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ABSTRACT

Corticosteroids have profound effects on bone metabolism, though the underlying mechanisms remain unclear. They are also known to alter glucose metabolism, in part by induction of insulin resistance. To determine whether corticosteroids impair glucose metabolism in bone cells, we have examined the actions of dexamethasone (DEX) on glucose transport and insulin receptor expression using osteoblast-like UMR 106-01 cells. DEX was shown to inhibit basal 2-deoxyglucose uptake by up to 30% in a time- and dose-dependent manner. It inhibited insulin-stimulated glucose transport by 13%. By Northern and Western blot analysis, DEX was shown to stimulate insulin receptor mRNA and protein by up to 5·6-fold, but it had no effect on expression of the glucose transporter GLUT 1 mRNA or protein under basal conditions. However, DEX augmented insulin-stimulated GLUT 1 mRNA and protein levels. By Scatchard analysis of labelled insulin binding, DEX increased insulin receptor number per cell by 54%. Subcellular fractionation and Western blot analysis demonstrated that DEX caused a redistribution of immunoreactive GLUT 1 from plasma membrane to intracellular microsomes, resulting in a 21% decrease in GLUT 1 at the plasma membrane. These data suggest that (i) DEX impairs basal glucose transport by post-translational mechanisms in UMR 106-01 cells, (ii) DEX increases insulin receptor mRNA, protein and insulin binding and (iii) the inhibition of glucose transport by DEX dominates its effects on the insulin receptor. It is possible that DEX inhibition of glucose transport in osteoblasts may contribute to steroid-induced osteoporosis.

ABSTRACT

This study characterizes the actions of insulin and parathyroid hormone (PTH) on the glucose transport system in the rat osteogenic sarcoma cell line UMR 106–01, which expresses a number of features of the osteoblast phenotype. Using [1,2-³H]2-deoxyglucose (2-DOG) as a label, UMR 106–01 cells were shown to possess a glucose transport system which was enhanced by insulin. In contrast, PTH influenced glucose transport in a biphasic manner with a stimulatory effect at 1 h and a more potent inhibitory effect at 16 h on basal and insulin-stimulated 2-DOG transport. To explore the mechanism of PTH action, a direct agonist of cAMP-dependent protein kinase (PKA) was tested. 8-Bromo-cAMP had no acute stimulatory effect but inhibited basal and insulin-stimulated 2-DOG transport at 16 h. This result suggested that the prolonged, but not the acute, effect of PTH was mediated by the generation of cAMP. Further studies with the cell line UMR 4–7, a UMR 106–01 clone stably transfected with an inducible mutant inactive regulatory subunit of PKA, confirmed that the inhibitory but not the stimulatory effect of PTH was mediated by the PKA pathway. Northern blot data indicated that the prolonged inhibitory effects of PTH and 8-bromo-cAMP on glucose transport were likely to be mediated in part by reduction in the levels of GLUT1 (HepG2/brain glucose transporter) mRNA.

ABSTRACT

Rats were maintained in chambers and breathed air (control, n=8) or an atmosphere containing 10% oxygen (hypoxic, n=10) for 35 days. On completion of the experiment the hypoxic animals weighed less than the controls (hypoxic, 290 ± 11.7g; control, 339 ± 19.2g; means ± S.E.M., p<0.05). No differences in the left ventricular weights were found between groups but the right ventricular weights were greater in the hypoxic rats (hypoxic, 0.39 ± 0.02g; control, 0.27 ± 0.08g; p<0.01). The amount of mRNA for IGF-I in the ventricles was quantified by Northern blot analysis. There was no difference between groups in IGF-I mRNA levels in the left ventricles (hypoxic, 1.07 ± 0.41 absorbance units (AU); control, 0.73 ± 0.33 AU). In the right ventricles, IGF-I mRNA was greater in hypoxic than in control rats (hypoxic, 2.37 ± 0.75 AU; control, 0.64 ± 0.11 AU; p<0.05). This study demonstrates that expression of IGF-I mRNA is increased in the hypertrophied right ventricle of hypoxic rats; IGF-I may play a central role in the initiation and maintenance of this process.

ABSTRACT

Thyroid hormones are essential for the normal growth and development of many tissues. In the rat, hypothyroidism is associated with growth impairment, and hyperthyroidism with the development of a hypercatabolic state and skeletal muscle wasting but, paradoxically, cardiac hypertrophy. The mechanism by which thyroid hormone produces cardiac hypertrophy and myosin isoenzyme changes remains unclear. The role of IGF-I, an anabolic hormone with both paracrine and endocrine actions, in producing cardiac hypertrophy was investigated during this study in hyperthyroid, hypothyroid and control rats. A treated hypothyroid group was also included in order to assess the effect of acute normalization of thyroid function.

Body weight was significantly lower in the hyperthyroid (mean±s.e.m.; 535·5±24·9 g, P<0·05), hypothyroid (245·3±9·8 g, P<0·001) and treated hypothyroid (265·3±9·8 g, P<0·001) animals when compared with controls (618·5±28·6 g). Heart weight/body weight ratios were, however, significantly increased in the hyperthyroid (2·74 ± 0·11×10⁻³, P<0·01) and treated hypothyroid (2·87±0·07 ×10⁻³, P<0·001) animals when compared with controls (2·26±0·03 × 10⁻³). Serum IGF-I concentrations were similar in the control and hyperthyroid rats (0·91±0·07 vs 0·78±0·04 U/ml, P=0·26), but bioactivity was reduced by 70% in hyperthyroid serum, suggesting a circulating inhibitor of IGF. Serum IGF-I levels (0·12±0·03 U/ml, P<0·001) and bioactivity (0·12±0·04 U/ml, P<0·001) were significantly lower in the hypothyroid group. Liver IGF-I mRNA levels were not statistically different in the control and hyperthyroid animals, but were significantly reduced in the hypothyroid animals (P<0·05 vs control). Heart IGF-I mRNA levels were similar in the control and hypothyroid rats, but were significantly increased in the hyperthyroid and treated hypothyroid animals (increased by 32% in hyperthyroidism, P<0·05; increased by 57% in treated hypothyroidism, P<0·01). Cardiac IGF-I was significantly elevated in hyperthyroidism (0·16±0·01 U/mg heart tissue, P<0·01), was low in hypothyroidism (0·08±0·01 U/mg, P<0·01) and was normalized in the treated hypothyroid group (0·11 ± 0·01 U/mg vs control, 0·13±0·01 U/mg).

Low body mass during both hypothyroidism and hyperthyroidism is therefore associated with reduced systemic IGF bioactivity. In hypothyroidism there is a primary defect in the endocrine function of IGF-I, while in hyperthyroidism serum IGF bioactivity is reduced in the presence of normal endocrine production of this anabolic hormone. In contrast, the paracrine actions of IGF-I are increased in the heart during hyperthyroidism, and this hormone appears to play a part in the development of hyperthyroid cardiac hypertrophy.

ABSTRACT

There is evidence that the hormonal control of hepatic IGF-I production is mediated by GH and insulin. To elucidate the role of these hormones further we administered s.c. or i.p. insulin (at 2·5 and 5·0 IU/day) and/or GH (0·8 IU/day) to rats made diabetic with streptozotocin 16 days previously. Hepatic IGF-I production was then assessed by quantifying hepatic IGF-I mRNA levels by autoradiography of Northern blots. Diabetes resulted in a fivefold reduction in hepatic IGF-I mRNA levels (optical density (OD) of the 0·7–1·1 kb band: controls, 1·3±0·09; diabetics, 0·28±0·08; P<0·01), which was not significantly changed by treatment with s.c. insulin (OD: low dose, 0·55±0·05; high dose, 0·58±0·05) or low dose i.p. insulin (OD: 0·40±0·03). High dose i.p. insulin enhanced hepatic IGF-I mRNA levels (OD: 0·93±0·23) compared with diabetic rats (P<0·01) and those given high dose s.c. insulin (P<0·04), despite the blood glucose values being similar in the treated groups (i.p., 4·72±0·29 mmol/l; s.c., 3·32±0·03 mmol/l). Administration of GH alone partially restored the hepatic IGF-I mRNA level (OD: GH-treated, 1·00±0·05; diabetic, 0·28±0·08; P<0·01), whilst having no effect on blood glucose values (diabetic, 36·35±0·45 mmol/l; GH-treated, 38·65±2·39 mmol/l). Additional administration of s.c. insulin completely restored IGF-I mRNA levels to those of controls (OD: low dose, 1·35±0·14; high dose, 1·27 ± 0·18). These observations indicate that insulin and GH are required for full expression of hepatic IGF-I mRNA and that insulin given i.p. is more potent than that given s.c. at stimulating hepatic synthesis of IGF-I.