mice. We discovered that GPR30 deficiency had limited effects on energy metabolism in mice fed a standard chow diet (STD). However, deletion of GPR30 promoted metabolic flexibility in both genders fed a HFD regardless of the presence of estrogen, suggesting that GPR30 may not solely act as an ER. Consistent with our previous findings, GPRKO mice had higher body temperature, indicating that GPR30 deficiency may promote thermogenesis and energy metabolism, resulting in the reduced fat depots and enhanced metabolic flexibility. For the third project, I further explored whether GPR30 is involved in regulating browning of adipose tissue and thermogenesis in mice. The results show that the expression of UCP-1, the key regulator of thermogenic browning, was higher in the adipose tissue of HFD-fed GPRKO female mice as compared with that of WT mice. Consistently, deletion of GPR30 enhanced mitochondrial respiration in brown adipose tissue (BAT), suggesting that GPR30 deficiency at least partially suppressed the fat accumulation by promoting thermogenesis and dissipating energy. Ex vivo, the expression of thermogenic genes and UCP-1 protein level were upregulated in beige adipocytes differentiated from GPR30-deficient stromal vascular fraction (SVF) cells. These findings provide evidence for the first time that deletion of GPR30 reduces adiposity, promotes white adipose beigeing and thermogenesis, therefore preventing the development of obesity in female mice exposed to excess energy. Further investigations elucidating the underlying mechanism by which GPR30 promotes obesity in females could provide a novel therapeutic target to fight obesity in females.