Mechanism of anthracycline-mediated down-regulation of GATA4 in the heart.

AIMS: Anthracyclines such as daunorubicin (DNR) and doxorubicin are effective cancer chemotherapeutic agents, but can induce cardiotoxicity. GATA4 has been shown to serve as a survival factor of cardiac muscle cells, and anthracyclines promote apoptosis in part by down-regulating GATA4. The present study investigated the mechanism of anthracycline action to down-regulate GATA4. METHODS AND RESULTS: DNR inhibited the transcriptional activity exhibited by the 250 bp conserved region immediately upstream from the transcriptional start site of the Gata4 gene. Mapping this region identified that the CCAAT-binding factor/nuclear factor-Y (CBF/NF-Y) binding to the CCAAT box was inhibited by DNR in HL-1 cardiac muscle cells and in perfused isolated mouse hearts. The DNR action on the Gata4 promoter was found to be dependent on p53, since DNR promoted nuclear binding of p53 to CBF/NF-Y and pifithrin-α (a p53 inhibitor) attenuated DNR down-regulation of GATA4. CONCLUSION: Anthracycline down-regulation of GATA4 is mediated by the inhibition of Gata4 gene transcription via a novel mechanism that involves the p53-dependent inhibition of CBF/NF-Y binding to the CCAAT box within the Gata4 promoter.

Macrophage-derived human resistin exacerbates adipose tissue inflammation and insulin resistance in mice.

Resistin is an adipokine that contributes to insulin resistance in mice. In humans, however, studies investigating the link between resistin and metabolic disease are conflicting. Further complicating the matter, human resistin is produced mainly by macrophages rather than adipocytes. To address this important issue, we generated mice that lack adipocyte-derived mouse resistin but produce human resistin in a pattern similar to that found in humans, i.e., in macrophages (humanized resistin mice). When placed on a high-fat diet, the humanized resistin mice rapidly developed accelerated white adipose tissue (WAT) inflammation, leading to increased lipolysis and increased serum free fatty acids. Over time, these mice accumulated lipids, including diacylglycerols, in muscle. We found that this resulted in increased Pkcq pathway activity, leading to increased serine phosphorylation of Irs-1 and insulin resistance. Thus, although the site of resistin production differs between species, human resistin exacerbates WAT inflammation and contributes to insulin resistance.

Retinol saturase promotes adipogenesis and is downregulated in obesity.

Adipocyte differentiation is controlled by many transcription factors, but few known downstream targets of these factors are necessary for adipogenesis. Here we report that retinol saturase (RetSat), which is an enzyme implicated in the generation of dihydroretinoid metabolites, is induced during adipogenesis and is directly regulated by the transcription factor peroxisome proliferator activated receptor gamma (PPARgamma). Ablation of RetSat dramatically inhibited adipogenesis but, surprisingly, this block was not overcome by the putative product of RetSat enzymatic activity. On the other hand, ectopic RetSat with an intact, but not a mutated, FAD/NAD dinucleotide-binding motif increased endogenous PPARgamma transcriptional activity and promoted adipogenesis. Indeed, RetSat was not required for adipogenesis when cells were provided with exogenous PPARgamma ligands. In adipose tissue, RetSat is expressed in adipocytes but is unexpectedly downregulated in obesity, most likely owing to infiltration of macrophages that we demonstrate to repress RetSat expression. Thiazolidinedione treatment reversed low RetSat expression in adipose tissue of obese mice. Thus, RetSat plays an important role in the biology of adipocytes, where it favors normal differentiation, yet is reduced in the obese state. RetSat is thus a novel target for therapeutic intervention in metabolic disease.

Rev-erbalpha, a heme sensor that coordinates metabolic and circadian pathways.

The circadian clock temporally coordinates metabolic homeostasis in mammals. Central to this is heme, an iron-containing porphyrin that serves as prosthetic group for enzymes involved in oxidative metabolism as well as transcription factors that regulate circadian rhythmicity. The circadian factor that integrates this dual function of heme is not known. We show that heme binds reversibly to the orphan nuclear receptor Rev-erbalpha, a critical negative component of the circadian core clock, and regulates its interaction with a nuclear receptor corepressor complex. Furthermore, heme suppresses hepatic gluconeogenic gene expression and glucose output through Rev-erbalpha-mediated gene repression. Thus, Rev-erbalpha serves as a heme sensor that coordinates the cellular clock, glucose homeostasis, and energy metabolism.