Traditionally, binding proteins are known to regulate the activity of ligands by prolonging their half-life, and insulin-like growth factor (IGF)-binding proteins (IGFBPs) are no exception to this. The IGFBP family contains six high-affinity members with variable functions and mechanisms of actions. In addition to functioning as simple carrier proteins, IGFBPs in serum function to regulate the endocrine actions of IGFs by regulating the amount of IGF available to bind to signaling IGF-I receptors, whereas locally produced IGFBPs act as autocrine/paracrine regulators of IGF action. Furthermore, recent in vitro and in vivo findings that IGFBPs function independently of the IGFs as growth modulators are particularly exciting. Regarding the role of IGFBPs as ligand-independent growth modulators, our recent data that IGFBP-5 stimulates markers of bone formation in osteoblasts lacking functional IGFs provide evidence that IGFBP-5 itself is a growth factor that can act independently of IGFs to regulate bone formation. In terms of the mechanism by which certain IGFBPs mediate their effects in a ligand-independent manner, the binding of IGFBP to its putative receptor on the cell membrane may stimulate the signaling pathway independent of an IGF receptor, to mediate the effects of IGFBPs in certain target cell types. IGFBPs may also exert IGF-independent effects by transcriptional activation of genes by IGFBPs transported into the nucleus via their nuclear localization signal. In conclusion, IGFBPs are unusually pleotrophic molecules with functions ranging from the traditional role of prolonging the half-life of the IGFs to functioning as growth factors independent of the IGFs. In this regard, it was surprising to find that the human genome contains only about 35 000 genes. One mechanism to account for such complexity with a relatively small number of genes is strikingly illustrated by the multifunctional IGFBP class of proteins.

A new radioreceptor assay was used to quantify changes in serum concentration of 1alpha,25-dihydroxyvitamin D3 in rats with low calcium or low phosphate diets. Low availability of either ion elicits a fivefold increase in the circulating concentration of 1alpha,25-dihydroxyvitamin D3. The enhancement of 1alpha,25-dihydroxyvitamin D3 concentration in response to calcium deficiency is dependent on the presence of the parathyroid or thyroid glands (or both), suggesting that this effect is mediated by parathyroid hormone. In contrast, the response of phosphate deficiency is independent of these glands and may result from an action of low serum phosphate concentration or some factor associated with phosphate depletion on the renal synthesis of the 1alpha,25-dihydroxyvitamin D3 hormone.

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Cobalamin deficiency has well-known hematologic and neurologic effects, but little is known about its other effects. We therefore studied the effect of cobalamin on osteoblast-related proteins. We found that mean (+/- 1 SD) levels of skeletal alkaline phosphatase in the blood were lower in 12 cobalamin-deficient patients (3.89 +/- 2.19 units per liter) than in 5 nondeficient and 5 iron-deficient control subjects (7.55 +/- 3.99 units per liter). The degree of the megaloblastic anemia correlated with the reduction in skeletal alkaline phosphatase levels (r = 0.67, P less than 0.01). With cobalamin therapy, levels of skeletal alkaline phosphatase rose in 11 of the 12 cobalamin-deficient subjects but not in the controls. The cobalamin-deficient patients also had significantly lower osteocalcin levels than the control subjects (1.11 +/- 0.77 vs. 1.84 +/- 0.49 nmol per liter). During cobalamin therapy, these levels rose in the cobalamin-deficient patients but not in the controls. In contrast to the levels of osteoblast-related proteins, hepatic alkaline phosphatase levels were similar in the patients and controls and were usually unaffected by cobalamin therapy. In vitro studies of calvarial cells from chicken embryos showed that their alkaline phosphatase content was cobalamin-dependent, thus supporting our in vivo observations in humans. Our findings suggest that osteoblast activity depends on cobalamin and that bone metabolism is affected by cobalamin deficiency, but we do not yet know whether cobalamin deficiency produces clinically important bone disease.

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In both ground sections and demineralized frozen sections of the rat tibial cortex, osteoid but not mature bone matrix stained for proteinpolysaccharides with the Alcian Blue and toluidine blue techniques. The loss of proteinpolysaccharide staining occurred precisely at the mineralizing front, which was identified by in vivo lead or procion markers, not only in normal animals but also in animals in which osteoid width was either increasing or decreasing. In vitro, both proteases and saccharidases abolished proteinpolysaccharide staining of osteoid. Critical electrolyte concentration and other procedures indicated that the major acid polysaccharide component in osteoid is chondroitin sulfate. Consistent with these findings, electron microprobe analyses revealed that sulfur concentration was high in osteoid but dropped abruptly as calcium concentration increased at the mineralizing front. The precise synchronization between loss of proteinpolysaccharides and onset of mineralization under various experimental conditions provides strong indirect evidence that the loss of these macromolecules is somehow involved in initiation of mineralization in bone.

In the adult skeleton, bone formation is regulated by an event referred to as the coupling of bone formation to resorption (i.e. formation is quantitatively linked to resorption), which is thought to be mediated in part by osteogenic growth factor molecules produced by bone cells. Of the various osteogenic growth factors identified in bone, there is sufficient evidence to document an important role for the insulin-like growth factor (IGF) system in mediating this coupling process. Studies on the basic aspects of the IGF system reveal that it is complex and involves a number of components which include the inhibitory IGF binding protein (IGFBP)-4 and stimulatory IGFBP-5 and their corresponding proteases. This complex regulatory scheme of the IGF component system, together with the finding that all of the IGF system components are regulated in bone cells by both local and systemic effectors of bone metabolism, underscores the importance of the IGFs in mediating coupling of bone formation to resorption. In addition, a number of in vitro and in vivo findings also suggest that the IGF system could play a role in the positive (net gain of bone) or negative (net loss of bone) uncoupling of bone formation to resorption.

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