Human thyrotropin (TSH), luteotropin (LH), follitropin (FSH), and chorionic gonadotropin are members of the heterodimeric glycoprotein hormone family. The common α subunit forms noncovalent heterodimers with different β subunits. Two novel human glycoprotein hormonelike genes, α2 (A2) and β5 (B5), recently have been identified. Using a yeast two-hybrid assay, the two subunits were found as potential heterodimerization partners. Immunological analyses confirmed the heterodimerization of A2 and B5 in transfected cells and their colocalization in the anterior pituitary. Recombinant A2/B5 heterodimeric glycoproteins, purified using cation exchange and size fractionation chromatography, activated human TSH receptors, but not LH and FSH receptors, and showed high affinity to TSH receptors in a radioligand receptor assay. The heterodimer also stimulated cAMP production and thymidine incorporation by cultured thyroid cells and increased serum thyroxine levels in TSH-suppressed rats in vivo. This new heterodimeric glycoprotein hormone was named as thyrostimulin based on its thyroid-stimulating activity. The expression of thyrostimulin in the anterior pituitary known to express TSH receptors suggested a paracrine mechanism. The present discovery of a new ligand based on genomic approaches could facilitate the understanding of the physiological roles of extra-thyroid TSH receptor systems and the structural-functional basis of receptor signaling by related glycoprotein hormones.
Koji Nakabayashi, Hirotaka Matsumi, Alka Bhalla, Jeehyeon Bae, Sietse Mosselman, Sheau Yu Hsu, Aaron J.W. Hsueh
Adiponectin, an adipocyte-derived hormone, was recently shown to have potential therapeutic applications in diabetes and obesity because of its influence on glucose and lipid metabolism. We found that brown fat in normal human bone marrow contains this protein and used marrow-derived preadipocyte lines and long-term cultures to explore potential roles in hematopoiesis. Recombinant adiponectin blocked fat cell formation in long-term bone marrow cultures and inhibited the differentiation of cloned stromal preadipocytes. Adiponectin also caused elevated expression of cyclooxygenase-2 (COX-2) by these stromal cells and induced release of prostaglandin E2 (PGE2). The COX-2 inhibitor Dup-697 prevented the inhibitory action of adiponectin on preadipocyte differentiation, suggesting involvement of stromal cell–derived prostanoids. Furthermore, adiponectin failed to block fat cell generation when bone marrow cells were derived from B6,129SPtgs2tm1Jed (COX-2+/–) mice. These observations show that preadipocytes represent direct targets for adiponectin action, establishing a paracrine negative feedback loop for fat regulation. They also link adiponectin to the COX-2–dependent PGs that are critical in this process.
Takafumi Yokota, C.S. Reddy Meka, Kay L. Medina, Hideya Igarashi, Phillip C. Comp, Masahiko Takahashi, Makoto Nishida, Kenji Oritani, Jun-ichiro Miyagawa, Tohru Funahashi, Yoshiaki Tomiyama, Yuji Matsuzawa, Paul W. Kincade
Recent studies have implicated fatty acid-dependent activation of the serine kinase IKKβ, which plays a key role in tissue inflammation, in the pathogenesis of insulin resistance. High doses of salicylates have recently been shown to inhibit IKKβ activity and might therefore ameliorate insulin resistance and improve glucose tolerance in patients with type 2 diabetes. To test this hypothesis, we studied nine type 2 diabetic subjects before and after 2 weeks of treatment with aspirin (∼7 g/d). Subjects underwent mixed-meal tolerance tests and hyperinsulinemic-euglycemic clamps with [6,6-2H2]glucose to assess glucose turnover before and after treatment. High-dose aspirin treatment resulted in a ∼25% reduction in fasting plasma glucose, associated with a ∼15% reduction in total cholesterol and C-reactive protein, a ∼50% reduction in triglycerides, and a ∼30% reduction in insulin clearance, despite no change in body weight. During a mixed-meal tolerance test, the areas under the curve for plasma glucose and fatty acid levels decreased by ∼20% and ∼50%, respectively. Aspirin treatment also resulted in a ∼20% reduction in basal rates of hepatic glucose production and a ∼20% improvement in insulin-stimulated peripheral glucose uptake under matched plasma insulin concentrations during the clamp. In conclusion, these data support the hypothesis that IKKβ represents a new target for treating type 2 diabetes mellitus.
Ripudaman S. Hundal, Kitt F. Petersen, Adam B. Mayerson, Pritpal S. Randhawa, Silvio Inzucchi, Steven E. Shoelson, Gerald I. Shulman
Lipodystrophy is a rare disorder that is characterized by selective loss of subcutaneous and visceral fat and is associated with hypertriglyceridemia, hepatomegaly, and disordered glucose metabolism. It has recently been shown that chronic leptin treatment ameliorates these abnormalities. Here we show that chronic leptin treatment improves insulin-stimulated hepatic and peripheral glucose metabolism in severely insulin-resistant lipodystrophic patients. This improvement in insulin action was associated with a marked reduction in hepatic and muscle triglyceride content. These data suggest that leptin may represent an important new therapy to reverse the severe hepatic and muscle insulin resistance and associated hepatic steatosis in patients with lipodystrophy.
Kitt Falk Petersen, Elif Arioglu Oral, Sylvie Dufour, Douglas Befroy, Charlotte Ariyan, Chunli Yu, Gary W. Cline, Alex M. DePaoli, Simeon I. Taylor, Phillip Gorden, Gerald I. Shulman
G protein–coupled receptors (GPCRs) play a key role in regulating bone remodeling. Whether GPCRs exert anabolic or catabolic osseous effects may be determined by the rate of receptor desensitization in osteoblasts. Receptor desensitization is largely mediated by direct phosphorylation of GPCR proteins by a family of enzymes termed GPCR kinases (GRKs). We have selectively manipulated GRK activity in osteoblasts in vitro and in vivo by overexpressing a GRK inhibitor. We found that expression of a GRK inhibitor enhanced parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor-stimulated cAMP generation and inhibited agonist-induced phosphorylation of this receptor in cell culture systems, consistent with attenuation of receptor desensitization. To determine the effect of GRK inhibition on bone formation in vivo, we targeted the expression of a GRK inhibitor to mature osteoblasts using the mouse osteocalcin gene 2 (OG2) promoter. Transgenic mice demonstrated enhanced bone remodeling as well as enhanced urinary excretion of the osteoclastic activity marker dexoypyridinoline. Both osteoprotegrin and OPG ligand mRNA levels were altered in calvaria of transgenic mice in a pattern that would promote osteoclast activation. The predominant effect of the transgene, however, was anabolic, as evidenced by an increase in bone density and trabecular bone volume in the transgenic mice compared with nontransgenic littermate controls.
Robert F. Spurney, Patrick J. Flannery, Sanford C. Garner, Krairerk Athirakul, Shiguang Liu, Farshid Guilak, L. Darryl Quarles
Patients with type 1 diabetes are identified after the onset of the disease, when β cell destruction is almost complete. β cell regeneration from islet cell precursors might reverse this disease, but factors that can induce β cell neogenesis and replication and prevent a new round of autoimmune destruction remain to be identified. Here we show that expression of IGF-I in β cells of transgenic mice (in both C57BL/6–SJL and CD-1 genetic backgrounds) counteracts cytotoxicity and insulitis after treatment with multiple low doses of streptozotocin (STZ). STZ-treated nontransgenic mice developed high hyperglycemia and hypoinsulinemia, lost body weight, and died. In contrast, STZ-treated C57BL/6–SJL transgenic mice showed mild hyperglycemia for about 1 month, after which they normalized glycemia and survived. After STZ treatment, all CD-1 mice developed high hyperglycemia, hypoinsulinemia, polydipsia, and polyphagia. However, STZ-treated CD-1 transgenic mice gradually normalized all metabolic parameters and survived. β cell mass increased in parallel as a result of neogenesis and β cell replication. Thus, our results indicate that local expression of IGF-I in β cells regenerates pancreatic islets and counteracts type 1 diabetes, suggesting that IGF-I gene transfer to the pancreas might be a suitable therapy for this disease.
Mónica George, Eduard Ayuso, Alba Casellas, Cristina Costa, Jean Christophe Devedjian, Fatima Bosch
Parathyroid hormone (PTH) is a potent pharmacologic inducer of new bone formation, but no physiologic anabolic effect of PTH on adult bone has been described. We investigated the role of PTH in fetal skeletal development by comparing newborn mice lacking either PTH, PTH-related peptide (PTHrP), or both peptides. PTH-deficient mice were dysmorphic but viable, whereas mice lacking PTHrP died at birth with dyschondroplasia. PTH-deficient mice uniquely demonstrated diminished cartilage matrix mineralization, decreased neovascularization with reduced expression of angiopoietin-1, and reduced metaphyseal osteoblasts and trabecular bone. Compound mutants displayed the combined cartilaginous and osseous defects of both single mutants. These results indicate that coordinated action of both PTH and PTHrP are required to achieve normal fetal skeletal morphogenesis, and they demonstrate an essential function for PTH at the cartilage-bone interface. The effect of PTH on fetal osteoblasts may be relevant to its postnatal anabolic effects on trabecular bone.
Dengshun Miao, Bin He, Andrew C. Karaplis, David Goltzman
The homeodomain transcription factor Pdx1 is required for pancreas development, including the differentiation and function of β cells. Mutations in Pdx1 or upstream hepatocyte nuclear factors cause autosomal forms of early-onset diabetes (maturity-onset diabetes of the young [MODY]). In mice, the Irs2 branch of the insulin/Igf signaling system mediates peripheral insulin action and pancreatic β cell growth and function. To investigate whether β cell failure in Irs2–/– mice might be related to dysfunction of MODY-related transcription factors, we measured the expression of Pdx1 in islets from young Irs2–/– mice. Before the onset of diabetes, Pdx1 was reduced in islets from Irs2–/– mice, whereas it was expressed normally in islets from wild-type or Irs1–/– mice, which do not develop diabetes. Whereas male Irs2–/–Pdx1+/+ mice developed diabetes between 8 and 10 weeks of age, haploinsufficiency for Pdx1 caused diabetes in newborn Irs2–/– mice. By contrast, transgenic expression of Pdx1 restored β cell mass and function in Irs2–/– mice and promoted glucose tolerance throughout life, as these mice survived for at least 20 months without diabetes. Our results suggest that dysregulation of Pdx1 might represent a common link between ordinary type 2 diabetes and MODY.
Jake A. Kushner, Jing Ye, Markus Schubert, Deborah J. Burks, Matthew A. Dow, Carrie L. Flint, Sanjoy Dutta, Christopher V.E. Wright, Marc R. Montminy, Morris F. White
Glucocorticoids depress bone formation by inhibiting osteoblastogenesis and increasing osteoblast apoptosis. However, the role of bone resorption in the initial rapid phase of bone loss characteristic of glucocorticoid-induced osteoporosis is unexplained, and the reason for the efficacy of bisphosphonates in this condition remains unknown. We report that in murine osteoclast cultures, glucocorticoids prolonged the baseline survival of osteoclasts and antagonized bisphosphonate-induced caspase activation and apoptosis by a glucocorticoid receptor–mediated action. Consistent with the in vitro evidence, in a murine model of glucocorticoid-induced osteoporosis, the number of cancellous osteoclasts increased, even though osteoclast progenitor number was reduced. Moreover, in mice receiving both glucocorticoids and bisphosphonates, the expected proapoptotic effect of bisphosphonates on osteoclasts was abrogated, as evidenced by maintenance of osteoclast numbers and, additionally, loss of bone density. In contrast, bisphosphonate administration prevented glucocorticoid-induced osteoblast apoptosis. These results indicate that the early loss of bone with glucocorticoid excess is caused by extension of the life span of pre-existing osteoclasts, an effect not preventable by bisphosphonates. Therefore, the early beneficial effects of these agents must be due, in part, to prolonging the life span of osteoblasts.
Robert S. Weinstein, Jin-Ran Chen, Cara C. Powers, Scott A. Stewart, Reid D. Landes, Teresita Bellido, Robert L. Jilka, A. Michael Parfitt, Stavros C. Manolagas
Acyl coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) is one of two known DGAT enzymes that catalyze the final step in mammalian triglyceride synthesis. DGAT1-deficient mice are resistant to diet-induced obesity through a mechanism involving increased energy expenditure. Here we show that these mice have decreased levels of tissue triglycerides, as well as increased sensitivity to insulin and to leptin. Importantly, DGAT1 deficiency protects against insulin resistance and obesity in agouti yellow mice, a model of severe leptin resistance. In contrast, DGAT1 deficiency did not affect energy and glucose metabolism in leptin-deficient (ob/ob) mice, possibly due in part to a compensatory upregulation of DGAT2 expression in the absence of leptin. Our results suggest that inhibition of DGAT1 may be useful in treating insulin resistance and leptin resistance in human obesity.
Hubert C. Chen, Steven J. Smith, Zuleika Ladha, Dalan R. Jensen, Luis D. Ferreira, Leslie K. Pulawa, James G. McGuire, Robert E. Pitas, Robert H. Eckel, Robert V. Farese Jr.