Pioglitazone attenuates TGF-beta(1)-induction of fibronectin synthesis and its splicing variant in human mesangial cells via activation of peroxisome proliferator-activated receptor (PPAR)gamma

The peroxisome proliferator-activated receptor (PPAR)gamma is expressed not only in adipose tissue but also in macrophages/monocytes and plays important roles in acute/chronic inflammation. Transforming growth factor (TGF)-beta is a common pathogenic indicator of sclerosis because it induces the accumulation of extracellular matrix (ECM) in the glomerular mesangium of the kidney. Among components of the ECM, fibronectin (FN) is an acute reactant in inflammation, and isoforms of it produced by splicing of gene variants appear during abnormal conditions such as wound healing. In this study, we examined the effects of pioglitazone, a PPARgamma agonist, on TGF-beta(1)-induced FN synthesis in cultured mesangial cells using RT-PCR and Western blot analysis. We also analyzed its splicing variant, extra domain (ED) A, containing FN (EDA(+)FN). TGF-beta(1) enhanced the production of both FN and EDA(+) FN and down-regulated PPARgamma expression. Pioglitazone reversed both these effects of TGF-beta(1). These findings suggest that PPARgamma activation by pioglitazone may affect the TGF-beta(1)-induced FN accumulation observed in the glomerular mesangium in cases of glomerulosclerosis, although further in vivo experiments are needed to evaluate this inference.     

Peroxisome proliferator-activated receptor gamma controls Muc1 transcription in trophoblasts

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is essential for placental development. Here, we show that the mucin gene Muc1 is a PPARgamma target, whose expression is lost in PPARgamma null placentas. During differentiation of trophoblast stem cells, PPARgamma is strongly induced, and Muc1 expression is upregulated by the PPARgamma agonist rosiglitazone. Muc1 promoter is activated strongly and specifically by liganded PPARgamma but not PPARalpha or PPARdelta. A PPAR binding site (DR1) in the proximal Muc1 promoter acts as a basal silencer in the absence of PPARgamma, and its cooperation with a composite upstream enhancer element is both necessary and sufficient for PPARgamma-dependent induction of Muc1. In the placenta, MUC1 protein is localized exclusively to the apical surface of the labyrinthine trophoblast around maternal blood sinuses, resembling its luminal localization on secretory epithelia. Last, variably penetrant maternal blood sinus dilation in Muc1-deficient placentas suggests that Muc1 regulation by PPARgamma contributes to normal placental development but also that the essential functions of PPARgamma in the organ are mediated by other targets.     

Induction of peroxisome proliferator-activated receptor gamma during the conversion of 3T3 fibroblasts into adipocytes is mediated by C/EBPbeta, C/EBPdelta, and glucocorticoids.

The differentiation of 3T3 preadipocytes into adipocytes is accompanied by a transient induction of C/EBPbeta and C/EBPdelta expression in response to treatment of the cells with methylisobutylxanthine (MIX) and dexamethasone (DEX), respectively. In this report, we demonstrate that peroxisome proliferator-activated receptor gamma (PPARgamma) expression in 3T3-L1 preadipocytes is induced by MIX and DEX, suggesting that C/EBPbeta and C/EBPdelta may be involved in this process. Using a tetracycline-responsive expression system, we have recently shown that the conditional ectopic expression of C/EBPbeta in NIH 3T3 fibroblasts (beta2 cells) in the presence of DEX activates the synthesis of peroxisome PPARgamma mRNA. Subsequent exposure of these cells to PPAR activators stimulates their conversion into adipocytes; however, neither the expression of C/EBPbeta nor exposure to DEX alone is capable of inducing PPARgamma expression in the beta2 cell line. We find that unlike the case for 3T3 preadipocytes, C/EBPdelta is not induced by DEX in these 3T3 fibroblasts and therefore is not relaying the effect of this glucocorticoid to the PPARgamma gene. To define the role of glucocorticoids in regulating PPARgamma expression and the possible involvement of C/EBPdelta, we have established an additional set of NIH 3T3 cell lines expressing either C/EBPdelta alone (delta23 cells) or C/EBPdelta and C/EBPbeta together (beta/delta39 cells), using the tetracycline-responsive system. Culture of these cells in tetracycline-deficient medium containing DEX, MIX, insulin, and fetal bovine serum shows that the beta/delta39 cells express PPARgamma and aP2 mRNAs at levels that are almost equivalent to those observed in fully differentiated 3T3-L1 adipocytes. These levels are approximately threefold higher than their levels of expression in the beta2 cells. Despite the fact that these beta/delta39 cells produce abundant amounts of C/EBPbeta and C/EBPdelta (in the absence of tetracycline), they still require glucocorticoids to attain maximum expression of PPARgamma mRNA. Furthermore, the induction of PPARgamma mRNA by exposure of these cells to DEX occurs in the absence of ongoing protein synthesis. The delta23 cells, on the other hand, are not capable of activating PPARgamma gene expression when exposed to the same adipogenic inducers. Finally, attenuation of ectopic C/EBPbeta production at various stages during the differentiation process results in a concomitant inhibition of PPARgamma and the adipogenic program. These data strongly suggest that the induction of PPARgamma gene expression in multipotential mesenchymal stem cells (NIH 3T3 fibroblasts) is dependent on elevated levels of C/EBPbeta throughout the differentiation process, as well as an initial exposure to glucocorticoids. C/EBPdelta may function by synergizing with C/EBPbeta to enhance the level of PPARgamma expression.     

Adipocyte-specific gene expression and adipogenic steatosis in the mouse liver due to peroxisome proliferator-activated receptor gamma1 (PPARgamma1) overexpression

Peroxisome proliferator activated-receptor (PPAR) isoforms, alpha and gamma, function as important coregulators of energy (lipid) homeostasis. PPARalpha regulates fatty acid oxidation primarily in liver and to a lesser extent in adipose tissue, whereas PPARgamma serves as a key regulator of adipocyte differentiation and lipid storage. Of the two PPARgamma isoforms, PPARgamma1 and PPARgamma2 generated by alternative splicing, PPARgamma1 isoform is expressed in liver and other tissues, whereas PPARgamma2 isoform is expressed exclusively in adipose tissue where it regulates adipogenesis and lipogenesis. Since the function of PPARgamma1 in liver is not clear, we have, in this study, investigated the biological impact of overexpression of PPARgamma1 in mouse liver. Adenovirus-PPARgamma1 injected into the tail vein induced hepatic steatosis in PPARalpha(-/-) mice. Northern blotting and gene expression profiling results showed that adipocyte-specific genes and lipogenesis-related genes are highly induced in PPARalpha(-/-) livers with PPARgamma1 overexpression. These include adipsin, adiponectin, aP2, caveolin-1, fasting-induced adipose factor, fat-specific gene 27 (FSP27), CD36, Delta(9) desaturase, and malic enzyme among others, implying adipogenic transformation of hepatocytes. Of interest is that hepatic steatosis per se, induced either by feeding a diet deficient in choline or developing in fasted PPARalpha(-/-) mice, failed to induce the expression of these PPARgamma-regulated adipogenesis-related genes in steatotic liver. These results suggest that a high level of PPARgamma in mouse liver is sufficient for the induction of adipogenic transformation of hepatocytes with adipose tissue-specific gene expression and lipid accumulation. We conclude that excess PPARgamma activity can lead to the development of a novel type of adipogenic hepatic steatosis.