Heat stress enhances adipogenic differentiation of subcutaneous fat depot-derived porcine stromovascular cells

Heat stress (HS) results from excessive heat load on animals such that all adaptive mechanisms used to dissipate the heat do not return the body to normal body temperature. In pigs, HS results in increased fat deposition compared with pair-fed animals in a thermoneutral environment. Although there is evidence that HS increases activity of lipoprotein lipase (LPL) in adipose tissue of heat stressed pigs, the fundamental causes of the increased adiposity are still unknown. It remains unclear whether HS directly alters metabolism in adipocytes. Therefore, to understand the mechanism of HS effects on porcine adipocytes, we used an in vitro adipocyte differentiation model to characterize cellular responses that occur during differentiation of pig adipocytes. Preadipocytes (stromovascular cells) were differentiated for 9 d at a normal (37 degrees C) or HS (41.5 degrees C) temperature under 5% CO2. Expressions of HS genes such as heat shock proteins (HSP; HSP27, HSP60, HSP70, and HSP90), adipogenic markers peroxisome proliferator-activated receptor gamma (PPAR gamma), CCAAT/enhancer binding proteins alpha (C/EBP alpha), fatty acid synthase (FAS), adipocyte protein 2 (aP2), fatty acid translocase 36 (CD36), fatty acid transport protein 4 (FATP4), fatty acid transport protein 6 (FATP6), LPL, glucose transporter protein type 4 (GLUT4), phosphoenolpyruvate carboxykinase 1 (PCK1 or PEPCK-C), and glycerol kinase (GK) and adipokines (adiponectin and leptin) were determined by real-time-PCR and immunoblotting or ELISA. Cellular triglyceride (TAG) and ATP concentrations were also determined. As expected, HS increased (P < 0.05) the expressions of HSP genes. There was no HS treatment effect on the level of PPAR., although C/EBPa was induced (P < 0.05) in HS. So it remains unclear whether HS affects adipocyte differentiation. However, HS leads to increased expressions of genes involved in fatty acid uptake and TAG synthesis (FAS, aP2, CD36, FATP4, FATP6, LPL, GLUT4, PCK1, and GK). This is supported by increased cellular TAG under HS. Therefore, HS promotes increased adipocyte TAG storage, perhaps through upregulation of genes involved in fatty acid uptake and TAG synthesis.