Activation of the hypoxia-inducible factor (HIF) pathway reprograms energy metabolism. Hemoglobin (Hb) is the main carrier of oxygen. Using its normal variation as a surrogate measure for hypoxia, we explored whether lower Hb levels could lead to healthier metabolic profiles in mice and humans (n = 7175) and used Mendelian randomization (MR) to evaluate potential causality (n = 173,480). The results showed evidence for lower Hb levels being associated with lower body mass index, better glucose tolerance and other metabolic profiles, lower inflammatory load, and blood pressure. Expression of the key HIF target genes SLC2A4 and Slc2a1 in skeletal muscle and adipose tissue, respectively, associated with systolic blood pressure in MR analyses and body weight, liver weight, and adiposity in mice. Last, manipulation of murine Hb levels mediated changes to key metabolic parameters. In conclusion, low-end normal Hb levels may be favorable for metabolic health involving mild chronic activation of the HIF response.

Non-alcoholic fatty liver disease (NAFLD) parallels the global obesity epidemic with unmet therapeutic needs. We investigated whether inhibition of hypoxia-inducible factor prolyl 4-hydroxylase-2 (HIF-P4H-2), a key cellular oxygen sensor whose inhibition stabilizes HIF, would protect from NAFLD by subjecting HIF-P4H-2-deficient (Hif-p4h-2 gt/gt ) mice to a high-fat, highfructose (HFHF) or high-fat, methionine-choline-deficient (HF-MCD) diet. On both diets, the Hif-p4h-2 gt/gt mice gained less weight and had less white adipose tissue (WAT) and its inflammation, lower serum cholesterol levels, and lighter livers with less steatosis and lower serum ALT levels than the wild type (WT). The intake of fructose in majority of the Hif-p4h-2 gt/gt tissues, including the liver, was 15-35% less than in the WT. We found upregulation of the key fructose transporter and metabolizing enzyme mRNAs, Slc2a2, Khka, and Khkc, and higher ketohexokinase activity in the Hif-p4h-2 gt/gt small intestine relative to the WT, suggesting enhanced metabolism of fructose in the former. On the HF-MCD diet, the Hif-p4h-2 gt/gt mice showed more browning of the WAT and increased thermogenesis. A pharmacological pan-HIF-P4H inhibitor protected WT mice on both diets against obesity, metabolic dysfunction, and liver damage. These data suggest that HIF-P4H-2 inhibition could be studied as a novel, comprehensive treatment strategy for NAFLD. Key messages• HIF-P4H-2 inhibition enhances intestinal fructose metabolism protecting the liver.• HIF-P4H-2 inhibition downregulates hepatic lipogenesis.• Induced browning of WAT and increased thermogenesis can also mediate protection.• HIF-P4H-2 inhibition offers a novel, comprehensive treatment strategy for NAFLD.

The aim of this study was to cross-sectionally and longitudinally examine whether higher hemoglobin (Hb) levels within the normal variation associate with key components of metabolic syndrome and total and cardiovascular mortality. The study included 967 Finnish subjects (age 40–59 years) followed for ≥ 20 years. The focus was on Hb levels, cardiovascular diseases (CVDs) and mortality rates. Higher Hb levels associated positively with key anthropometric and metabolic parameters at baseline. At the follow-up similar associations were seen in men. The highest Hb quartile showed higher leptin levels and lower adiponectin levels at baseline and follow-up (p < 0.05) and lower plasma ghrelin levels at baseline (p < 0.05). Higher baseline Hb levels associated independently with prevalence of type 2 diabetes at follow-up (p < 0.01). The highest Hb quartile associated with higher serum alanine aminotransferase levels (p < 0.001) and independently with increased risk for liver fat accumulation (OR 1.63 [1.03; 2.57]) at baseline. The highest Hb quartile showed increased risk for total (HR = 1.48 [1.01; 2.16]) and CVD-related mortality (HR = 2.08 [1.01; 4.29]). Higher Hb levels associated with an adverse metabolic profile, increased prevalence of key components of metabolic syndrome and higher risk for CVD-related and total mortality.

We have previously demonstrated protection against obesity, metabolic dysfunction, atherosclerosis and cardiac ischemia in a hypoxia-inducible factor (HIF) prolyl 4-hydroxylase-2 (Hif-p4h-2) deficient mouse line, attributing these protective effects to activation of the hypoxia response pathway in a normoxic environment. We intended here to find out whether the Hif-p4h-2 deficiency affects the cardiac health of these mice upon aging. Methods and results: When the Hif-p4h-2 deficient mice and their wild-type littermates were monitored during normal aging, the Hif-p4h-2 deficient mice had better preserved diastolic function than the wild type at one year of age and less cardiomyocyte hypertrophy at two years. On the mRNA level, downregulation of hypertrophyassociated genes was detected and shown to be associated with upregulation of Notch signaling, and especially of the Notch target gene and transcriptional repressor Hairy and enhancer-of-split-related basic helix-loophelix (Hey2). Blocking of Notch signaling in cardiomyocytes isolated from Hif-p4h-2 deficient mice with a gamma-secretase inhibitor led to upregulation of the hypertrophy-associated genes. Also, targeting Hey2 in isolated wild-type rat neonatal cardiomyocytes with siRNA led to upregulation of hypertrophic genes and increased leucine incorporation indicative of increased protein synthesis and hypertrophy. Finally, oral treatment of wild-type mice with a small molecule inhibitor of HIF-P4Hs phenocopied the effects of Hif-p4h-2 deficiency with less cardiomyocyte hypertrophy, upregulation of Hey2 and downregulation of the hypertrophyassociated genes. Conclusions: These results indicate that activation of the hypoxia response pathway upregulates Notch signaling and its target Hey2 resulting in transcriptional repression of hypertrophy-associated genes and less cardiomyocyte hypertrophy. This is eventually associated with better preserved cardiac function upon aging. Activation of the hypoxia response pathway thus has therapeutic potential for combating age-induced cardiac hypertrophy.