Insulin and glucose regulation of glycogen synthase in rat calvarial osteoblastlike cells

Bone Cell Bioenergetics and Skeletal Energy Homeostasis

The rising incidence of metabolic diseases worldwide has prompted renewed interest in the study of intermediary metabolism and cellular bioenergetics. The application of modern biochemical methods for quantitating fuel substrate metabolism with advanced mouse genetic approaches has greatly increased understanding of the mechanisms that integrate energy metabolism in the whole organism. Examination of the intermediary metabolism of skeletal cells has been sparked by a series of unanticipated observations in genetically modified mice that suggest the existence of novel endocrine pathways through which bone cells communicate their energy status to other centers of metabolic control. The recognition of this expanded role of the skeleton has in turn led to new lines of inquiry directed at defining the fuel requirements and bioenergetic properties of bone cells. This article provides a comprehensive review of historical and contemporary studies on the metabolic properties of bone cells and the mechanisms that control energy substrate utilization and bioenergetics. Special attention is devoted to identifying gaps in our current understanding of this new area of skeletal biology that will require additional research to better define the physiological significance of skeletal cell bioenergetics in human health and disease.

Morbid obesity attenuates the skeletal abnormalities associated with leptin deficiency in mice

Leptin-deficient ob/ob mice are morbidly obese and exhibit low total bone mass and mild osteopetrosis. In order to disassociate the skeletal effects of leptin deficiency from those associated with morbid obesity, we evaluated bone mass, architecture, gene expression, and indices of bone turnover in WT mice, ob/ob mice allowed to feed ad libitum (ob/ob), and ob/ob mice pair-fed equivalent to WT mice (pair-fed ob/ob). Mice were maintained at 32 °C (thermoneutral) from 6 to 18 weeks of age to minimize differences in resting energy expenditure. ob/ob mice were heavier, had more abdominal white adipose tissue (WAT), and were hyperglycemic compared with WT mice. Femur length, bone mineral content (BMC) and bone mineral density, and midshaft femur cortical thickness were lower in ob/ob mice than in WT mice. Cancellous bone volume (BV) fraction was higher but indices of bone formation and resorption were lower in ob/ob mice compared with WT mice; reduced bone resorption in ob/ob mice resulted in pathological retention of calcified cartilage. Pair-fed ob/ob mice were lighter and had lower WAT, uterine weight, and serum glucose than ob/ob mice. Similarly, femoral length, BMC, and cortical thickness were lower in pair-fed ob/ob mice compared with ob/ob mice, as were indices of cancellous bone formation and resorption. In contrast, bone marrow adiposity, calcified cartilage, and cancellous BV fraction were higher at one or more cancellous sites in pair-fed ob/ob mice compared with ob/ob mice. These findings indicate that the skeletal abnormalities caused by leptin deficiency are markedly attenuated in morbidly obese ob/ob mice.

Skeletal effects of sodium fluoride during hypokinesia

This study tested the capacity of fluoride (F) to prevent the disuse-associated reduction in bone formation/growth. Suspending young male Wistar rats by the tail for 2-2.5 weeks reduced femoral cortical (P less than 0.05) and trabecular (P less than 0.01) bone areas. Tetracycline labelling showed that the decrement in cortical area was largely due to a reduction in the percent periosteal mineralizing surface (PsMS). Periosteal mineral apposition rate (PsMAR) was not affected. Endosteal mineralizing surface (EsMS) and mineral apposition rate (EsMAR) were significantly stimulated spontaneously during the second week of suspension. F treatment (5 mg/kg/day i.p.) prevented the loss in bone area, and established a trend toward increased PsMS without affecting EsMS and EsMAR. None of these changes are associated with alterations in serum Ca, P or osteocalcin. F treatment in hypokinetic animals caused a decrease in serum PTH (-21% compared to control; P = 0.001). We conclude that F prevents the development of hypokinetic osteopenia in rats.

Glucose transport system in UMR-106-01 osteoblastic osteosarcoma cells: regulation by insulin

Insulin is a potent stimulator of collagen synthesis and other osteoblastic cell functions. In various insulin-sensitive tissues, stimulation of glucose transport and glycolytic metabolism are hallmarks of insulin action and may play a role in insulin regulation of cellular function. However, the effects of insulin on glucose metabolism in osteoblast-like cells have not been defined. We therefore characterized 2-deoxy-D-glucose (2-DG) transport in UMR-106-01 rat osteoblastic osteosarcoma cells and examined its regulation by insulin. 2-DG (0.1 mM) uptake was shown to be linear with time over 45 minutes, temperature-sensitive, and inhibited by phloridzin. Competitive inhibition studies against other hexoses demonstrated a transport system stereospecificity for 2-DG similar to that previously demonstrated in fat and muscle cells. Kinetic analysis of 15 minute 2-DG uptake at 25 degrees C demonstrated a saturable transport mechanism with a Km (1.9 mM) similar to that observed for 2-DG transport in other tissues. Insulin stimulated 2-DG transport in a dose-related manner, with significant stimulation observed at 0.5 nM and maximal effect observed at 50 nM insulin. The stimulatory effect of insulin was reversibly inhibited by cytochalasin B (50 microM). Insulin stimulation of 2-DG transport was associated with a 1.7-fold increase in Vmax, while Km remained constant. When insulin effects on glucose transport were inhibited by the addition of 5 mM phloridzin, stimulatory effects on DNA and collagen synthesis were diminished, suggesting that stimulation of glucose transport may play a role in insulin effects on replication and function in osteoblast-like cells.