The retinoblastoma protein (RB) has previously been shown to facilitate adipocyte differentiation by inducing cell cycle arrest and enhancing the transactivation by the adipogenic CCAAT/enhancer binding proteins (C/EBP). We show here that the peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor pivotal for adipogenesis, promotes adipocyte differentiation more efficiently in the absence of RB. PPARgamma and RB were shown to coimmunoprecipitate, and this PPARgamma-RB complex also contains the histone deacetylase HDAC3, thereby attenuating PPARgamma's capacity to drive gene expression and adipocyte differentiation. Dissociation of the PPARgamma-RB-HDAC3 complex by RB phosphorylation or by inhibition of HDAC activity stimulates adipocyte differentiation. These observations underscore an important function of both RB and HDAC3 in fine-tuning PPARgamma activity and adipocyte differentiation.

Highlights d A therapeutic dose (50 mg) of mirabegron does not stimulate human BAT thermogenesis d Human brown adipocytes lack b 3 -AR and do not respond to mirabegron in vitro d Norepinephrine-induced respiration is driven by b 2 -AR, which co-localizes with UCP1 d b 2 -AR is the main target for pharmacological activation of human brown adipocytes

In addition to their role in cell cycle progression, new data reveal an emerging role of D-type cyclins in transcriptional regulation and cellular differentiation processes. Using 3T3-L1 cell lines to study adipogenesis, we observed an up-regulation of cyclin D3 expression throughout the differentiation process. Surprisingly, cyclin D3 was only minimally expressed during the initial stages of adipogenesis, when mitotic division is prevalent. This seemingly paradoxical expression led us to investigate a potential cell cycle-independent role for cyclin D3 during adipogenesis. We show here a direct interaction between cyclin D3 and the nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥). Our experiments reveal cyclin D3 acts as a ligand-dependent PPAR␥ coactivator, which, together with its cyclin-dependent kinase partner, phosphorylates the A-B domain of the nuclear receptor. Overexpression and knockdown studies with cyclin D3 had marked effects on PPAR␥ activity and subsequently on adipogenesis. Chromatin immunoprecipitation assays confirm the participation of cyclin D3 in the regulation of PPAR␥ target genes. We show that cyclin D3 mutant mice are protected from diet-induced obesity, display smaller adipocytes, have reduced adipogenic gene expression, and are insulin sensitive. Our results indicate that cyclin D3 is an important factor governing adipogenesis and obesity.

Peroxisome proliferator-activated receptor ␥ (PPAR␥) might not be permissive to ligand activation in prostate cancer cells. Association of PPAR␥ with repressing factors or posttranslational modifications in PPAR␥ protein could explain the lack of effect of PPAR␥ ligands in a recent randomized clinical trial. Using cells and prostate cancer xenograft mouse models, we demonstrate in this study that a combination treatment using the PPAR␥ agonist pioglitazone and the histone deacetylase inhibitor valproic acid is more efficient at inhibiting prostate tumor growth than each individual therapy. We show that the combination treatment impairs the bone-invasive potential of prostate cancer cells in mice. In addition, we demonstrate that expression of E-cadherin, a protein involved in the control of cell migration and invasion, is highly up-regulated in the presence of valproic acid and pioglitazone. We show that E-cadherin expression responds only to the combination treatment and not to single PPAR␥ agonists, defining a new class of PPAR␥ target genes. These results open up new therapeutic perspectives in the treatment of prostate cancer.Prostate cancer is the most common form of cancer in men and the second leading cause of cancer deaths. Tumor growth is originally androgen dependent. Androgens exert their effects through activation of the androgen receptor (AR), a member of the hormone nuclear receptor superfamily. In the mature prostatic gland, the AR regulates the expression of genes involved in cell division and proliferation of the epithelial cells (26). The AR is also involved in several other aspects of prostate cellular metabolism, including lipid biosynthesis, and controls the production of specialized secretory proteins with prostate-restricted expression, such as prostate-specific antigen (PSA) (26). When prostate cancer is still hormone dependent, androgen ablation therapy causes regression of the tumor (18), likely through inactivation of the transcription of the AR target genes. However, the durability of this response is inadequate and many men develop recurrent androgen-independent prostate cancer, which has a very poor prognosis (see reference 11 for a review). Other nuclear receptors or locally produced factors that interact with nuclear receptors are likely involved in cell proliferation, differentiation, and apoptosis in the prostate. The peroxisome proliferator-activated receptor ␥ (PPAR␥) is one such factor. PPAR␥ is another member of the hormone nuclear receptor superfamily. As for most of the other members of this family, its activity is regulated by ligands. Prostaglandin J2 and the antidiabetic drugs thiazolidinediones have been determined to be natural and synthetic ligands of PPAR␥, respectively (for a review, see reference 9). PPAR␥ is highly expressed in the adipose tissue and is required for its development through regulation of the expression of adipocyte-specific genes, such as lipoprotein lipase or the fatty acid transport protein aP2. PPAR␥ is expressed in several other tissues in add...

We evaluated the effects of E2F1 on glucose homeostasis using E2F1 -/-mice. E2F1 -/-mice show an overall reduction in pancreatic size as the result of impaired postnatal pancreatic growth. Furthermore, these animals have dysfunctional β cells, linked to impaired PDX-1 activity. Because of the disproportionate small pancreas and dysfunctional islets, E2F1 -/-mice secrete insufficient amounts of insulin in response to a glucose load, resulting in glucose intolerance. Despite this glucose intolerance, E2F1 -/-mice do not develop overt diabetes mellitus because they have insulin hypersensitivity, which is secondary to a diminished adipose tissue mass and altered adipocytokine levels, which compensates for the defect in insulin secretion. These data demonstrate that factors controlling cell proliferation, such as E2F1, determine pancreatic growth and function, subsequently affecting metabolic homeostasis. 1288The Nonstandard abbreviations used: dimerization partner (DP); dual x-ray absorptiometry (DEXA); hepatocyte nuclear factor-1A (HNF-A); insulin promoter factor 1 (IPF-1); insulin-receptor substrate-1 (IRS-1); intraperitoneal glucose-tolerance test (IPGTT); maturity-onset diabetes of the young (MODY); postcoitum day (dpc); protein tyrosine phosphatase 1B (PTP1B); retinoblastoma (RB); type 2 diabetes mellitus (T2D); white adipose tissue (WAT).

Increased expression levels of the long non-coding RNA metastasis-associated lung adenocarcinoma transcript 1 (Malat1) have been associated with enhanced proliferation and metastasis of several cancer cell types. Hypoxia, a hallmark characteristic of solid tumors, has been linked to an increase in the activity of the ATP-generating AMPK protein. Since Malat1 was recently shown to be upregulated during hypoxia, the objective of this study was to determine the contribution of AMPK in the mechanistic pathways regulating Malat1 expression in low oxygen conditions. Compared to those cultured in 21% O2 conditions, HeLa cells incubated in 1.5% O2 expressed more Malat1 transcripts. This observation was mimicked in HEK293T cells using a synthetic reporter construct containing 5.6 kb of the human Malat1 promoter, suggesting that hypoxia directly impacted Malat1 gene transcription. Interestingly, pharmacological stimulation of AMPK increased Malat1 promoter transactivation in 21% O2 conditions, whereas inhibition of either AMPK or its upstream activator CaMKK completely abolished the augmentation of Malat1 under hypoxia. Pharmacological modulation of LKB1, another major regulator of AMPK, had no impact on Malat1 promoter transactivation, suggesting that calcium inputs are important in the control of Malat1 expression by AMPK. Overexpression of hypoxia-inducible factor-1α (HIF-1α) increased Malat1 expression in 21% O2 conditions, whereas pharmacological inhibition of HIF-1α blocked the impact of hypoxia on the Malat1 promoter. Taken together, these findings strongly suggest that Malat1 expression is regulated in hypoxic conditions by a CaMKK/AMPK/HIF-1α axis. More research is needed in physiological settings to test the clinical relevance of this pathway.

The nuclear receptor PPARc is implicated in the control of cell proliferation and apoptosis. However, the molecular mechanisms by which it controls these processes remain largely elusive. We show here that PPARc activation in the presence of the retinoblastoma protein (RB) results in the arrest of cells at the G1 phase of the cell cycle, whereas in the absence of RB, cells accumulate in G2/M, endoreduplicate, and undergo apoptosis. Through the use of HDAC inhibitors and coimmunoprecipitations, we furthermore demonstrate that the effects of RB on PPARc-mediated control of the cell cycle and apoptosis depend on the recruitment of histone deacetylase 3 (HDAC3) to PPARc. In combination, these data hence demonstrate that the effects of PPARc on cell proliferation and apoptosis are dependent on the presence of an RB-HDAC3 complex.

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