Role of Mcpip1 in obesity‐induced hepatic steatosis as determined by myeloid and liver‐specific conditional knockouts

Monocyte chemoattractant protein‐induced protein 1 (MCPIP1, alias Regnase 1) is a negative regulator of inflammation, acting through cleavage of transcripts coding for proinflammatory cytokines and by inhibition of NFκB activity. Moreover, it was demonstrated that MCPIP1 regulates lipid metabolism both in adipose tissue and in hepatocytes. In this study, we investigated the effects of tissue‐specific Mcpip1 deletion on the regulation of hepatic metabolism and development of nonalcoholic fatty liver disease (NAFLD). We used control Mcpip1(fl/fl) mice and animals with deletion of Mcpip1 in myeloid leukocytes (Mcpip1(fl/fl)LysM(Cre)) and in hepatocytes (Mcpip1(fl/fl)Alb(Cre)), which were fed chow or a high‐fat diet (HFD) for 12 weeks. Mcpip1(fl/fl)LysM(Cre) mice fed a chow diet were characterized by a significantly reduced hepatic expression of genes regulating lipid and glucose metabolism, which subsequently resulted in low plasma glucose level and dyslipidemia. These animals also displayed systemic inflammation, demonstrated by increased concentrations of cytokines in the plasma and high Tnfa, Il6, IL1b mRNA levels in the liver and brown adipose tissue (BAT). Proinflammatory leukocyte infiltration into BAT, together with low expression of Ucp1 and Ppargc1a, resulted in hypothermia of 22‐week‐old Mcpip1(fl/fl)LysM(Cre) mice. On the other hand, there were no significant changes in phenotype in Mcpip1(fl/fl)Alb(Cre) mice. Although we detected a reduced hepatic expression of genes regulating glucose metabolism and β‐oxidation in these mice, they remained asymptomatic. Upon feeding with a HFD, Mcpip1(fl/fl)LysM(Cre) mice did not develop obesity, glucose intolerance, nor hepatic steatosis, but were characterized by low plasma glucose level and dyslipidemia, along with proinflammatory phenotype. Mcpip1(fl/fl)Alb(Cre) animals, following a HFD, became hypercholesterolemic, but accumulated lipids in the liver at the same level as Mcpip1(fl/fl) mice, and no changes in the level of soluble factors tested in the plasma were detected. We have demonstrated that Mcpip1 protein plays an important role in the liver homeostasis. Depletion of Mcpip1 in myeloid leukocytes, followed by systemic inflammation, has a more pronounced effect on controlling liver metabolism and homeostasis than the depletion of Mcpip1 in hepatocytes

Transcriptional Regulation of Fatty Acid Translocase/CD36 Expression by CCAAT/Enhancer-binding Protein α*

Fatty acid translocase (FAT/CD36) plays an important role in facilitating long chain fatty acid transport. FAT/CD36 gene deletion protects mice from high fat diet-induced obesity. In this study we have investigated the regulatory mechanism of FAT/CD36 expression at the transcription level. FAT/CD36 expression was activated during 3T3-L1 adipocyte differentiation, and FAT/CD36 protein levels were positively correlated with CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ. However, a negative correlation was detected between FAT/CD36 and C/EBPβ. Overexpression of C/EBPα or C/EBPβ increased FAT/CD36 mRNA and protein levels in several types of cells. Restoration of C/EBPα or C/EBPβ expression in C/EBPα- or C/EBPβ-deficient mouse embryonic fibroblasts increased FAT/CD36 expression. However, in mouse embryonic fibroblasts C/EBPα was a more potent activator of FAT/CD36 expression than was C/EBPβ. Expression of C/EBPα robustly increased FAT/CD36 proximal promoter-directed luciferase expression in human embryonic kidney 293 cells. A C/EBP-responsive element was identified in the FAT/CD36 promoter by using 5′ and specific site mutations. The binding of C/EBPα in the FAT/CD36 promoter was detected by chromatin immunoprecipitation in 3T3-L1 adipocytes. These results demonstrated that C/EBPα regulates FAT/CD36 gene expression at the transcriptional level

Regulation of the Human Prostacyclin Receptor Gene by the Cholesterol-responsive Sterol Response Element Binding Protein (SREBP) 1.

Prostacyclin and its prostacyclin receptor, the IP, play essential roles in regulating haemostasis and vascular tone and have also been implicated in a range cardio-protective effects, but through largely unknown mechanisms. In this study, the influence of cholesterol on human (h)IP gene expression was investigated in cultured vascular endothelial and platelet-progenitor megakaryocytic cells. Cholesterol-depletion increased hIP mRNA, hIP promoter-directed reporter gene expression and hIP-induced cAMP generation in all cell types. Furthermore, the constitutively active SREBP1a, but not SREBP2, increased hIP mRNA and promoter-directed gene expression while deletional and mutational analysis uncovered an evolutionary conserved sterol-response element (SRE), adjacent to a known functional Sp1 element, within the core hIP promoter. Moreover, chromatin immunoprecipitation assays confirmed direct cholesterol-regulated binding of SREBP1a to this hIP promoter region in vivo, while immunofluorescence microscopy corroborated that cholesterol-depletion significantly increases hIP expression levels. In conclusion, the hIP gene is directly regulated by cholesterol-depletion that occurs through binding of SREBP1a to a functional SRE within its core promoter. Mechanistically, these data establish that cholesterol can regulate hIP expression which may, at least in part, account for the combined cardio-protective actions of low serum cholesterol through its regulation of prostacyclin receptor (IP) expression within the human vasculature.Science Foundation IrelandOther funderProgramme for Research in Third Level Institutions (PRTLI) 1DG - 15/10/2012Update rights statement with correct citation when availabl