Insulin resistance (IR) in skeletal muscle is a key defect mediating the link between obesity and type 2 diabetes, a disease that typically affects people in later life. Sarcopenia (age-related loss of muscle mass and quality) is a risk factor for a number of frailty-related conditions that occur in the elderly. In addition, a syndrome of ‘sarcopenic obesity’ (SO) is now increasingly recognised, which is common in older people and is applied to individuals that simultaneously show obesity, IR and sarcopenia. Such individuals are at an increased risk of adverse health events compared with those who are obese or sarcopenic alone. However, there are no licenced treatments for sarcopenia or SO, the syndrome is poorly defined clinically and the mechanisms that might explain a common aetiology are not yet well characterised. In this review, we detail the nature and extent of the clinical syndrome, highlight some of the key physiological processes that are dysregulated and discuss some candidate molecular pathways that could be implicated in both metabolic and anabolic defects in skeletal muscle, with an eye towards future therapeutic options. In particular, the potential roles of Akt/mammalian target of rapamycin signalling, AMP-activated protein kinase, myostatin, urocortins and vitamin D are discussed.
Mark E Cleasby, Pauline M Jamieson, and Philip J Atherton
Nele Cielen, Nele Heulens, Karen Maes, Geert Carmeliet, Chantal Mathieu, Wim Janssens, and Ghislaine Gayan-Ramirez
Chronic obstructive pulmonary disease (COPD) is associated with skeletal muscle dysfunction. Vitamin D plays an important role in muscle strength and performance in healthy individuals. Vitamin D deficiency is highly prevalent in COPD, but its role in skeletal muscle dysfunction remains unclear. We examined the time-course effect of vitamin D deficiency on limb muscle function in mice with normal or deficient vitamin D serum levels exposed to air or cigarette smoke for 6, 12 or 18 weeks. The synergy of smoking and vitamin D deficiency increased lung inflammation and lung compliance from 6 weeks on with highest emphysema scores observed at 18 weeks. Smoking reduced body and muscle mass of the soleus and extensor digitorum longus (EDL), but did not affect contractility, despite type II atrophy. Vitamin D deficiency did not alter muscle mass but reduced muscle force over time, downregulated vitamin D receptor expression, and increased muscle lipid peroxidation but did not alter actin and myosin expression, fiber dimensions or twitch relaxation time. The combined effect of smoking and vitamin D deficiency did not further deteriorate muscle function but worsened soleus mass loss and EDL fiber atrophy at 18 weeks. We conclude that the synergy of smoking and vitamin D deficiency in contrast to its effect on lung disease, had different, independent but important noxious effects on skeletal muscles in a mouse model of mild COPD.
Virginia Rider, Alex Talbott, Anuradha Bhusri, Zach Krumsick, Sierra Foster, Joshua Wormington, and Bruce F Kimler
Preparation of mammalian uterus for embryo implantation requires a precise sequence of cell proliferation. In rodent uterus, estradiol stimulates proliferation of epithelial cells. Progesterone operates as a molecular switch and redirects proliferation to the stroma by down-regulating glycogen synthase kinase-3β (GSK-3β) and stimulating β-catenin accumulation in the periluminal stromal cells. In this study, the WNT signal involved in the progesterone-dependent proliferative switch was investigated. Transcripts of four candidate Wnt genes were measured in the uteri from ovariectomized (OVX) rats, progesterone-pretreated (3 days of progesterone, 2mg/daily) rats, and progesterone-pretreated rats given a single dose (0.2µg) of estradiol. The spatial distribution of the WNT proteins was determined in the uteri after the same treatments. Wnt5a increased in response to progesterone and the protein emerged in the periluminal stromal cells of progesterone-pretreated rat uteri. To investigate whether WNT5A was required for proliferation, uterine stromal cell lines were stimulated with progesterone (1µM) and fibroblast growth factor (FGF, 50ng/mL). Proliferating stromal cells expressed a two-fold increase in WNT5A protein at 12h post stimulation. Stimulated stromal cells were cultured with actinomycin D (25µg/mL) to inhibit new RNA synthesis. Relative Wnt5a expression increased at 4 and 6 h of culture, suggesting that progesterone plus FGF preferentially increased Wnt5a mRNA stability. Knockdown of Wnt5a in uterine stromal cell lines inhibited stromal cell proliferation and decreased Wnt5a mRNA. The results indicate that progesterone initiates and synchronizes uterine stromal cell proliferation by increasing WNT5A expression and signaling.
Nami Kim, Jung Ok Lee, Hye Jeong Lee, Yong Woo Lee, Hyung Ip Kim, Su Jin Kim, Sun Hwa Park, Chul Su Lee, Sun Woo Ryoo, Geum-Sook Hwang, and Hyeon Soo Kim
Isoeugenol exerts various beneficial effects on human health. However, the mechanisms underlying these effects are poorly understood. In this study, we observed that isoeugenol activated AMP-activated protein kinase (AMPK) and increased glucose uptake in rat L6 myotubes. Isoeugenol-induced increase in intracellular calcium concentration and glucose uptake was inhibited by STO-609, an inhibitor of calcium/calmodulin-dependent protein kinase kinase (CaMKK). Isoeugenol also increased the phosphorylation of protein kinase C-α (PKCα). Chelation of calcium with BAPTA-AM blocked isoeugenol-induced AMPK phosphorylation and glucose uptake. Isoeugenol stimulated p38MAPK phosphorylation that was inhibited after pretreatment with compound C, an AMPK inhibitor. Isoeugenol also increased glucose transporter type 4 (GLUT4) expression and its translocation to the plasma membrane. GLUT4 translocation was not observed after the inhibition of AMPK and CaMKK. In addition, isoeugenol activated the Akt substrate 160 (AS160) pathway, which is downstream of the p38MAPK pathway. Knockdown of the gene encoding AS160 inhibited isoeugenol-induced glucose uptake. Together, these results indicate that isoeugenol exerts beneficial health effects by activating the AMPK/p38MAPK/AS160 pathways in skeletal muscle.
Saeed Alshahrani, Mohammed Mashari Almutairi, Shams Kursan, Eduardo Dias-Junior, Mohamed Mahmoud Almiahuob, Lydia Aguilar-Bryan, and Mauricio Di Fulvio
The products of the Slc12a1 and Slc12a2 genes, commonly known as Na+-dependent K+2Cl− co-transporters NKCC2 and NKCC1, respectively, are the targets for the diuretic bumetanide. NKCCs are implicated in the regulation of intracellular chloride concentration ([Cl−]i) in pancreatic β-cells, and as such, they may play a role in glucose-stimulated plasma membrane depolarization and insulin secretion. Unexpectedly, permanent elimination of NKCC1 does not preclude insulin secretion, an event potentially linked to the homeostatic regulation of additional Cl− transporters expressed in β-cells. In this report we provide evidence for such a mechanism. Mice lacking a single allele of Slc12a2 exhibit lower fasting glycemia, increased acute insulin response (AIR) and lower blood glucose levels 15–30 min after a glucose load when compared to mice harboring both alleles of the gene. Furthermore, heterozygous expression or complete absence of Slc12a2 associates with increased NKCC2 protein expression in rodent pancreatic β-cells. This has been confirmed by using chronic pharmacological down-regulation of NKCC1 with bumetanide in the mouse MIN6 β-cell line or permanent molecular silencing of NKCC1 in COS7 cells, which results in increased NKCC2 expression. Furthermore, MIN6 cells chronically pretreated with bumetanide exhibit increased initial rates of Cl− uptake while preserving glucose-stimulated insulin secretion. Together, our results suggest that NKCCs are involved in insulin secretion and that a single Slc12a2 allele may protect β-cells from failure due to increased homeostatic expression of Slc12a1.
Nadia Schoenmakers, Kyriaki S Alatzoglou, V Krishna Chatterjee, and Mehul T Dattani
Central congenital hypothyroidism (CCH) may occur in isolation, or more frequently in combination with additional pituitary hormone deficits with or without associated extrapituitary abnormalities. Although uncommon, it may be more prevalent than previously thought, affecting up to 1:16 000 neonates in the Netherlands. Since TSH is not elevated, CCH will evade diagnosis in primary, TSH-based, CH screening programs and delayed detection may result in neurodevelopmental delay due to untreated neonatal hypothyroidism. Alternatively, coexisting growth hormones or ACTH deficiency may pose additional risks, such as life threatening hypoglycaemia. Genetic ascertainment is possible in a minority of cases and reveals mutations in genes controlling the TSH biosynthetic pathway (TSHB, TRHR, IGSF1) in isolated TSH deficiency, or early (HESX1, LHX3, LHX4, SOX3, OTX2) or late (PROP1, POU1F1) pituitary transcription factors in combined hormone deficits. Since TSH cannot be used as an indicator of euthyroidism, adequacy of treatment can be difficult to monitor due to a paucity of alternative biomarkers. This review will summarize the normal physiology of pituitary development and the hypothalamic–pituitary–thyroid axis, then describe known genetic causes of isolated central hypothyroidism and combined pituitary hormone deficits associated with TSH deficiency. Difficulties in diagnosis and management of these conditions will then be discussed.
David R Grattan
The hypothalamic control of prolactin secretion is different from other anterior pituitary hormones, in that it is predominantly inhibitory, by means of dopamine from the tuberoinfundibular dopamine neurons. In addition, prolactin does not have an endocrine target tissue, and therefore lacks the classical feedback pathway to regulate its secretion. Instead, it is regulated by short loop feedback, whereby prolactin itself acts in the brain to stimulate production of dopamine and thereby inhibit its own secretion. Finally, despite its relatively simple name, prolactin has a broad range of functions in the body, in addition to its defining role in promoting lactation. As such, the hypothalamo-prolactin axis has many characteristics that are quite distinct from other hypothalamo-pituitary systems. This review will provide a brief overview of our current understanding of the neuroendocrine control of prolactin secretion, in particular focusing on the plasticity evident in this system, which keeps prolactin secretion at low levels most of the time, but enables extended periods of hyperprolactinemia when necessary for lactation. Key prolactin functions beyond milk production will be discussed, particularly focusing on the role of prolactin in inducing adaptive responses in multiple different systems to facilitate lactation, and the consequences if prolactin action is impaired. A feature of this pleiotropic activity is that functions that may be adaptive in the lactating state might be maladaptive if prolactin levels are elevated inappropriately. Overall, my goal is to give a flavour of both the history and current state of the field of prolactin neuroendocrinology, and identify some exciting new areas of research development.
Farhana Naznin, Koji Toshinai, T M Zaved Waise, Cherl NamKoong, Abu Saleh Md Moin, Hideyuki Sakoda, and Masamitsu Nakazato
Ghrelin, a stomach-derived orexigenic peptide, transmits starvation signals to the hypothalamus via the vagus afferent nerve. Peripheral administration of ghrelin does not induce food intake in high fat diet (HFD)-induced obese mice. We investigated whether this ghrelin resistance was caused by dysfunction of the vagus afferent pathway. Administration (s.c.) of ghrelin did not induce food intake, suppression of oxygen consumption, electrical activity of the vagal afferent nerve, phosphorylation of ERK2 and AMP-activated protein kinase alpha in the nodose ganglion, or Fos expression in hypothalamic arcuate nucleus of mice fed a HFD for 12 weeks. Administration of anti-ghrelin IgG did not induce suppression of food intake in HFD-fed mice. Expression levels of ghrelin receptor mRNA in the nodose ganglion and hypothalamus of HFD-fed mice were reduced. Inflammatory responses, including upregulation of macrophage/microglia markers and inflammatory cytokines, occurred in the nodose ganglion and hypothalamus of HFD-fed mice. A HFD blunted ghrelin signaling in the nodose ganglion via a mechanism involving in situ activation of inflammation. These results indicate that ghrelin resistance in the obese state may be caused by dysregulation of ghrelin signaling via the vagal afferent.
Kunihisa Hamano, Yuko Nakagawa, Yoshiaki Ohtsu, Longfei Li, Johan Medina, Yuji Tanaka, Katsuyoshi Masuda, Mitsuhisa Komatsu, and Itaru Kojima
Glucose activates the glucose-sensing receptor T1R3 and facilitates its own metabolism in pancreatic β-cells. An inhibitor of this receptor would be helpful in elucidating the physiological function of the glucose-sensing receptor. The present study was conducted to examine whether or not lactisole can be used as an inhibitor of the glucose-sensing receptor. In MIN6 cells, in a dose-dependent manner, lactisole inhibited insulin secretion induced by sweeteners, acesulfame-K, sucralose and glycyrrhizin. The IC50 was ∼4 mmol/l. Lactisole attenuated the elevation of cytoplasmic Ca2 + concentration ([Ca2 +]c) evoked by sucralose and acesulfame-K but did not affect the elevation of intracellular cAMP concentration ([cAMP]c) induced by these sweeteners. Lactisole also inhibited the action of glucose in MIN6 cells. Thus, lactisole significantly reduced elevations of intracellular [NADH] and intracellular [ATP] induced by glucose, and also inhibited glucose-induced insulin secretion. To further examine the effect of lactisole on T1R3, we prepared HEK293 cells stably expressing mouse T1R3. In these cells, sucralose elevated both [Ca2 +]c and [cAMP]c. Lactisole attenuated the sucralose-induced increase in [Ca2 +]c but did not affect the elevation of [cAMP]c. Finally, lactisole inhibited insulin secretion induced by a high concentration of glucose in mouse islets. These results indicate that the mouse glucose-sensing receptor was inhibited by lactisole. Lactisole may be useful in assessing the role of the glucose-sensing receptor in mouse pancreatic β-cells.
Corinne Caillaud, Mie Mechta, Heidi Ainge, Andreas N Madsen, Patricia Ruell, Emilie Mas, Catherine Bisbal, Jacques Mercier, Stephen Twigg, Trevor A Mori, David Simar, and Romain Barrès
Erythropoietin (EPO) ameliorates glucose metabolism through mechanisms not fully understood. In this study, we investigated the effect of EPO on glucose metabolism and insulin signaling in skeletal muscle. A 2-week EPO treatment of rats fed with a high-fat diet (HFD) improved fasting glucose levels and glucose tolerance, without altering total body weight or retroperitoneal fat mass. Concomitantly, EPO partially rescued insulin-stimulated AKT activation, reduced markers of oxidative stress, and restored heat-shock protein 72 expression in soleus muscles from HFD-fed rats. Incubation of skeletal muscle cell cultures with EPO failed to induce AKT phosphorylation and had no effect on glucose uptake or glycogen synthesis. We found that the EPO receptor gene was expressed in myotubes, but was undetectable in soleus. Together, our results indicate that EPO treatment improves glucose tolerance but does not directly activate the phosphorylation of AKT in muscle cells. We propose that the reduced systemic inflammation or oxidative stress that we observed after treatment with EPO could contribute to the improvement of whole-body glucose metabolism.