CREB1 and Obesity: Collectively, downregulation of Creb1 and Enpp2 and an inverse relationship between serum leptin concentration and the expression of these genes by diet type, suggest that different concentration ranges of circulating leptin would affect the regulation of Creb1 and Enpp2 differently, presumably explaining the relationship between obesity-induced increases in serum leptin and the pathophysiological events in the colon.