The opposite modulation observed for circulating LPI levels
The opposite modulation observed for circulating LPI levels and WAT GPR55 (S)-Mephenytoin by nutritional status suggests that increased levels of LPI may down-regulate the expression of its receptor in WAT, whereas in conditions of low serum LPI levels, the expression of GPR55 is stimulated. However, during gestation, we failed to find an inverse correlation between WAT GPR55 and serum LPI, likely indicating that other factors are important for the regulation of the LPI/GPR55 system during this period. In this sense it is also intriguing the significant decrease in LPI 16:0 levels at day 12 of gestation with a restoration at the end of this state. The physiological meaning of this finding is unclear but suggests a specific regulation of the different LPI species. It is important to note that even though GPR55 is regulated identically to CB1 in response to nutritional status and pregnancy, circulating levels of LPI and endocannabinoids (Moreno-Navarrete et al., 2012, Bluher et al., 2006) are differentially regulated. Whereas we found decreased serum LPI levels after fasting and a recovery by leptin administration, it has been reported that leptin reduces levels of endocannabinoids in serum and tissues (Di Marzo et al., 2001, Buettner et al., 2008), indicating that LPI and endocannabinoids probably have different actions. Since alterations in the hypothalamic-pituitary axis causes important metabolic alterations, including GH, gonadal and thyroid axis (Pinilla et al., 1999, Zengin et al., 2010), we assessed the influence of gonadal function on CB1 and GPR55. Estrogen deficiency in female animals is associated with hyperphagia, increased body weight and adiposity (Clegg et al., 2006) and serum testosterone levels are inversely associated with whole body and fat mass (Marin et al., 1993). Our data indicate that gWAT CB1 and GPR55 gene expression is regulated by both testosterone and estrogens and support the existence of a feedback mechanism between LPI and its receptor. Although the causes for the different regulation of WAT CB1 and GPR55 after ovariectomy need further clarification, it might be possible that estrogens have a different binding site promoter in the GPR55 and CB1 sequences and therefore differentially modulate both receptors. The existence of an important interrelationship among glucocorticoids and cannabinoids has been shown in many homeostatic processes such as stress (O’Sullivan et al., 2012) and memory (Atsak et al., 2012). In this work we showed the existence of this kind of interaction at least in relation to the regulation of CB1 and GPR55 in adipose tissue by physiological levels of glucocorticoids. Changes in thyroid hormone levels are frequently associated with metabolic shifts that affect adipocyte metabolism (Pontikides and Krassas, 2007). Moreover, an inverse relationship between thyroid hormone levels and sensitivity to cannabinoid agonists has been described (Gine et al., 2013). Our findings suggest that the thyroid status modulates the expression of CB1 and GPR55. GH is also known to exert marked effects on adipose tissue and body composition (Park et al., 2008, Perrini et al., 2008, Davies et al., 2007). Our data suggest that CB1 and GPR55 are differentially modulated by GH levels. Finally, it is important to note that changes in energy balance are associated with other biological responses, for instance, when there is a gain in fat mass there is also a stimulation of angiogenesis and inflammation. Since GPR55 mediates angiogenesis (Zhang et al., 2010) and inflammation (Staton et al., 2008), it might be also possible that these processes indirectly affect WAT GPR55 expression. In summary, our findings are the first demonstrating that: (a) CB1 and GPR55 are similarly regulated in WAT, (b) gWAT GPR55 and CB1 expression are regulated by nutritional status and pregnancy (c) CB1 expression is regulated by testosterone and estrogens and GPR55 is specifically modulated by testosterone only, (d) gWAT CB1 and GPR55 mRNA levels are regulated by changes in the pituitary function, and (e) the endogenous ligand of GPR55, LPI, is inversely modulated by fasting and gonadal hormones. Our findings suggest that the LPI/GPR55 system is correlated to energy balance status and its detailed investigation might be of interest to verify its potential value to become a target to treat obesity. Indeed, the development of novel compounds targeting specifically GPR55 will be needed to clarify this aspect, because current literature (Ross, 2009) suggests that compounds acting through CB1 can also exert part of their biological actions through GPR55 and vice versa. The present study together with a previous study performed in humans (Moreno-Navarrete et al., 2012) demonstrates a clear involvement of GPR55 in states associated with energy imbalance.