Characterization of zinc effect to inhibit osteoclast-like cell formation in mouse marrow culture: interaction with dexamethasone

Zinc as a Therapeutic Agent in Bone Regeneration

Zinc is an essential mineral that is required for normal skeletal growth and bone homeostasis. Furthermore, zinc appears to be able to promote bone regeneration. However, the cellular and molecular pathways through which zinc promotes bone growth, homeostasis, and regeneration are poorly understood. Zinc can positively affect chondrocyte and osteoblast functions, while inhibiting osteoclast activity, consistent with a beneficial role for zinc in bone homeostasis and regeneration. Based on the effects of zinc on skeletal cell populations and the role of zinc in skeletal growth, therapeutic approaches using zinc to improve bone regeneration are being developed. This review focuses on the role of zinc in bone growth, homeostasis, and regeneration while providing an overview of the existing studies that use zinc as a bone regeneration therapeutic.

Long-term marginal zinc supply is not detrimental to the skeleton of aged female rats

In this experiment, we investigated the long-term effects of a marginal zinc (Zn) supply on bone metabolism in aged rats. Nine-mo-old female Fischer-344 rats were divided into 8 weight-matched groups of 8 rats each. All rats were adapted for 1 mo to restrictive feeding (7.5 g/d) of a purified diet containing 8 g/kg sodium phytate and 64 mg/kg Zn. Control rats were pair-fed throughout the experiment. During the 1-mo depletion phase, controls received the Zn-replete diet with 64 mg/kg Zn, whereas Zn-deficient rats were fed the same diet with 2.2 mg/kg Zn. The depletion phase was followed by a 3-mo marginal phase in which the rats fed the diet with 2.2 mg/kg Zn received an additional daily Zn supplement of 75 microg Zn/rat by gavage. In the following 2-mo repletion phase, a marginal group was switched to the Zn-replete diet, while the other groups were maintained on marginal Zn supply or on the Zn-replete diet. Zn depletion and marginal Zn reduced serum and bone Zn and serum alkaline phosphatase activity. Zn repletion normalized serum Zn. However, apart from subtle changes in bone mineralization density distribution, Zn deficiency was not associated with detrimental effects on bone mineral density, turnover, architecture, or biomechanics relative to control rats at any time point. Our data suggest that Zn does not play an essential role in bone metabolism in aged rats and cast doubt on the hypthosis that Zn deficiency is a risk factor for osteoporosis.

Expression of the zinc transporter ZIP1 in osteoclasts

Zinc has been previously demonstrated to be a potent inhibitor of osteoclastogenesis and osteoclast function. The mechanisms for cellular uptake of zinc into osteoclasts have not been characterized. We have corroborated previous studies on the reduction of osteoclastogenesis in the presence of extracellular zinc. We demonstrate that osteoclasts express a ubiquitous plasma membrane zinc transporter, namely ZIP1, which was diffusely distributed throughout the cytoplasm. Following an adenoviral-mediated overexpression of ZIP1 in murine osteoclasts, ZIP1 was predominantly colocalized with actin at the sealing zone and significantly inhibited osteoclast function, as assessed by resorptive activity. Finally, overexpression of ZIP1 negatively impacted NF-kappaB binding activity, as assessed by electrophoretic mobility shift assays. In conclusion, these data both corroborate previous studies on regulation of osteoclast formation and activity by zinc and reveal the presence of a zinc uptake mechanism that exerts an important effect on osteoclast activity.

Regulatory roles of zinc in matrix vesicle-mediated mineralization of growth plate cartilage

Zinc (Zn2+) has long been known to play important roles in mineralization and ossification of skeletal tissues, but the mechanisms of Zn2+ action are not well understood. In this study we investigated the effects of Zn2+ on mineralization in a cell culture system in which terminal differentiation and mineralization of hypertrophic growth plate chondrocytes was induced by retinoic acid (RA) treatment. Addition of Zn2+ to RA-treated cultures decreased mineralization in a dose-dependent manner without affecting alkaline phosphatase (APase) activity. Characterization of matrix vesicles (MVs), particles that initiate the mineralization process, revealed that vesicles isolated from RA-treated and RA/Zn2+-treated cultures showed similar APase activity, but vesicles from RA/Zn2+-treated cultures contained significantly less Ca2+ and Pi. MVs isolated from RA-treated cultures were able to take up Ca2+ and mineralize in vitro, whereas vesicles isolated from RA/Zn2+-treated cultures were not able to do so. Detergent treatment, which ruptures the MV membrane and exposes preformed intravesicular Ca2+-Pi-phospholipid complexes, did not restore the Ca2+ uptake abilities of MVs isolated from RA/Zn2+-treated cultures, suggesting that vesicles from RA/Zn2+-treated cultures did not contain functional Ca2+-Pi-phospholipid complexes. Zn2+ treatment did not affect the content of annexins II, V, and VI in MVs or the Ca2+-dependent, EDTA-reversible binding of these molecules to the membrane surface. However, Zn2+ treatment did affect the EDTA-nonreversible binding of these molecules to the MV membrane, suggesting that Zn2+ interferes with the assembly of annexins in the MV membrane. In addition, Zn2+ inhibited annexin II-, V-, and VI-mediated Ca2+ influx into liposomes. In conclusion, Zn2+ inhibits the mineralizing competence of intravesicular Ca2+-Pi-phospholipid complexes and function of annexin channels, thereby controlling Ca2+ influx into MVs, the formation of the first crystal phase inside the vesicles and initiation of mineralization.

Skeletal effects of zinc deficiency in growing rats

There is ample evidence that zinc plays an important role in bone metabolism and zinc deficiency has been implicated as a risk factor in the development of osteoporosis. It was the aim of the present study to investigate the skeletal effects of alimentary zinc deficiency in growing rats using quantitative bone histomorphometry. Twenty-four male Sprague Dawley rats with a mean initial body weight of 101 +/- 2 g were allocated in two groups of 12 rats each and had free access to a semi-synthetic, casein-based, zinc-deficient diet (0.76 mg zinc/kg) or to the same diet supplemented with 60 mg zinc per kg. All rats were sacrificed 42 days after the start of the experiment and the right distal femur was removed for bone histomorphometry. Relative to controls (+Zn), the zinc-deficient rats (-Zn) had a significantly lower body weight and about an 80% reduction in plasma and femur zinc concentration. The histomorphometric evaluation of the distal femoral metaphysis showed that zinc deficiency led to a 45% reduction (p < 0.01) in cancellous bone mass and to a deterioration of trabecular bone architecture, with fewer and thinner trabeculae. The osteopenia in -Zn rats was accompanied by significant reductions in osteoid perimeter (-31%, p < 0.05), osteoblast perimeter (-30%, p < 0.05), and osteoclast number (-38%, p < 0.01) relative to +Zn controls. We conclude that zinc deficiency induced low turnover osteopenia in femoral cancellous bone of growing rats. These results support the hypothesis that zinc deficiency during growth may impair the accumulation of maximal bone mass in humans; additionally, they suggest that zinc deficiency may play a role as a risk factor in the pathogenesis of osteoporosis.