The gene encoding for transcription factor 7-like 2 () is the strongest type 2 diabetes mellitus (T2DM) candidate gene discovered to date. The TCF7L2 protein is a key transcriptional effector of the Wnt/β-catenin signaling pathway, which is an important developmental pathway that negatively regulates adipogenesis. However, the precise role that TCF7L2 plays in the development and function of adipocytes remains largely unknown. Using a combination of in vitro approaches, we first show that TCF7L2 protein is increased during adipogenesis in 3T3-L1 cells and primary adipocyte stem cells and that TCF7L2 expression is required for the regulation of Wnt signaling during adipogenesis. Inactivation of TCF7L2 protein by removing the high-mobility group (HMG)-box DNA binding domain in mature adipocytes in vivo leads to whole-body glucose intolerance and hepatic insulin resistance. This phenotype is associated with increased subcutaneous adipose tissue mass, adipocyte hypertrophy, and inflammation. Finally, we demonstrate that mRNA expression is downregulated in humans with impaired glucose tolerance and adipocyte insulin resistance, highlighting the translational potential of these findings. In summary, our data indicate that TCF7L2 has key roles in adipose tissue development and function that may reveal, at least in part, how TCF7L2 contributes to the pathophysiology of T2DM.

Dynamic adjustment of insulin secretion to compensate for changes of insulin sensitivity that result from alteration of nutritional or metabolic status is a fundamental aspect of glucose homeostasis. To investigate the role of the brain in this coupling process, we used cold exposure as an experimental paradigm because the sympathetic nervous system (SNS) helps to coordinate the major shifts of tissue glucose utilization needed to ensure that increased thermogenic needs are met. We found that glucose-induced insulin secretion declined by 50% in rats housed at 5°C for 28 h, and yet, glucose tolerance did not change, owing to a doubling of insulin sensitivity. These potent effects on insulin secretion and sensitivity were fully reversed by returning animals to room temperature (22°C) for 4 h or by intravenous infusion of the α-adrenergic receptor antagonist phentolamine for only 30 min. By comparison, insulin clearance was not affected by cold exposure or phentolamine infusion. These findings offer direct evidence of a key role for the brain, acting via the SNS, in the rapid, highly coordinated, and reciprocal changes of insulin secretion and insulin sensitivity that preserve glucose homeostasis in the setting of cold exposure.

In the liver Wnt-signaling contributes to the metabolic fate of hepatocytes, but the precise role of the TCF7L2 in this process is unknown. We employed a temporal RNA-Seq approach to examine gene expression 3–96 h following Tcf7l2 silencing in rat hepatoma cells, and combined this with ChIP-Seq to investigate mechanisms of target gene regulation by TCF7L2. Silencing Tcf7l2 led to a time-dependent appearance of 406 differentially expressed genes (DEGs), including key regulators of cellular growth and differentiation, and amino acid, lipid and glucose metabolism. Direct regulation of 149 DEGs was suggested by strong proximal TCF7L2 binding (peak proximity score > 10) and early mRNA expression changes (≤18 h). Indirect gene regulation by TCF7L2 likely occurred via alternate transcription factors, including Hnf4a, Foxo1, Cited2, Myc and Lef1, which were differentially expressed following Tcf7l2 knock-down. Tcf7l2-silencing enhanced the expression and chromatin occupancy of HNF4α, and co-siRNA experiments revealed that HNF4α was required for the regulation of a subset of metabolic genes by TCF7L2, particularly those involved in lipid and amino-acid metabolism. Our findings suggest TCF7L2 is an important regulator of the hepatic phenotype, and highlight novel mechanisms of gene regulation by TCF7L2 that involve interplay between multiple hepatic transcriptional pathways.

We recently reported that in rodent models of type 2 diabetes (T2D), a single intracerebroventricular (icv) injection of fibroblast growth factor 1 (FGF1) induces remission of hyperglycemia that is sustained for weeks. To clarify the peripheral mechanisms underlying this effect, we used the Zucker diabetic fatty fa/fa rat model of T2D, which, like human T2D, is characterized by progressive deterioration of pancreatic b-cell function after hyperglycemia onset. We report that although icv FGF1 injection delays the onset of b-cell dysfunction in these animals, it has no effect on either glucose-induced insulin secretion or insulin sensitivity. These observations suggest that FGF1 acts in the brain to stimulate insulinindependent glucose clearance. On the basis of our finding that icv FGF1 treatment increases hepatic glucokinase gene expression, we considered the possibility that increased hepatic glucose uptake (HGU) contributes to the insulin-independent glucose-lowering effect of icv FGF1. Consistent with this possibility, we report that icv FGF1 injection increases liver glucokinase activity by approximately twofold. We conclude that sustained remission of hyperglycemia induced by the central action of FGF1 involves both preservation of b-cell function and stimulation of HGU through increased hepatic glucokinase activity.Unlike other therapies for type 2 diabetes (T2D), remission of hyperglycemia lasting for weeks or longer can be induced by a single intracerebroventricular (icv) injection of fibroblast growth factor 1 (FGF1) (1). Although this prolonged antidiabetic effect of icv FGF1 was observed in both ob/ob mice and Zucker diabetic fatty fa/fa (ZDF) rats, it lasted longer (.4 months) in the former than in the latter animals (;4 weeks), potentially owing to the progressive deterioration of b-cell function characteristic of ZDF rats after hyperglycemia onset (2). Because this deterioration does not occur in ob/ob mice (on the C57BL/6 background), the ZDF rat model more closely approximates human T2D, which is also characterized by a progressive loss of b-cell function and mass over time (3).We therefore selected the ZDF model for the current studies, which were undertaken to identify peripheral mechanisms responsible for sustained glucose lowering induced by the central action of FGF1. Our results suggest that in these animals, icv FGF1 injection induces remission of hyperglycemia both by delaying the onset of b-cell failure and by increasing hepatic glucose uptake (HGU) through increased hepatic glucokinase (GCK) activity.

Hajnal A, Acharya NK, Grigson PS, Covasa M, Twining RC. Obese OLETF rats exhibit increased operant performance for palatable sucrose solutions and differential sensitivity to D2 receptor antagonism. Am J Physiol Regul Integr Comp Physiol 293: R1846-R1854, 2007. First published September 5, 2007; doi:10.1152/ajpregu.00461.2007.-CCK-1-receptor-deficient Otsuka Long-Evans Tokushima fatty (OLETF) rats are hyperphagic and exhibit a greater preference for sucrose compared with lean controls [Long-Evans Tokushima Otsuka (LETO)]. To directly assess motivation to work for sucrose reward in this model of obesity and type 2 diabetes, we examined the operant performance of OLETF rats at nondiabetic and prediabetic stages (14 and 24 wk of age, respectively) on fixed-ratio (FR) and progressiveratio (PR) schedules of reinforcement. To evaluate the involvement of dopamine systems, the effects of the D1 receptor antagonist SCH23390 (100 and 200 nmol/kg ip) and the D2 receptor antagonist raclopride (200 and 400 nmol/kg ip), were also tested on PR responding for sucrose. Compared with age-matched LETO rats, 14-wk-old OLETF rats emitted more licks on the "active" empty spout operant on the FR-10 schedule of reinforcement to obtain 0.01 M and 0.3 M sucrose and completed higher ratio requirements on the PR schedule to gain access to 0.3 M and 1.0 M sucrose. At 24 wk, this effect was limited to 1.0 M sucrose. Both antagonists were potent in reducing operant responding to 0.3 M sucrose in both strains at both ages, and there was no strain effect to SCH23390 at either age. OLETF rats, on the other hand, showed an increased sensitivity to the higher dose of raclopride, resulting in reduced responding to sucrose reinforcement at 24 wk. Taken together, these findings provide the first direct evidence for an increased motivation for sucrose reward in the OLETF rats and suggest altered D2 receptor regulation with the progression of obesity and prediabetes. dietary obesity; food reward; appetite; palatability; operant lick task THE EPIDEMIC OF OBESITY AND its associated health consequences represent a major cause of preventable morbidity and mortality in the United States and worldwide (31). Although the etiology of obesity is complex, the motivation to respond to palatable food correlates with obesity (3, 52, 60). Despite this relationship, little is known about food reward functions in the obese. The common view is that obese individuals are presumed to show enhanced liking for palatable foods. Several human studies investigating the relationship between preference and obesity support this notion, whereas others debate it (3,19,20,38).Due to the complexity of human eating behavior there has been growing interest in using animal models to decipher basic neural processes underlying the development of obesity. Our laboratory has been investigating the Otsuka Long-Evans Tokushima fatty (OLETF) rats that lack the CCK-1 receptor, are hyperphagic, obese, and gradually develop non-insulin-dependent diabetes mellitus (36). Unlike in other geneticall...