WuQiang Fan

GPR120 Is an Omega-3 Fatty Acid Receptor Mediating Potent Anti-inflammatory and Insulin-Sensitizing Effects

Omega-3 fatty acids (omega-3 FAs), DHA and EPA, exert anti-inflammatory effects, but the mechanisms are poorly understood. Here, we show that the G protein-coupled receptor 120 (GPR120) functions as an omega-3 FA receptor/sensor. Stimulation of GPR120 with omega-3 FAs or a chemical agonist causes broad anti-inflammatory effects in monocytic RAW 264.7 cells and in primary intraperitoneal macrophages. All of these effects are abrogated by GPR120 knockdown. Since chronic macrophage-mediated tissue inflammation is a key mechanism for insulin resistance in obesity, we fed obese WT and GPR120 knockout mice a high-fat diet with or without omega-3 FA supplementation. The omega-3 FA treatment inhibited inflammation and enhanced systemic insulin sensitivity in WT mice, but was without effect in GPR120 knockout mice. In conclusion, GPR120 is a functional omega-3 FA receptor/sensor and mediates potent insulin sensitizing and antidiabetic effects in vivo by repressing macrophage-induced tissue inflammation.

Brain PPAR-γ promotes obesity and is required for the insulin–sensitizing effect of thiazolidinediones

In adipose tissue, muscle, liver and macrophages, signaling by the nuclear receptor peroxisome proliferator–activated receptor-γ (PPAR-γ) is a determinant of insulin sensitivity and this receptor mediates the insulin–sensitizing effects of thiazolidinediones (TZDs). As PPAR-γ is also expressed in neurons, we generated mice with neuron-specific Pparg knockout (Pparg brain knockout (BKO)) to determine whether neuronal PPAR-γ signaling contributes to either weight gain or insulin sensitivity. During high-fat diet (HFD) feeding, food intake was reduced and energy expenditure increased in Pparg-BKO mice compared to Ppargf/f mice, resulting in reduced weight gain. Pparg-BKO mice also responded better to leptin administration than Ppargf/f mice. When treated with the TZD rosiglitazone, Pparg-BKO mice were resistant to rosiglitazone-induced hyperphagia and weight gain and, relative to rosiglitazone-treated Ppargf/f mice, experienced only a marginal improvement in glucose metabolism. Hyperinsulinemic euglycemic clamp studies showed that the increase in hepatic insulin sensitivity induced by rosiglitazone treatment during HFD feeding was completely abolished in Pparg-BKO mice, an effect associated with the failure of rosiglitazone to improve liver insulin receptor signal transduction. We conclude that excess weight gain induced by HFD feeding depends in part on the effect of neuronal PPAR-γ signaling to limit thermogenesis and increase food intake. Neuronal PPAR-γ signaling is also required for the hepatic insulin sensitizing effects of TZDs.

Adipocyte NCoR Knockout Decreases PPARγ Phosphorylation and Enhances PPARγ Activity and Insulin Sensitivity

Insulin resistance, tissue inflammation, and adipose tissue dysfunction are features of obesity and Type 2 diabetes. We generated adipocyte-specific Nuclear Receptor Corepressor (NCoR) knockout (AKO) mice to investigate the function of NCoR in adipocyte biology, glucose and insulin homeostasis. Despite increased obesity, glucose tolerance was improved in AKO mice, and clamp studies demonstrated enhanced insulin sensitivity in liver, muscle, and fat. Adipose tissue macrophage infiltration and inflammation were also decreased. PPARγ response genes were upregulated in adipose tissue from AKO mice and CDK5-mediated PPARγ ser-273 phosphorylation was reduced, creating a constitutively active PPARγ state. This identifies NCoR as an adaptor protein that enhances the ability of CDK5 to associate with and phosphorylate PPARγ. The dominant function of adipocyte NCoR is to transrepress PPARγ and promote PPARγ ser-273 phosphorylation, such that NCoR deletion leads to adipogenesis, reduced inflammation, and enhanced systemic insulin sensitivity, phenocopying the TZD-treated state.

Insulin-like Growth Factor 1/Insulin Signaling Activates Androgen Signaling through Direct Interactions of Foxo1 with Androgen Receptor

The androgen-androgen receptor (AR) system plays vital roles in a wide array of biological processes, including prostate cancer development and progression. Several growth factors, such as insulin-like growth factor 1 (IGF1), can induce AR activation, whereas insulin resistance and hyperinsulinemia are correlated with an elevated incidence of prostate cancer. Here we report that Foxo1, a downstream molecule that becomes phosphorylated and inactivated by phosphatidylinositol 3-kinase/Akt kinase in response to IGF1 or insulin, suppresses ligand-mediated AR transactivation. Foxo1 reduces androgen-induced AR target gene expressions and suppresses the in vitro growth of prostate cancer cells. These inhibitory effects of Foxo1 are attenuated by IGF1 but are enhanced when it is rendered Akt-nonphosphorylatable. Foxo1 interacts directly with the C terminus of AR in a ligand-dependent manner and disrupts ligand-induced AR subnuclear compartmentalization. Foxo1 is recruited by liganded AR to the chromatin of AR target gene promoters, where it interferes with AR-DNA interactions. IGF1 or insulin abolish the Foxo1 occupancy of these promoters. Of interest, a positive feedback circuit working locally in an autocrine/intracrine manner may exist, because liganded AR up-regulates IGF1 receptor expression in prostate cancer cells, presumably resulting in higher IGF1 signaling tension and further enhancing the functions of the receptor itself. Thus, Foxo1 is a novel corepressor for AR, and IGF1/insulin signaling may confer stimulatory effects on AR by attenuating Foxo1 inhibition. These results highlight the potential involvement of metabolic syndrome and hyperinsulinemia in prostate diseases and further suggest that intervention of IGF1/insulin-phosphatidylinositol 3-kinase-Akt signaling may be of clinical value for prostate diseases.

FoxO1 regulates Tlr4 inflammatory pathway signalling in macrophages

The macrophage‐mediated inflammatory response is a key etiologic component of obesity‐related tissue inflammation and insulin resistance. The transcriptional factor FoxO1 is a key regulator of cell metabolism, cell cycle and cell death. Its activity is tightly regulated by the phosphoinositide‐3‐kinase‐AKT (PI3K‐Akt) pathway, which leads to phosphorylation, cytoplasmic retention and inactivation of FoxO1. Here, we show that FoxO1 promotes inflammation by enhancing Tlr4‐mediated signalling in mature macrophages. By means of chromatin immunoprecipitation (ChIP) combined with massively parallel sequencing (ChIP‐Seq), we show that FoxO1 binds to multiple enhancer‐like elements within the Tlr4 gene itself, as well as to sites in a number of Tlr4 signalling pathway genes. While FoxO1 potentiates Tlr4 signalling, activation of the latter induces AKT and subsequently inactivates FoxO1, establishing a self‐limiting mechanism of inflammation. Given the central role of macrophage Tlr4 in transducing extrinsic proinflammatory signals, the novel functions for FoxO1 in macrophages as a transcriptional regulator of the Tlr4 gene and its inflammatory pathway, highlights FoxO1 as a key molecular adaptor integrating inflammatory responses in the context of obesity and insulin resistance.

FOXO1 Transrepresses Peroxisome Proliferator-activated Receptor γ Transactivation, Coordinating an Insulin-induced Feed-forward Response in Adipocytes

The transcriptional factor FoxO1 plays an important role in metabolic homeostasis. Herein we identify a novel transrepressional function that converts FoxO1 from an activator of transcription to a promoter-specific repressor of peroxisome proliferator-activated receptor γ (PPARγ) target genes that regulate adipocyte biology. FoxO1 transrepresses PPARγ via direct protein-protein interactions; it is recruited to PPAR response elements (PPRE) on PPARγ target genes by PPARγ bound to PPRE and interferes with promoter DNA occupancy of the receptor. The FoxO1 transrepressional function, which is independent and dissectible from the transactivational effects, does not require a functional FoxO1 DNA binding domain, but dose require an evolutionally conserved 31 amino acids LXXLL-containing domain. Insulin induces FoxO1 phosphorylation and nuclear exportation, which prevents FoxO1-PPARγ interactions and rescues transrepression. Adipocytes from insulin resistant mice show reduced phosphorylation and increased nuclear accumulation of FoxO1, which is coupled to lowered expression of endogenous PPARγ target genes. Thus the innate FoxO1 transrepression function enables insulin to augment PPARγ activity, which in turn leads to insulin sensitization, and this feed-forward cycle represents positive reinforcing connections between insulin and PPARγ signaling.

Androgens and metabolic syndrome: Lessons from androgen receptor knock out (ARKO) mice

Testosterone (T) is an important factor for determining body composition in males. Abdominal obesity is inversely correlated with serum T levels in men, leading to greater mortality. Pathologically hypogonadal men also have a significantly higher fat mass, which is reversed by T administration. However, the mechanism for such anti-obesity effect of androgen has not been well clarified. Androgen receptor (AR) null male mice revealed late-onset obesity. Male ARKO mice were euphagic compared to the wild-type male controls, but also less dynamic and less oxygen consuming. Transcript profiling indicated that male ARKO mice had lower transcripts for the thermogenetic uncoupling protein 1 (UCP1). We also found enhanced secretion of adiponectin, which is insulin-sensitizing, from adipose tissue in comparison to wild type, which might partly explain why the overall insulin sensitivity of male ARKO mice remained almost intact despite their apparent obesity. In addition, decreased lipolysis rather than increased lipid synthesis was observed, which might also account for the increased adiposity in male ARKO mice. The results revealed that AR plays important roles in male metabolism by affecting the energy balance, and is negative to both adiposity and insulin sensitivity.

Glucocorticoids and Thiazolidinediones Interfere with Adipocyte-mediated Macrophage Chemotaxis and Recruitment

The link between intra-abdominal adiposity and type II diabetes has been known for decades, and adipose tissue macrophage (ATM)-associated inflammation has recently been linked to insulin resistance. However, the mechanisms associated with ATM recruitment remain ill defined. Herein, we describe in vitro chemotaxis studies, in which adipocyte conditioned medium was used to stimulate macrophage migration. We demonstrate that tumor necrosis factor α and free fatty acids, key inflammatory stimuli involved in obesity-associated autocrine/paracrine inflammatory signaling, stimulate adipocyte expression and secretion of macrophage chemoattractants. Pharmacological studies showed that peroxisome proliferator-activated receptor γ agonists and glucocorticoids potently inhibit adipocyte- induced recruitment of macrophages. This latter effect was mediated by the glucocorticoid receptor, which led to decreased chemokine secretion and expression. In vivo results were quite comparable; treatment of high fat diet-fed mice with dexamethasone prevented ATM accumulation in epididymal fat. This decrease in ATM was most pronounced for the proinflammatory F4/80+, CD11b+, CD11c+ M-1-like ATM subset. Overall, our results elucidate a beneficial function of peroxisome proliferator-activated receptor γ activation and glucocorticoid receptor/glucocorticoids in adipose tissue and indicate that pharmacologic prevention of ATM accumulation could be beneficial.

Free fatty acids induce Lhb mRNA but suppress Fshb mRNA in pituitary LβT2 gonadotropes and diet-induced obesity reduces FSH levels in male mice and disrupts the proestrous LH/FSH surge in female mice.

Female obesity is associated with insulin resistance, hyperandrogenemia, and reproductive dysfunction. We hypothesized that elevated free fatty acids (FFAs) might directly modulate pituitary gonadotropin production. FFAs caused a time- and dose-dependent increase in phosphorylation of the MAPKs p38MAPK, c-Jun N-terminal kinase (JNK)-1/2, and ERK1/2 in LβT2 gonadotrope cells. Furthermore, FFAs up-regulated Lhb mRNA expression acutely, an effect that was blocked by JNK inhibition, but suppressed Fshb mRNA expression, an effect that was independent of MAPK signaling. FFAs enhanced the activation of the MAPKs in the presence of GnRH, although the cotreatment did not alter Lhb induction but did eliminate the GnRH induction of Fshb. FFAs also suppressed activin-induced Fshb expression. Knockdown experiments showed that the FFA effect on the inflammatory kinases p38MAPK and JNK and on Lhb, but not Fshb, mRNA expression is mediated via toll-like receptor-2 and toll-like receptor-4 and was mimicked by lipopolysaccharide stimulation. In vivo, male C57BL/6 mice on a high-fat diet showed reduced FSH levels consistent with the suppression of Fshb seen in vitro. Histological analysis of the testes showed an increased number of abnormal seminiferous tubules. Female mice on a high-fat diet lacked the expected proestrus LH and FSH surge and exhibited an increase in the number of days at estrus and a reduced number of days at proestrus, and ovaries had significantly fewer corpora lutea. Taken together, our findings suggest that lipid excess can lead to reproductive defects in both male and female mice.

Modification of androgen receptor function by IGF-1 signaling implications in the mechanism of refractory prostate carcinoma.

The androgen-androgen receptor (AR) system plays important roles in a variety of biological processes, including prostate cancer (PC) development and progression. Insulin and Insulin-like growth factor-1 (IGF-1) signaling negatively regulate a member of the forkhead box-containing protein O subfamily (FoxO), Foxo-1, and associated biological functions. IGF-1 can potentiate androgen signaling through AR activation. Foxo-1, phosphorylated and inactivated by phosphatidylinositol-3-kinase (PI3K)/Akt kinase induced by IGF-1 or insulin, suppresses ligand-mediated AR transactivation. Foxo-1 reduces expression of androgen-induced AR target genes and suppresses in vitro growth of PC cells. These inhibitory effects of Foxo-1 are attenuated by IGF-1, but enhanced when it was rendered Akt-non-phosphorylatable. Foxo-1 directly interacts with the C-terminus of AR in a ligand-dependent manner, and disrupts ligand-induced AR subnuclear compartmentalization. Foxo-1 is recruited by liganded AR to the chromatin of the AR target gene promoter, while IGF-1 or insulin abolishes the Foxo-1 occupancy on the promoter. Liganded AR stimulates IGF-1 receptor expression, suggesting the presence of local positive feedback between IGF-1 and AR signaling in PC cells, presumably resulting in higher IGF-1 signaling tension and further enhancing the functions of the receptor itself. Thus, Foxo-1 is a novel corepressor for AR and IGF-1/insulin signaling may confer stimulatory effects on AR by attenuating Foxo-1 inhibition. Positive feedback between the growth factor and androgen in the local cellular environment may play important roles in AR transactivation regulation in several clinical situations including refractory PC.