Energy Balance, Myostatin, and GILZ: Factors Regulating Adipocyte Differentiation in Belly and Bone

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Abstract

Peroxisome proliferator-activated receptor gamma (PPAR- γ ) belongs to the nuclear hormone receptor subfamily of transcription factors. PPARs are expressed in key target tissues such as liver, fat, and muscle and thus they play a major role in the regulation of energy balance. Because of PPAR- γ 's role in energy balance, signals originating from the gut (e.g., GIP), fat (e.g., leptin), muscle (e.g., myostatin), or bone (e.g., GILZ) can in turn modulate PPAR expression and/or function. Of the two PPAR- γ isoforms, PPAR- γ 2 is the key regulator of adipogenesis and also plays a role in bone development. Activation of this receptor favors adipocyte differentiation of mesenchymal stem cells, while inhibition of PPAR- γ 2 expression shifts the commitment towards the osteoblastogenic pathway. Clinically, activation of this receptor by antidiabetic agents of the thiazolidinedione class results in lower bone mass and increased fracture rates. We propose that inhibition of PPAR- γ 2 expression in mesenchymal stem cells by use of some of the hormones/factors mentioned above may be a useful therapeutic strategy to favor bone formation.

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An antidiabetic thiazolidinedione is a high affinity ligand for peroxisome proliferator-activated receptor gamma (PPAR gamma).

J Lehmann, L B Moore, T A Smith-Oliver … (1995)

Thiazolidinedione derivatives are antidiabetic agents that increase the insulin sensitivity of target tissues in animal models of non-insulin-dependent diabetes mellitus. In vitro, thiazolidinediones promote adipocyte differentiation of preadipocyte and mesenchymal stem cell lines; however, the molecular basis for this adipogenic effect has remained unclear. Here, we report that thiazolidinediones are potent and selective activators of peroxisome proliferator-activated receptor gamma (PPAR gamma), a member of the nuclear receptor superfamily recently shown to function in adipogenesis. The most potent of these agents, BRL49653, binds to PPAR gamma with a Kd of approximately 40 nM. Treatment of pluripotent C3H10T1/2 stem cells with BRL49653 results in efficient differentiation to adipocytes. These data are the first demonstration of a high affinity PPAR ligand and provide strong evidence that PPAR gamma is a molecular target for the adipogenic effects of thiazolidinediones. Furthermore, these data raise the intriguing possibility that PPAR gamma is a target for the therapeutic actions of this class of compounds.

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PPAR gamma is required for the differentiation of adipose tissue in vivo and in vitro.

E D Rosen, P Sarraf, A E Troy … (1999)

The process of adipogenesis is known to involve the interplay of several transcription factors. Activation of one of these factors, the nuclear hormone receptor PPAR gamma, is known to promote fat cell differentiation in vitro. Whether PPAR gamma is required for this process in vivo has remained an open question because a viable loss-of-function model for PPAR gamma has been lacking. We demonstrate here that mice chimeric for wild-type and PPAR gamma null cells show little or no contribution of null cells to adipose tissue, whereas most other organs examined do not require PPAR gamma for proper development. In vitro, the differentiation of ES cells into fat is shown to be dependent on PPAR gamma gene dosage. These data provide direct evidence that PPAR gamma is essential for the formation of fat.

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PPAR gamma is required for placental, cardiac, and adipose tissue development.

Y Barak, M. C. Nelson, E Ong … (1999)

The nuclear hormone receptor PPAR gamma promotes adipogenesis and macrophage differentiation and is a primary pharmacological target in the treatment of type II diabetes. Here, we show that PPAR gamma gene knockout results in two independent lethal phases. Initially, PPAR gamma deficiency interferes with terminal differentiation of the trophoblast and placental vascularization, leading to severe myocardial thinning and death by E10.0. Supplementing PPAR gamma null embryos with wild-type placentas via aggregation with tetraploid embryos corrects the cardiac defect, implicating a previously unrecognized dependence of the developing heart on a functional placenta. A tetraploid-rescued mutant surviving to term exhibited another lethal combination of pathologies, including lipodystrophy and multiple hemorrhages. These findings both confirm and expand the current known spectrum of physiological functions regulated by PPAR gamma.