Role of bone morphogenetic protein-9 in the regulation of glucose and lipid metabolism.

Bone morphogenetic protein (BMP)-9 has been reported to regulate energy balance in vivo. However, the mechanisms underlying BMP9-mediated regulation of energy balance remain incompletely understood. Here, we investigated the role of BMP9 in energy metabolism. In the current study, we found that hepatic BMP9 expression was down-regulated in insulin resistance (IR) mice and in patients who are diabetic. In mice fed a high-fat diet (HFD), the overexpression of hepatic BMP9 improved glucose tolerance and IR. The expression of gluconeogenic genes was down-regulated, whereas the level of insulin signaling molecule phosphorylation was increased in the livers of Adenovirus-BMP9-treated mice and glucosamine-treated hepatocytes. Furthermore, BMP9 overexpression ameliorated triglyceride accumulation and inhibited the expression of lipogenic genes in both human hepatocellular carcinoma HepG2 cells treated with a fatty acid mixture as well as the livers of HFD-fed mice. In hepatocytes isolated from sterol regulatory element-binding protein (SREBP)-1c knockout mice, the effects of BMP9 were ablated. Mechanistically, BMP9 inhibited SREBP-1c expression through the inhibition of liver X receptor response element 1 activity in the SREBP-1c promoter. Taken together, our results show that BMP9 is an important regulator of hepatic glucose and lipid metabolism.-Yang, M., Liang, Z., Yang, M., Jia, Y., Yang, G., He, Y., Li, X., Gu, H. F., Zheng, H., Zhu, Z., Li, L. Role of bone morphogenetic protein-9 in the regulation of glucose and lipid metabolism.

An important role of SREBP-1 in HBV and HCV co-replication inhibition by PTEN.

HBV HCV co-infection leads to more severe liver diseases including liver cancer than mono-infections. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor, inhibits sterol regulatory element binding protein-1 (SREBP-1). In this study, we characterized the effect of the PTEN - SREBP-1 pathway on HBV HCV co-replication in a cellular model. We found that HBV and HCV can co-replicate in Huh-7 cells with no interference. Overexpression of PTEN inhibits, whereas PTEN knockdown enhances, HBV replication as well as HBV and HCV co-replication. Knocking down SREBP-1 decreases HBV replication in an HBx-dependent manner. SREBP-1 knockdown also decreases HCV replication. PTEN knockdown is concomitant with increased nuclear SREBP-1 levels. PTEN and SREBP-1 double knockdown results in intermediate levels of HBV and HCV replication in mono- and co-replication scenarios. Taken together, we demonstrated, for the first time, that the PTEN - SREBP-1 pathway can regulate HBV HCV co-replication.

Sterol regulatory element binding protein-1C knockout mice show altered neuroactive steroid levels in sciatic nerve.

Neuroactive steroid levels are altered in several experimental models of peripheral neuropathy, and on this basis, they have been proposed as protective agents. For the first time, the levels of these molecules were assessed here in sterol regulatory element binding protein -1c knock-out male mice (i.e., an experimental model of peripheral neuropathy) and compared with observations in wild type animals. The levels of neuroactive steroids have been evaluated by liquid chromatography-tandem mass spectrometry in plasma and sciatic nerve at 2 and 10 months of age and these analyses were implemented analyzing the gene expression of crucial steroidogenic enzymes in sciatic nerve. Data obtained at 2 months of age showed high levels of pregnenolone in sciatic nerve, associated with low levels of its first metabolite, progesterone, and further metabolites (i.e., 5α-pregnane-3,20-dione and 5α-pregnan-3ß-ol-20-one). High levels of testosterone and 17ß-estradiol were also observed. At 10 months of age, the neuroactive steroid profile showed some differences. Indeed, low levels of pregnenolone and high levels of 5α-pregnan-3α-ol-20-one and 5α-pregnan-3ß-ol-20-one were observed. The analysis of the gene expression of steroidogenic enzymes considered here generally followed these changes. Interestingly, the levels of pregnenolone and progesterone were unmodified in plasma suggesting a specific effect of sterol regulatory element binding protein-1c on neurosteroidogenesis. Because this peripheral neuropathy is due to altered fatty acid biosynthesis, data reported here support the belief that the cross-talk between this biosynthetic pathway and neuroactive steroids may represent a possible therapeutic strategy for peripheral neuropathy.

Sterol regulatory element-binding protein-1c orchestrates metabolic remodeling of white adipose tissue by caloric restriction.

Caloric restriction (CR) can delay onset of several age-related pathophysiologies and extend lifespan in various species, including rodents. CR also induces metabolic remodeling involved in activation of lipid metabolism, enhancement of mitochondrial biogenesis, and reduction of oxidative stress in white adipose tissue (WAT). In studies using genetically modified mice with extended lifespans, WAT characteristics influenced mammalian lifespans. However, molecular mechanisms underlying CR-associated metabolic remodeling of WAT remain unclear. Sterol regulatory element-binding protein-1c (Srebp-1c), a master transcription factor of fatty acid (FA) biosynthesis, is responsible for the pathogenesis of fatty liver (steatosis). Our study showed that, under CR conditions, Srebp-1c enhanced mitochondrial biogenesis via increased expression of peroxisome proliferator-activated receptor gamma coactivator-1α (Pgc-1α) and upregulated expression of proteins involved in FA biosynthesis within WAT. However, via Srebp-1c, most of these CR-associated metabolic alterations were not observed in other tissues, including the liver. Moreover, our data indicated that Srebp-1c may be an important factor both for CR-associated suppression of oxidative stress, through increased synthesis of glutathione in WAT, and for the prolongevity action of CR. Our results strongly suggested that Srebp-1c, the primary FA biosynthesis-promoting transcriptional factor implicated in fatty liver disease, is also the food shortage-responsive factor in WAT. This indicated that Srebp-1c is a key regulator of metabolic remodeling leading to the beneficial effects of CR.

Thyrotropin increases hepatic triglyceride content through upregulation of SREBP-1c activity.

BACKGROUND & AIMS: Hallmarks of non-alcoholic fatty liver disease (NAFLD) are increased triglyceride accumulation within hepatocytes. The prevalence of NAFLD increases steadily with increasing thyrotropin (TSH) levels. However, the underlying mechanisms are largely unknown. Here, we focused on exploring the effect and mechanism of TSH on the hepatic triglyceride content. METHODS: As the function of TSH is mediated through the TSH receptor (TSHR), Tshr(-/-) mice (supplemented with thyroxine) were used. Liver steatosis and triglyceride content were analysed in Tshr(-/-) and Tshr(+/+) mice fed a high-fat or normal chow diet, as well as in Srebp-1c(-/-) and Tshr(-/-)Srebp-1c(-/-) mice. The expression levels of proteins and genes involved in liver triglyceride metabolism was measured. RESULTS: Compared with control littermates, the high-fat diet induced a relatively low degree of liver steatosis in Tshr(-/-) mice. Even under chow diet, hepatic triglyceride content was decreased in Tshr(-/-) mice. TSH caused concentration- and time-dependent effects on intracellular triglyceride contents in hepatocytes in vitro. The activity of SREBP-1c, a key regulator involved in triglyceride metabolism and in the pathogenesis of NAFLD, was significantly lower in Tshr(-/-) mice. In Tshr(-/-)Srebp-1c(-/-) mice, the liver triglyceride content showed no significant difference compared with Tshr(+/+)Srebp-1c(-/-) mice. When mice were injected with forskolin (cAMP activator), H89 (inhibitor of PKA) or AICAR (AMPK activator), or HeG2 cells received MK886 (PPARα inhibitor), triglyceride contents presented in a manner dependent on SREBP-1c activity. The mechanism, underlying TSH-induced liver triglyceride accumulation, involved that TSH, through its receptor TSHR, triggered hepatic SREBP-1c activity via the cAMP/PKA/PPARα pathway associated with decreased AMPK, which further increased the expression of genes associated with lipogenesis. CONCLUSIONS: TSH increased the hepatic triglyceride content, indicating an essential role for TSH in the pathogenesis of NAFLD.

SREBP-1c gene silencing can decrease lipid deposits in bovine hepatocytes cultured in vitro.

BACKGROUND: Fatty liver is a major metabolic disorder that occurs during early lactation in high-producing dairy cows. Sterol regulatory element-binding protein-1c (SREBP-1c) is an important transcription factor that regulates lipid synthesis by regulating the expression of lipid metabolism genes. METHODS: In this study, we reduced the expression of SREBP-1c by adenovirus-mediated SREBP-1c with a low expression vector (AD-GFP-SREBP-1c) to study the effects of SREBP-1c on lipid deposits in bovine hepatocytes. The expression levels and enzyme activities of SERBP-1c and its target genes were determined by real-time PCR, western blot, and ELISA. RESULTS: These results showed that Ad-GFP-SREBP-1c could inhibit SREBP-1c expression. The expression of the lipid synthesis enzyme acetyl-CoA carboxylase (ACC) was down-regulated. The expression levels of the lipid oxidation enzymes long-chain fatty acyl-COA synthetase (ACSL-1), carnitine palmitoyltransferase І (CPT-І), carnitine palmitoyltransferase II (CPT- II), and ß-hydroxyacyl-CoA-DH (HADH) were significantly elevated. Furthermore, the expression levels of factors involved in the assembly and transport of very low-density lipoproteins (VLDLs), such as apolipoprotein B100 (ApoB), apolipoprotein E (ApoE), and microsomal triglyceride transfer protein (MTTP) were decreased comparison with the negative control and the blank control groups, but the low-density lipoprotein receptor (LDLR) was elevated. The concentrations of TG (triglyceride) and VLDL were also reduced. CONCLUSION: These data suggest that low SREBP-1c expression can decrease lipid synthesis, increase lipid oxidation, and decrease the TG and VLDL content in bovine hepatocytes.

Sterol regulatory element-binding protein-1 determines plasma remnant lipoproteins and accelerates atherosclerosis in low-density lipoprotein receptor-deficient mice.

OBJECTIVE: Sterol regulatory element-binding protein-1 (SREBP-1) is nutritionally regulated and is known to be a key transcription factor regulating lipogenic enzymes. The goal of this study was to evaluate the roles of SREBP-1 in dyslipidemia and atherosclerosis. METHODS AND RESULTS: Transgenic mice that overexpress SREBP-1c in the liver and SREBP-1-deficient mice were crossed with low-density lipoprotein receptor (LDLR)-deficient mice, and the plasma lipids and atherosclerosis were analyzed. Hepatic SREBP-1c overexpression in LDLR-deficient mice caused postprandial hypertriglyceridemia, increased very-low-density lipoprotein (VLDL) cholesterol, and decreased high-density lipoprotein cholesterol in plasma, which resulted in accelerated aortic atheroma formation. Conversely, absence of SREBP-1 suppressed Western diet-induced hyperlipidemia in LDLR-deficient mice and ameliorated atherosclerosis. In contrast, bone marrow-specific SREBP-1 deficiency did not alter the development of atherosclerosis. The size of nascent VLDL particles secreted from the liver was increased in SREBP-1c transgenic mice and reduced in SREBP-1-deficient mice, accompanied by upregulation and downregulation of phospholipid transfer protein expression, respectively. CONCLUSIONS: Hepatic SREBP-1c determines plasma triglycerides and remnant cholesterol and contributes to atherosclerosis in hyperlipidemic states. Hepatic SREBP-1c also regulates the size of nascent VLDL particles.