Previous in vivo studies have shown that growth hormone (GH) affects vitamin D and mineral metabolism. Insulin-like growth factor-I (IGF-I) was recently reported to be a regulator of renal 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) production, suggesting that it mediates the effects of GH on vitamin D metabolism. However, there is no direct evidence to support this. The present study was designed to investigate the in vitro effects of GH and IGF-I on the renal production of 1,25-(OH)2D3 and 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3) in a pig kidney cell line, LLC-PK1. Confluent cells were preincubated in serum-free medium with hormone (GH or IGF-I) or vehicle, and then incubated with 25-[3H]OHD3. The levels of 1,25-[3H](OH)2D3 and 24,25-[3H](OH)2D3 produced were determined after lipid extraction and HPLC purification. Production of 1,25-(OH)2D3 and 24,25-(OH)2D3 was increased after both IGF-I and GH preincubation in a dose-dependent manner. Significant increases were found after preincubation with 13 nmol/l IGF-I (1,25-(OH)2D3, 1.8-fold: 24,25-(OH)2D3, 1.5-fold)or 0.9 or 9 nmol/lGH (1,25-(OH)2D3, 1.3-fold and 1.5-fold; 24.25-(OH)2D3, 1.4-fold and 1.5-fold respectively). Furthermore, the effect of 9 nmol/l GH on 1.25-(OH)2D3 and 24,25-(OH)2D3 production was blocked in the presence of IGF-I receptor monoclonal antibody. These results confirm that IGF-I acts on renal tubules, resulting in induction of 1,25-(OH)2D3 and 24,25-(OH)2D3 production, and the findings suggest that GH stimulates 1.25-(OH)2D3 and 24,2 5-(OH)2D3 production by increasing local IGF-I production in the kidney.
T Miura, W Suzuki, E Ishihara, I Arai, H Ishida, Y Seino and K Tanigawa
BACKGROUND: In skeletal muscle and adipocytes, insulin-stimulated glucose transport has been known to occur through the translocation of glucose transporter (GLUT) 4 from the intracellular pool to the plasma membrane. The Tsumura Suzuki obese diabetic (TSOD) mouse, a new genetic animal model of type 2 diabetes, develops moderate degrees of obesity and diabetes that are especially apparent in animals more than 11 weeks old. A defect in insulin stimulation of GLUT4 translocation also contributes to the characteristics of type 2 diabetes. OBJECTIVE: To characterize this mouse further, we examined the alteration in insulin-stimulated GLUT4 translocation in the skeletal muscle and adipose tissue. METHODS: For glucose and insulin tolerance tests, the mice were given glucose or insulin and blood samples were collected. After isolation of low-density microsomal membrane and plasma membrane from skeletal muscle and adipose tissue, insulin-stimulated translocation of GLUT4 in these TSOD mice was examined by Western blot. RESULTS AND CONCLUSIONS: TSOD mice showed a significant increase in blood glucose after the glucose load, and exhibited a significantly attenuated decrease in blood glucose concentrations after administration of insulin, compared with that in control Tsumura Suzuki non-obese (TSNO) mice. The insulin-stimulated translocation of GLUT4 from low-density microsomal membranes to plasma membrane was significantly reduced in both skeletal muscle and adipose tissue of TSOD mice. These results indicate that the reduced insulin sensitivity in diabetic TSOD mice is presumably due, at least in part, to the impaired GLUT4 translocation by insulin in both skeletal muscle and adipocytes.
H Tanaka, T Kubo, T Yamate, T Ono, S Kanzaki and Y Seino
OBJECTIVE: Although there are a few reports on GH therapy in achondroplasia, these were based on a small sample and/or short-term observation. To clarify the effectiveness of GH treatment on short stature in achondroplasia and hypochondroplasia, a long-term treatment study in a larger number of patients was performed. METHOD: Forty-two children (16 males and 26 females, age 3-14 years) with achondroplasia were examined in this study. Initially, we evaluated hypothalamic-pituitary function and point mutation analysis as previously reported. After the evaluation, the children were treated with GH for more than 2 years; then post-treatment growth velocity and body proportion parameters were determined. RESULTS: The 35 typical variants of our achondroplasia patients showed previously reported point mutation in the fibroblast growth factor receptor 3 gene. The annual height gain during GH therapy was significantly greater than that before therapy (3.9 +/- 1.0 cm/year before treatment vs 6.5 +/- 1.8 cm/year for the first year and 4.6 +/- 1.6 cm/year for the second year of treatment). The body disproportion had not been aggravated during the treatment period. CONCLUSION: We conclude that GH might be beneficial in the treatment of short stature in children with achondroplasia in the first 2 years of treatment.
K Tsukiyama, Y Yamada, K Miyawaki, A Hamasaki, K Nagashima, M Hosokawa, S Fujimoto, A Takahashi, K Toyoda, S Toyokuni, Y Oiso and Y Seino
OBJECTIVE: ATP-sensitive K(+) (K(ATP)) channels in pancreatic beta-cells are crucial in the regulation of glucose-induced insulin secretion. Recently, K(ATP) channel-deficient mice were generated by genetic disruption of Kir6.2, the pore-forming component of K(ATP) channels, but the mice still showed a significant insulin response after oral glucose loading in vivo. Gastric inhibitory polypeptide (GIP) is a physiological incretin that stimulates insulin release upon ingestion of nutrients. To determine if GIP is the insulinotropic factor in insulin secretion in K(ATP) channel-deficient mice, we generated double-knockout Kir6.2 and GIP receptor null mice and compared them with Kir6.2 knockout mice. METHODS: Double-knockout mice were generated by intercrossing Kir6.2-knockout mice with GIP receptor-knockout mice. An oral glucose tolerance test, insulin tolerance test and batch incubation study of pancreatic islets were performed on double-knockout mice and Kir6.2-knockout mice. RESULTS: Fasting glucose and insulin levels were similar in both groups. After oral glucose loading, blood glucose levels of double-knockout mice became elevated compared with Kir6.2-knockout mice, especially at 15 min (345+/-10 mg/dl vs 294+/-20 mg/dl, P<0.05) and 30 min (453+/-20 mg/dl vs 381+/-26 mg/dl, P<0.05). The insulin response was almost completely lost in double-knockout mice, although insulin secretion from isolated islets was stimulated by another incretin, glucagon-like peptide-1 in the double-knockout mice. Double-knockout mice and Kir6.2-knockout mice were similarly insulin sensitive as assessed by the insulin tolerance test. CONCLUSION: GIP is the major insulinotropic factor in the secretion of insulin in response to glucose load in K(ATP) channel-deficient mice.