Characterization of the HCA2 Receptor in a Murine and a Feline Model of Human Obesity

Abstract

Background. There is mounting evidence that the hydroxycarboxyilic acid 2 (HCA2) receptor, which is present on both adipocytes and immune cells, plays an important role in modulating cross-talk between metabolic and immune functions. Previous work from our laboratory demonstrates that in addition to anti-inflammatory effects, niacin, a pharmacologic ligand of HCA2, increases serum concentrations of the anti-inflammatory adipokine, adiponectin. Therefore, it is unclear whether niacin’s anti-inflammatory effects are due to changes in adiponectin, or if they are adiponectin-independent. In addition, our lab has shown that HCA2 receptor expression is decreased in high fat diet (HFD)-fed rodent models of obesity, suggesting a possible mechanism for obesity-associated alterations in immune regulation. Cats provide an important translational model for human obesity. However, little is known about obesity-associated inflammation in cats and the feline HCA2 receptor has not been identified. Objectives. Therefore, the objectives of this study are to 1) investigate the underlying mechanisms in niacin-mediated activation of HCA2 in a HFD-fed mouse model of obesity; 2) characterize the HCA2 receptor in cats; and 3) evaluate obesity-associated inflammation and HCA2 receptor expression in cats during the development of obesity. Methods. To evaluate the relationship between adiponectin and niacin-mediated activation of the HCA2 receptor, 32 male C57BL/6J mice (WT) and 32 adiponectin null mice (AdipoQ-/-) were fed a control (chow) or HFD. After 6 weeks, half of the mice from each group received either vehicle (water) or niacin (360 mg/kg/day) in their drinking water for 5 weeks. To evaluate HCA2 receptor distribution and functionality in cats, tissues and primary adipocytes for culture were collected from lean, healthy, female cats. To investigate inflammation and changes in HCA2 receptor expression in a developmental model of feline obesity, twelve lean, male neutered cats, fed a chow diet, were allowed to gain weight owing to ad libitum feeding for 18 months. Results. In adipose tissue of HFD fed mice, there was increased gene expression of monocyte chemo-attractant protein -1 (MCP-1) and cluster differentiation 68 (CD68), and niacin treatment attenuated the HFD-induced increase in MCP-1, but had no effect on CD68. Similar findings were observed in AdipoQ-/- mice. HFD fed mice had increased gene expression of M1 and M2 macrophage markers. Niacin decreased expression of M2 markers and tended to decrease expression of M1 markers. Interestingly, in HFD-fed AdipoQ-/- mice, niacin significantly increased expression of both M1 and M2 markers. The in silico predicted feline HCA2 protein sequence exhibited 83.1% and 86.5% amino acid similarity to human and mouse sequences respectively. The feline HCA2 receptor is predominantly expressed in adipose tissue and spleen. Exposure of feline adipocytes in primary culture to niacin inhibited lipolysis to a similar degree as insulin. After 18 months of ad libitum feeding, lean cats had ~60% increase in bodyweight, body condition score (BCS) and fat mass, and there was evidence of decreased insulin sensitivity and altered lipid metabolism. Feline obesity was associated with a slight shift in the percent distribution of fat mass with a greater increase in the percent of subcutaneous adipose tissue compared to the percent increase in abdominal adipose tissue. However, there were no changes in systemic markers of inflammation, including inflammatory cytokines and circulating immune cells. Mean adipocyte diameter increased by ~66% in abdominal adipocytes and 79% in subcutaneous adipocytes. Rare adipose tissue macrophages (ATMs) and crown-like structures (CLS) were observed and distribution was independent of time point and location. During the development of feline obesity, HCA2 receptor expression tended to decrease in abdominal adipose tissue, but was unchanged in subcutaneous adipose tissue. Conclusion. In conclusion, niacin does not appear to alter macrophage number, but exerts adiponectin-independent effects on macrophage recruitment. Absence of adiponectin alters niacin’s effects on macrophage polarity, implicating adiponectin-dependent mechanisms. The feline HCA2 receptor is similar to rodent and human HCA2 in sequence, distribution and function, and similar to rodent models of obesity, feline HCA2 expression tends to decrease in abdominal adipose tissue. Cats have a unique immune response during the development of obesity that appears to protect them from certain obesity-associated sequelae such as atherosclerosis, while sustaining other metabolic and immunological pathologies such as type 2 diabetes mellitus (T2DM).