The heme oxygenase (HO) system is essential for heme and iron homeostasis and necessary for adaptation to cell stress. HO degrades heme to biliverdin (BV), carbon monoxide (CO) and ferrous iron. Although mostly beneficial, the HO reaction can also produce deleterious effects, predominantly attributed to excessive product formation. Underrated so far is, however, that HO may exert effects additionally via modulation of the cellular heme levels. Heme, besides being an often-quoted generator of oxidative stress, plays also an important role as a signaling molecule. Heme controls the anti-oxidative defense, circadian rhythms, activity of ion channels, glucose utilization, erythropoiesis, and macrophage function. This broad spectrum of effects depends on its interaction with proteins ranging from transcription factors to enzymes. In degrading heme, HO has the potential to exert effects also via modulation of heme-mediated pathways. In this review, we will discuss the multitude of pathways regulated by heme to enlarge the view on HO and its role in cell physiology. We will further highlight the contribution of HO to pathophysiology, which results from a dysregulated balance between heme and the degradation products formed by HO.

Increased visceral fat is associated with a high risk of diabetes and metabolic syndrome and is in part caused by excessive glucocorticoids (GCs). However, the molecular mechanisms remain undefined. We now identify the GC-dependent gene LIM domain only 3 (LMO3) as being selectively upregulated in a depot-specific manner in human obese visceral adipose tissue, localizing primarily in the adipocyte fraction. Visceral LMO3 levels were tightly correlated with expression of 11β-hydroxysteroid dehydrogenase type-1 (HSD11B1), the enzyme responsible for local activation of GCs. In early human adipose stromal cell differentiation, GCs induced LMO3 via the GC receptor and a positive feedback mechanism involving 11βHSD1. No such induction was observed in murine adipogenesis. LMO3 overexpression promoted, while silencing of LMO3 suppressed, adipogenesis via regulation of the proadipogenic PPARγ axis. These results establish LMO3 as a regulator of human adipogenesis and could contribute a mechanism resulting in visceral-fat accumulation in obesity due to excess glucocorticoids.

Context: Recent data suggest that circulating retinol-binding protein (RBP) might be involved in the pathogenesis of insulin resistance. Moreover, protein C inhibitor (PCI), which specifically binds retinoic acid, was found to be increased in myocardial infarction survivors who are also insulin resistant. Objective: The objective of this study was to investigate the association of insulin resistance with RBP factors and PCI active antigen. Design and Setting: This was a clinical study. Patients: Nondiabetic humans with high (IS; n = 20, 14 females, six males, aged 47.2 ± 1.9 yr, body mass index 26 ± 1 kg/m2) and low (IR; n = 20, 14 females, six males, aged 45.5 ± 1.7 yr, body mass index 28 ± 1 kg/m2) insulin-stimulated glucose-disposal (M) participated in this study. Main Outcome Measures: M was measured by 2-h hyperinsulinemic (40 mU·min−1·m−2)-isoglycemic clamp tests. Measurements of RBP were performed using a nephelometric method and validated using quantitative Western blotting. Results: M (80–120 min) was higher in IS (10.9 ± 0.6 mg·min−1·kg−1) than IR (4.0 ± 0.2; P < 10−12). Fasting plasma RBP concentrations were comparable between IS and IR measured by both nephelometry (IS: 4.4 ± 0.3; IR: 4.6 ± 0.3 mg/dl, P = 0.6) and quantitative Western blot (IS 7.9 ± 0.5, IR 8.3 ± 0.6 mg/dl; P = 0.6). Fasting plasma PCI active antigen was similar in both groups. Plasma RBP and PCI were not significantly related to M. RBP was positively correlated with uric acid (r = 0.488, P = 0.003), triglycerides (r = 0.592, P < 0.001), prealbumin (r = 0.63, P < 0.0001), and vitamin A (r = 0.75, P < 10−6). Conclusions: Our data demonstrate that healthy, insulin-resistant humans do not show altered plasma retinol binding factors, such as RBP and PCI. Both do not significantly correlate with insulin sensitivity. Thus, our findings do not support the hypothesis of insulin sensitivity modulation by proteins involved in retinol transport.

OBJECTIVE: T cells and level of the cytokine interferon-γ (IFN-γ) are increased in adipose tissue in obesity. Hedgehog (Hh) signaling has been shown to potently inhibit white adipocyte differentiation. In light of recent findings in neurons that IFN-γ and Hh signaling cross-talk, we examined their potential interaction in the context of adipogenesis. RESEARCH DESIGN AND METHODS: We used Hh reporter cells, cell lines, and primary adipocyte differentiation models to explore costimulation of IFN-γ and Hh signaling. Genetic dissection using Ifngr1−/− and Stat1−/− mouse embryonic fibroblasts, and ultimately, anti–IFN-γ neutralization and expression profiling in obese mice and humans, respectively, were used to place the findings into the in vivo context. RESULTS: T-cell supernatants directly inhibited hedgehog signaling in reporter and 3T3-L1 cells. Intriguingly, using blocking antibodies, Ifngr1−/− and Stat1−/− cells, and simultaneous activation of Hh and IFN-γ signaling, we showed that IFN-γ directly suppresses Hh stimulation, thus rescuing adipogenesis. We confirmed our findings using primary mouse and primary human (pre)adipocytes. Importantly, robust opposing signals for Hh and T-cell pathways in obese human adipose expression profiles and IFN-γ depletion in mice identify the system as intact in adipose tissue in vivo. CONCLUSIONS: These results identify a novel antagonistic cross-talk between IFN-γ and Hh signaling in white adipose tissue and demonstrate IFN-γ as a potent inhibitor of Hh signaling.

Peroxisome proliferator–activated receptor-γ coactivator-1 (PPARGC1/PGC-1) is a transcriptional coactivator of nuclear hormone receptors implicated in blood pressure regulation. We therefore ascertained whether the PPARGC1 gene locus is associated with hypertension. We studied associations of 3 polymorphisms in PPARGC1 transcripts with hypertension in 683 middle-aged men and 530 middle-aged women of a cross-sectional Austrian population. Hypertension was defined by average values of systolic or diastolic ambulatory blood pressure readings (taken between 7 am and 10 pm ) above 140 and/or 90 and/or use of antihypertensive medication. Among the 3 polymorphic sites, genotype distributions associated with Gly482Ser differed by hypertension status in men ( P =0.0038), but not in women. The less common Ser482 allele was associated with a modest, but significant, reduction in the prevalence of hypertension in men. The distribution of 3 loci haplotypes also differed in men with and without hypertension ( P =0.015). Despite its moderate effect, but because of its high frequency (≈64%), the more common risk allele contributed to hypertension in 35% (95% CI 16% to 54%) of our male population. These results suggest, but do not prove, that PPARGC1 participates in blood pressure control, and sequence substitutions at its gene locus confer an increased risk of hypertension to a substantial proportion of men.