Human Silent Information Regulator Type 1 (SIRT1) is an NAD+-dependent deacetylase protein which is an intermediary of cellular metabolism in gene silencing and aging. SIRT1 has been extensively investigated and shown to delay senescence; however, less is known about the regulation of SIRT1 during aging. In this study, we show that the peroxisome proliferator-activated receptor-γ (PPARγ), which is a ligand-regulated modular nuclear receptor that governs adipocyte differentiation and inhibits cellular proliferation, inhibits SIRT1 expression at the transcriptional level. Moreover, both PPARγ and SIRT1 can bind the SIRT1 promoter. PPARγ directly interacts with SIRT1 and inhibits SIRT1 activity, forming a negative feedback and self-regulation loop. In addition, our data show that acetylation of PPARγ increased with increasing cell passage number. We propose that PPARγ is subject to regulation by acetylation and deacetylation via p300 and SIRT1 in cellular senescence. These results demonstrate a mutual regulation between PPARγ and SIRT1 and identify a new posttranslational modification that affects cellular senescence.

IGFs have multiple functions regarding cellular growth, survival and differentiation under different physiological and pathological conditions. IGF effects are modulated systemically and locally by six high-affinity IGF-binding proteins (IGFBP-1 to -6). Despite their structural similarity, each IGFBP has unique properties and exhibits specific functions. IGFBP-4, the smallest IGFBP, exists in both non-glycosylated and N-glycosylated forms in all biological fluids. It is expressed by a wide range of cell types and tissues, and its expression is regulated by different mechanisms in a cell type-specific manner.IGFBP-4 binds IGF-I and IGF-II with similar affinities and inhibits their actions under almost all in vitro and in vivo conditions. In this review, we summarize the available data regarding the following aspects of IGFBP-4: genomic organization, protein structure-function relationship, expression and its regulation, as well as IGFdependent and -independent actions. The biological significance of IGFBP-4 for reproductive physiology, bone formation, renal pathophysiology and cancer is discussed.

Sirtuin 1 (SIRT1) regulates adipocyte and osteoblast differentiation. However, the underlying mechanism should be investigated. This study revealed that SIRT1 acts as a crucial repressor of adipogenesis. RNA-interference-mediated SIRT1 knockdown or genetic ablation enhances adipogenic potential, whereas SIRT1 overexpression inhibits adipogenesis in mesenchymal stem cells (MSCs). SIRT1 also deacetylates the histones of sFRP1, sFRP2, and Dact1 promoters; inhibits the mRNA expression of sFRP1, sFRP2, and Dact1; activates Wnt signaling pathways; and suppresses adipogenesis. SIRT1 deacetylates β-catenin to promote its accumulation in the nucleus and thus induces the transcription of genes that block MSC adipogenesis. In mice, the partial absence of SIRT1 promotes the formation of white adipose tissues without affecting the development of the body of mice. Our study described the regulatory role of SIRT1 in Wnt signaling and proposed a regulatory mechanism of adipogenesis.

Peroxisome proliferator-activated receptor γ (PPARγ) plays an important role in the inhibition of cell growth by promoting cell-cycle arrest, and PPARγ activation induces the expression of p16INK4α (CDKN2A), an important cell-cycle inhibitor that can induce senescence. However, the role of PPARγ in cellular senescence is unknown. Here, we show that PPARγ promotes cellular senescence by inducing p16INK4α expression. We found several indications that PPARγ accelerates cellular senescence, including enhanced senescence-associated (SA)-β-galactosidase staining, increased G1 arrest and delayed cell growth in human fibroblasts. Western blotting studies demonstrated that PPARγ activation can upregulate the expression of p16INK4α. PPARγ can bind to the p16 promoter and induce its transcription, and, after treatment with a selective PPARγ agonist, we observed more-robust expression of p16INK4α in senescent cells than in young cells. In addition, our data indicate that phosphorylation of PPARγ decreased with increased cell passage. Our results provide a possible molecular mechanism underlying the regulation of cellular senescence.

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