The sodium-phosphate co-transporter NPT2a plays a key role in reabsorbing filtered phosphate in proximal renal tubules thereby critically contributing to phosphate homeostasis. Inadequate urinary phosphate excretion can lead to severe hyperphosphatemia as in tumoral calcinosis, and in chronic kidney disease (CKD). Pharmacological inhibition of NPT2a may therefore represent a novel approach for treating hyperphosphatemic conditions. The NPT2a-selective small molecule inhibitor, PF-06869206, was previously shown to reduce phosphate uptake in human proximal tubular cells in vitro. We now investigated the acute and chronic effects of the inhibitor in vivo and report that administration of PF-06869206 was well-tolerated and elicited a dose-dependent increase in fractional phosphate excretion. This phosphaturic effect lowered plasma phosphate levels in wild-type mice and in rats with CKD due to subtotal nephrectomy. PF-06869206 had no effect in Npt2a-null mice, but promoted phosphate excretion and reduced plasma phosphate in normophophatemic mice lacking Npt2c and in hyperphosphatemic mice lacking Fgf23 or Galnt3. In CKD rats, once daily administration of PF-06869206 for eight weeks induced an unabated acute phosphaturic and hypophosphatemic effect, but had no significant effect on FGF23 or PTH levels. Selective pharmacological inhibition of NPT2a thus holds promises as a novel therapeutic option for genetic and acquired hyperphosphatemic disorders.
Valerie Clerin, Hiroshi Saito, Kevin J. Filipski, An Hai Nguyen, Jeonifer Garren, Janka Kisucka, Monica Reyes, Harald Jüppner
Hypoxia can be defined as a relative deficiency in the amount of oxygen reaching the tissues. Hypoxia inducible factors (HIFs) are critical regulators of the mammalian response to hypoxia. In normal circumstances, HIF-1α protein turnover is rapid, and hyperglycemia further destabilizes the protein. In addition to their role in diabetes pathogenesis, HIFs are implicated in development of the microvascular and macrovascular complications of diabetes. Improving glucose control in people with diabetes increases HIF-1α protein and has wide-ranging benefits, some of which are at least partially mediated by HIF-1α. Despite this, most strategies to improve diabetes or its complications via regulating HIF-1α have not proven currently clinically useful. The intersection of HIF biology with diabetes is a complex area in which many further questions remain, especially around the well-conducted and clearly-described discrepant effects of different methods of increasing HIF-1α, even within the same tissues. This review will present a brief overview of HIFs, discuss the range of evidence implicating HIFs in β-cell dysfunction, diabetes pathogenesis, and diabetes complications, and examine the differing outcomes of HIF-targeting approaches in these conditions.
Jenny E. Gunton
Particulate matter < 2.5 micrometers (PM2.5) air pollution is the world’s leading environmental risk factor contributing to mortality through cardiometabolic pathways. In this study, we modeled early life exposure using chow-fed C57BL/6J male mice, exposed to real-world inhaled concentrated PM2.5 (~10 times ambient levels / ~60-120ug/m3) or filtered air over 14 weeks. We investigated PM2.5 effects on phenotype, transcriptome and chromatin accessibility, compared the effects with a prototypical high-fat diet (HFD) stimulus, and examined cessation of exposure on reversibility of phenotype. Exposure to PM2.5 impaired glucose and insulin tolerance, reduced energy expenditure and 18FDG-PET uptake in brown adipose tissue. Multiple differentially expressed gene (DEG) clusters in pathways involving metabolism and circadian rhythm were noted in insulin responsive tissues. Although the magnitude of transcriptional change seen with PM2.5 was lower than HFD, the degree of alteration in chromatin accessibility after PM2.5 exposure was significant. A novel chromatin remodeler SMARCA5 (SWI/SNF complex) was regulated in response to PM2.5 with cessation of exposure associated with reversal of insulin resistance, restoration of chromatin accessibility/nucleosome positioning near transcription start sites (TSS) and exposure induced changes in the transcriptome including SMARCA5, indicating pliable epigenetic control mechanisms following exposure cessation.
Sanjay Rajagopalan, Bongsoo Park, Rengasamy Palanivel, Vinesh Vinayachandran, Jeffrey A. Deiuliis, Roopesh Singh Gangwar, Lopa M. Das, Jinhu Yin, Youngshim Choi, Sadeer Al-Kindi, Mukesh K. Jain, Kasper D. Hansen, Shyam Biswal
Drivers of sporadic benign pituitary adenoma growth are largely unknown. Whole exome sequencing of 159 prospectively resected pituitary adenomas showed somatic copy number alteration (SCNA) rather than mutation is a hallmark of hormone-secreting adenomas and that SCNA correlate with adenoma phenotype. Using single-gene SCNA pathway analysis, we observed cAMP and Fanconi anemia DNA damage repair pathways both affected by SCNA in growth hormone (GH)-secreting somatotroph adenomas. As somatotroph differentiation and GH secretion is dependent on cAMP activation and we previously showed DNA damage, aneuploidy, and senescence in somatotroph adenomas, we studied links between cAMP signaling and DNA damage. Stimulation of cAMP in C57Bl/6 mouse primary pituitary cultures using forskolin or long-acting GH releasing hormone (GHRH) analogue increased GH production and DNA damage measured by phosphorylated H2AX and Comet assay. Octreotide, a somatostatin receptor ligand that targets somatotroph adenoma GH secretion in patients with acromegaly, inhibited cAMP and GH and reversed DNA damage induction. In vivo long-acting GHRH treatment also induced mouse pituitary DNA damage. We conclude that cAMP, which induces somatotroph proliferation and GH secretion, may concomitantly induce DNA damage, potentially linking hormone hypersecretion to SCNA and genome instability. These results elucidating somatotroph adenoma pathophysiology identify pathways for treatment targeting.
Anat Ben-Shlomo, Nan Deng, Evelyn Ding, Masaaki Yamamoto, Adam Mamelak, Vera Chesnokova, Artak Labadzhyan, Shlomo Melmed
Type 2 diabetes mellitus (T2DM) has become an expanding global public health problem. Although the glucocorticoid receptor (GR) is an important regulator of glucose metabolism, the relationship between circulating glucocorticoids (GCs) and the features of T2DM remains controversial. Here, we show that 17-hydroxyprogesterone (17-OHP), an intermediate steroid in the biosynthetic pathway that converts cholesterol to cortisol, binds to and stimulates the transcriptional activity of GR. Hepatic 17-OHP concentrations are increased in diabetic mice and patients due to aberrantly increased expression of Cyp17A1. Systemic administration of 17-OHP or overexpression of Cyp17A1 in the livers of lean mice promoted the pathogenesis of hyperglycemia and insulin resistance, whereas knockdown of Cyp17A1 abrogated metabolic disorders in obese mice. Therefore, our results identify a Cyp17A1/17-OHP/GR–dependent pathway in the liver that mediates obesity-induced hyperglycemia, suggesting that selectively targeting hepatic Cyp17A1 may provide a therapeutic avenue for treating T2DM.
Yan Lu, E Wang, Ying Chen, Bing Zhou, Jiejie Zhao, Liping Xiang, Yiling Qian, Jingjing Jiang, Lin Zhao, Xuelian Xiong, Zhiqiang Lu, Duojiao Wu, Bin Liu, Jing Yan, Rong Zhang, Huijie Zhang, Cheng Hu, Xiaoying Li
The identification of loss-of-function mutations in MKRN3 in patients with central precocious puberty (CPP) in association with the decrease in MKRN3 expression in the medial basal hypothalamus (MBH) of mice prior to the initiation of reproductive maturation suggest that MKRN3 is acting as a ‘brake’ on GnRH secretion during childhood. In the current study, we investigated the mechanism by which MKRN3 prevents premature manifestation of the pubertal process. We showed that, as in mice, MKRN3 expression is high in the hypothalamus of rats and nonhuman primates early in life, declining as puberty approaches, and is independent of sex steroid hormones. We demonstrated that Mkrn3 is expressed in Kiss1 neurons of the mouse hypothalamic arcuate nucleus (ARC) and that MKRN3 repressed promoter activity of human KISS1 and TAC3, two key stimulators of GnRH secretion. We further showed that MKRN3 has ubiquitinase activity, that this activity is reduced by MKRN3 mutations affecting the RING finger domain, and that these mutations compromised the ability of MKRN3 to repress KISS1 and TAC3 promoter activity. These results indicate that MKRN3 acts to prevent puberty initiation, at least in part, by repressing KISS1 and TAC3 transcription and that this action may involve a MKRN3-directed ubiquitination-mediated mechanism.
Ana Paula Abreu, Carlos A. Toro, Yong Bhum Song, Victor M Navarro, Martha A. Bosch, Aysegul Eren, Joy N. Liang, Rona S. Carroll, Ana Claudia Latronico, Oline K. Ronnekleiv, Carlos F Aylwin, Alejandro Lomniczi, Sergio R. Ojeda, Ursula B Kaiser
Diabetes, obesity and Alzheimer’s disease (AD) are associated with vascular complications and impaired nitric oxide (NO) production. Furthermore, increased β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1), APP and β-amyloid (Aβ) are linked with vascular disease development and raised BACE1 and Aβ accompany hyperglycemia and hyperlipidemia. However, the causal relationship between obesity and diabetes, raised Aβ and vascular dysfunction is unclear. We report that diet-induced obesity (DIO) in mice raised plasma and vascular Aβ42 that correlated with decreased NO bioavailability, endothelial dysfunction and raised blood pressure. Genetic or pharmacological reduction of BACE1 activity and Aβ42 prevented and reversed, respectively, these outcomes. In contrast, expression of human mutant APP in mice or Aβ42 infusion into control diet-fed mice to mimic obese levels impaired NO production, vascular relaxation and raised blood pressure. In humans, raised plasma Aβ42 correlated with diabetes and endothelial dysfunction. Mechanistically, higher Aβ42 reduced endothelial NO synthase (eNOS), cyclic GMP and protein kinase G (PKG) activity independently of diet whereas endothelin-1 was increased by diet and Aβ42. Lowering Aβ42 reversed the DIO deficit in the eNOS-cGMP-PKG pathway and decreased endothelin-1. Our findings suggest that BACE1 inhibitors may have therapeutic value in the treatment of vascular disease associated with diabetes.
Paul J. Meakin, Bethany M. Coull, Zofia Tuharska, Christopher McCaffery, Ioannis Akoumianakis, Charalambos Antoniades, Jane Brown, Kathryn J. Griffin, Fiona Platt, Claire H. Ozber, Nadira Y. Yuldasheva, Natallia Makava, Anna Skromna, Alan Russell Prescott, Alison D. McNeilly, Moneeza K. Siddiqui, Colin Neil Alexander Palmer, Faisel Khan, Michael LJ Ashford
Background. Insulin is a key regulator of metabolic function. The effects of excess adiposity, insulin resistance and hepatic steatosis on the complex integration of insulin secretion and hepatic and extrahepatic tissue extraction are not clear. Methods. A hyperinsulinemic-euglycemic clamp and a 3-hour oral glucose tolerance test were used to evaluate insulin sensitivity and insulin kinetics after glucose ingestion in three groups: i) lean with normal intrahepatic triglyceride (IHTG) and glucose tolerance (Lean-NL; n=14); ii) obese with normal IHTG and glucose tolerance (Obese-NL; n=24); and iii) obese with hepatic steatosis and prediabetes (Obese-NAFLD; n=22). Results. Insulin sensitivity progressively decreased and insulin secretion progressively increased from Lean-NL to Obese-NL to Obese-NAFLD. Fractional hepatic insulin extraction progressively decreased from Lean-NL to Obese-NL to Obese-NAFLD, whereas total hepatic insulin extraction (molar amount removed) was greater in Obese-NL and Obese-NAFLD than Lean-NL. Insulin appearance in the systemic circulation and extrahepatic insulin extraction progressively increased from Lean-NL to Obese-NL to Obese-NAFLD. Total hepatic insulin extraction plateaued at high rates of insulin delivery, whereas the relationship between systemic insulin appearance and total extrahepatic extraction was linear. Conclusion. Hyperinsulinemia after glucose ingestion in Obese-NL and Obese-NAFLD is due to an increase in insulin secretion, without a decrease in total hepatic or extrahepatic insulin extraction. However, the liver’s maximum capacity to remove insulin is limited because of a saturable extraction process. The increase in insulin delivery to the liver and extrahepatic tissues in Obese-NAFLD is unable to compensate for the increase in insulin resistance, resulting in impaired glucose homeostasis.
Gordon I. Smith, David C. Polidori, Mihoko Yoshino, Monica L. Kearney, Bruce W. Patterson, Bettina Mittendorfer, Samuel Klein
Background. Post-receptor insulin resistance (IR) is associated with hyperglycemia and hepatic steatosis. However, receptor-level IR (e.g. insulin receptor pathogenic variants, INSR) causes hyperglycemia without steatosis. We examined four pathologic conditions of IR in humans to examine pathways controlling lipid metabolism and gluconeogenesis. Methods. Cross-sectional study of severe, receptor IR (INSR, n=7), versus post-receptor IR that was severe (lipodystrophy, n=14), moderate (type 2 diabetes [T2D], n=9) or mild (obesity, n=8). Lipolysis (glycerol turnover), hepatic glucose production (HGP), gluconeogenesis (deuterium incorporation from body water into glucose), hepatic triglyceride (magnetic resonance spectroscopy), and hepatic fat oxidation (plasma β-hydroxybutyrate) were measured. Results. Lipolysis was 2-3-fold higher in INSR versus all other groups, and HGP 2-fold higher in INSR and lipodystrophy versus T2D and obesity (p<0.001) suggesting severe adipose and hepatic IR. INSR subjects had a higher contribution of gluconeogenesis to HGP, ~77%, versus 52-59% in other groups (p=0.0001). Despite high lipolysis, INSR subjects had low hepatic triglycerides (0.5 [0.1-0.5]), in contrast to lipodystrophy (10.6 [2.8-17.1], p<0.0001). β-hydroxybutyrate was 2-7-fold higher in INSR versus all other groups (p<0.0001) consistent with higher hepatic fat oxidation. Conclusion. These data support a key pathogenic role of adipose tissue IR to increase glycerol and FFA availability to the liver in both receptor and post-receptor IR. However, the fate of FFA diverges in these populations. In receptor-level IR, FFA oxidation drives gluconeogenesis rather than being reesterified to triglyceride. In contrast, in post-receptor IR, FFA contributes to both gluconeogenesis and hepatic steatosis. Trial registration. ClinicalTrials.gov NCT01778556; NCT00001987; NCT02457897 Funding. NIDDK, USDA ARS 58-3092-5-001
Hilal Sekizkardes, Stephanie T. Chung, Shaji Chacko, Morey Haymond, Megan Startzell, Mary Walter, Peter J. Walter, Marissa Lightbourne, Rebecca J. Brown
Meal ingestion increases body temperature in multiple species, an effect that is blunted by obesity. However, the mechanisms responsible for these phenomena remain incompletely understood. Here we show that refeeding increases plasma leptin concentrations approximately 8-fold in 48-hour-fasted lean rats, and this normalization of plasma leptin concentrations stimulates adrenomedullary catecholamine secretion. Increased adrenal medulla–derived plasma catecholamines were necessary and sufficient to increase body temperature postprandially, a process that required both fatty acids generated from adipose tissue lipolysis and β-adrenergic activation of brown adipose tissue (BAT). Diet-induced obese rats, which remained relatively hyperleptinemic while fasting, did not exhibit fasting-induced reductions in temperature. To examine the impact of feeding-induced increases in body temperature on energy balance, we compared rats fed chronically by either 2 carbohydrate-rich boluses daily or a continuous isocaloric intragastric infusion. Bolus feeding increased body temperature and reduced weight gain compared with continuous feeding, an effect abrogated by treatment with atenolol. In summary, these data demonstrate that leptin stimulates a hypothalamus–adrenal medulla–BAT axis, which is necessary and sufficient to induce lipolysis and, as a result, increase body temperature after refeeding.
Rachel J. Perry, Kun Lyu, Aviva Rabin-Court, Jianying Dong, Xiruo Li, Yunfan Yang, Hua Qing, Andrew Wang, Xiaoyong Yang, Gerald I. Shulman