Preferential accumulation in vivo of 24R,25-dihydroxyvitamin D(3) in growth plate cartilage of rats

24R,25-dihydroxyvitamin D3 modulates tumorigenicity in breast cancer in an estrogen receptor-dependent manner

Vitamin D has long been prescribed as a supplement to breast cancer patients. This is partially motivated by data indicating that low serum vitamin D, measured as 25-hydroxyvitamin D3 [25(OH)D₃], is associated with worsened cancer prognosis and decreased survival rates in cancer patients. However, clinical studies investigating the role of vitamin D supplementation in breast cancer treatment are largely inconclusive. One reason for this may be that many of these studies ignore the complexity of the vitamin D metabolome and the effects of these metabolites at the cellular level. Once ingested, vitamin D is metabolized into 37 different metabolites, including 25(OH)D₃, which is the metabolite actually measured clinically, as well as 1,25(OH)₂D₃ and 24,25(OH)₂D₃. Recent work by our lab and others has demonstrated a role for 24R,25(OH)₂D₃, in the modulation of breast cancer tumors via an estrogen receptor α-dependent mechanism. This review highlights the importance of considering estrogen receptor status in vitamin d-associated prognostic studies of breast cancer and proposes a potential mechanism for 24R,25(OH)₂D₃ signaling in breast cancer cells.

24R,25-Dihydroxyvitamin D3 Protects against Articular Cartilage Damage following Anterior Cruciate Ligament Transection in Male Rats

Osteoarthritis (OA) in humans is associated with low circulating 25-hydroxyvitamin D3 [25(OH)D3]. In vitamin D replete rats, radiolabeled 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] accumulates in articular cartilage following injection of [3H]-25(OH)D3. Previously, we showed that 24R,25(OH)2D3 blocks chondrocyte apoptosis via phospholipase D and p53, suggesting a role for 24R,25(OH)2D3 in maintaining cartilage health. We examined the ability of 24R,25(OH)2D3 to prevent degenerative changes in articular cartilage in an OA-like environment and the potential mechanisms involved. In vitro, rat articular chondrocytes were treated with IL-1β with and without 24R,25(OH)2D3 or 1α,25(OH)2D3. 24R,25(OH)2D3 but not 1α,25(OH)2D3 blocked the effects of IL-1β in a dose-dependent manner, and its effect was partially mediated through the TGF-β1 signaling pathway. In vivo, unilateral anterior cruciate ligament transections were performed in immunocompetent rats followed by intra-articular injections of 24R,25(OH)2D3 or vehicle (t = 0, 7, 14, 21 days). Tissues were harvested on day 28. Joints treated with vehicle had changes typical of OA whereas joints treated with 24R,25(OH)2D3 had less articular cartilage damage and levels of inflammatory mediators. These results indicate that 24R,25(OH)2D3 protects against OA, and suggest that it may be a therapeutic approach for preventing trauma-induced osteoarthritis.

Mechanism of Pdia3-dependent 1α,25-dihydroxy vitamin D3 signaling in musculoskeletal cells

1α,25-Dihydroxy vitamin D3 [1,25(OH)2D3] acts on cells through traditional steroid hormone receptor-mediated gene transcription and by initiating rapid membrane-associated signaling pathways. Two receptors have been implicated in rapid signaling by 1,25(OH)2D3, the classical nuclear vitamin D receptor (VDR) and the more recently identified protein disulfide isomerase, family A, member 3 (Pdia3). Our lab along with other groups has established various tools to investigate the role of these two receptors, including gene knock-out, conditional knock-out, silencing, and over-expression in various model systems (growth plate chondrocytes, osteoblastic cells, chick intestinal epithelial cells, mouse embryoid bodies, extracellular matrix vesicles and isolated cell membranes). The data demonstrate the requirement for Pdia3 in 1,25(OH)2D3 induced phospholipase A2 (PLA2) and protein kinase C (PKC) activation and downstream responses. Pdia3+/- heterozygote mice also exhibit both cartilage and bone defects. VDR is present on the plasma membrane and one VDR-/- mouse strain lacks transcaltachia, although 1,25(OH)2D3 induced PKC activation and transcaltachia are not affected in another VDR-/- mouse strain. In the context of osteoblast differentiation, both receptors are expressed during osteogenic commitment of embryoid bodies and silencing of each causes a more mature osteoblast phenotype in MC3T3-E1 pre-osteoblasts. Pdia3 exists in caveolae, where it interacts with PLA2 activating protein (PLAA) and caveolin-1 to initiate rapid signaling via PLA2, phospholipase C (PLC), PKC, and ultimately the ERK1/2 family of mitogen activated protein kinases (MAPK). Using the growth plate chondrocyte and matrix vesicle models, we have demonstrated that Pdia3-dependent signaling in response to 1,25(OH)2D3 regulates growth plate physiology.

Ligand structure-function relationships in the vitamin D endocrine system from the perspective of drug development (including cancer treatment)

It has become readily apparent to many scientists and pharmaceutical companies that the vitamin D endocrine system offers a wide array of drug development opportunities. There are already successes, as noted by 1alpha,25(OH)2D3 (Roche, and Abbott) for renal osteodystrophy and osteoporosis and 1alpha(OH)D3 (Leo, Chugai, Teijin) for renal osteodystrophy and (in Japan) osteoporosis, 1alpha,24(OH)2-24-cyclopropyl-D3 (Dovonex) and 1alpha,24(OH)2D3 (Teijin) for psoriasis, and 19-nor-1alpha,25(OH)2D2 (Abbott) for renal osteodystrophy, as well as drugs under active development. Yet there are still many important and challenging drug development frontiers, particularly in the area of cancer treatment and immune system disorders where exploration is only in the initial early stages. In addition, the application of vitamin D-related drugs in neurology and brain pathology should not be overlooked. It is to be hoped that the cellular and molecular basis for the vexing problem of analog-induced hypercalcemia will be elucidated. Given that there are believed to be over 2000 analogs of 1alpha,25(OH)2D3 already available for consideration, it is to be expected that over the next decade a significant number of new vitamin D structure-function drug development projects will be brought to conclusion.

Differential regulation of growth plate chondrocytes by 1alpha,25-(OH)2D3 and 24R,25-(OH)2D3 involves cell-maturation-specific membrane-receptor-activated phospholipid metabolism

This review discusses the regulation of growth plate chondrocytes by vitamin D(3). Over the past ten years, our understanding of how two vitamin D metabolites, 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3), exert their effects on endochondral ossification has undergone considerable advances through the use of cell biology and signal transduction methodologies. These studies have shown that each metabolite affects a primary target cell within the endochondral developmental lineage. 1alpha,25-(OH)(2)D(3) affects primarily growth zone cells, and 24R,25-(OH)(2)D(3) affects primarily resting zone cells. In addition, 24R,25-(OH)(2)D(3) initiates a differentiation cascade that results in down-regulation of responsiveness to 24R,25-(OH)(2)D(3) and up-regulation of responsiveness to 1alpha,25-(OH)(2)D(3). 1alpha,25-(OH)(2)D(3) regulates growth zone chondrocytes both through the nuclear vitamin D receptor, and through a membrane-associated receptor that mediates its effects via a protein kinase C (PKC) signal transduction pathway. PKCalpha is increased via a phosphatidylinositol-specific phospholipase C (PLC)-dependent mechanism, as well as through the stimulation of phospholipase A(2) (PLA(2)) activity. Arachidonic acid and its downstream metabolite prostaglandin E(2) (PGE(2)) also modulate cell response to 1alpha,25-(OH)(2)D(3). In contrast, 24R,25-(OH)(2)D(3) exerts its effects on resting zone cells through a separate, membrane-associated receptor that also involves PKC pathways. PKCalpha is increased via a phospholipase D (PLD)-mediated mechanism, as well as through inhibition of the PLA(2) pathway. The target-cell-specific effects of each metabolite are also seen in the regulation of matrix vesicles by vitamin D(3). However, the PKC isoform involved is PKCzeta, and its activity is inhibited, providing a mechanism for differential autocrine regulation of the cell and events in the matrix by these two vitamin D(3) metabolites.

24,25-(OH)(2)D(3) regulates cartilage and bone via autocrine and endocrine mechanisms

The purpose of this paper is to summarize recent advances in our understanding of the physiological role of 24(R),25(OH)(2)D(3) in bone and cartilage and its mechanism of action. With the identification of a target cell, the growth plate resting zone (RC) chondrocyte, we have been able to use cell biology methodology to investigate specific functions of 24(R),25(OH)(2)D(3) and to determine how 24(R),25(OH)(2)D(3) elicits its effects. These studies indicate that there are specific membrane-associated signal transduction pathways that mediate both rapid, nongenomic and genomic responses of RC cells to 24(R),25(OH)(2)D(3). 24(R),25(OH)(2)D(3) binds RC chondrocyte membranes with high specificity, resulting in an increase in protein kinase C (PKC) activity. The effect is stereospecific; 24R,25(OH)(2)D(3), but not 24S,25-(OH)(2)D(3), causes the increase, indicating a receptor-mediated response. Phospholipase D-2 (PLD2) activity is increased, resulting in increased production of diacylglycerol (DAG), which in turn activates PKC. 24(R),25(OH)(2)D(3) does not cause translocation of PKC to the plasma membrane, but activates existing PKCalpha. There is a rapid decrease in Ca(2+) efflux, and influx is stimulated. 24(R),25(OH)(2)D(3) also reduces arachidonic acid release by decreasing phospholipase A(2) (PLA(2)) activity, thereby decreasing available substrate for prostaglandin production via the action of cyclooxygenase-1. PGE(2) that is produced acts on the EP1 and EP2 receptors expressed by RC cells to downregulate PKC via protein kinase A, but the reduction in PGE(2) decreases this negative feedback mechanism. Both pathways converge on MAP kinase, leading to new gene expression. One consequence of this is production of new matrix vesicles containing PKCalpha and PKCzeta and an increase in PKC activity. The chondrocytes also produce 24(R),25(OH)(2)D(3), and the secreted metabolite acts directly on the matrix vesicle membrane. Only PKCzeta is directly affected by 24(R),25(OH)(2)D(3) in the matrix vesicles, and activity of this isoform is inhibited. This effect may be involved in the control of matrix maturation and turnover. 24(R),25(OH)(2)D(3) causes RC cells to mature along the endochondral developmental pathway, where they become responsive to 1alpha,25(OH)(2)D(3) and lose responsiveness to 24(R),25(OH)(2)D(3), a characteristic of more mature growth zone (GC) chondrocytes. 1alpha,25(OH)(2)D(3) elicits its effects on GC through different signal transduction pathways than those used by 24(R),25(OH)(2)D(3). These studies indicate that 24(R),25(OH)(2)D(3) plays an important role in endochondral ossification by regulating less mature chondrocytes and promoting their maturation in the endochondral lineage.

24R,25-(OH)(2)D(3) mediates its membrane receptor-dependent effects on protein kinase C and alkaline phosphatase via phospholipase A(2) and cyclooxygenase-1 but not cyclooxygenase-2 in growth plate chondrocytes

Recent studies have shown that 24R,25-(OH)(2)D(3) mediates its effects on growth plate chondrocytes via membrane receptors. This study examined the roles of phospholipase A(2) (PLA(2)) and cyclooxygenase (Cox) in the mechanism of action of 24R, 25-(OH)(2)D(3) in resting zone chondrocytes in order to determine whether the activity of one or both enzymes provides a regulatory checkpoint in the signaling pathway resulting in increased protein kinase C (PKC) activity. We also determined whether constitutive or inducible Cox is involved. Cultures were incubated with 24R, 25-(OH)(2)D(3) for 90 min to measure PKC or for 24 h to measure physiological responses ([(3)H]-thymidine incorporation, alkaline phosphatase-specific activity, [(35)S]-sulfate incorporation). Based on RT-PCR and Northern blot analysis, resting zone chondrocytes express mRNAs for both Cox-1 and Cox-2. Levels of mRNA for both proteins were unchanged from control levels after a 24-h incubation with 24R,25-(OH)(2)D(3). To examine the role of Cox, the cultures were also treated with resveratrol (a specific inhibitor of Cox-1), NS-398 (a specific inhibitor of Cox-2), or indomethacin (a general Cox inhibitor). Cox-1 inhibition resulted in effects on proliferation, differentiation, and matrix production typical of 24R, 25-(OH)(2)D(3). In contrast, inhibition of Cox-2 had no effect, indicating that 24R,25-(OH)(2)D(3) exerts its effects via Cox-1. Inhibition of Cox-1 also blocked 24R,25-(OH)(2)D(3)-dependent increases in PKC. Activation of PLA(2) with melittin inhibited 24R, 25-(OH)(2)D(3)-dependent stimulation of PKC, and inhibition of PLA(2) with quinacrine stimulated PKC in response to 24R, 25-(OH)(2)D(3). Inclusion of resveratrol reduced the melittin-dependent inhibition of PLA(2) and caused an increase in quinacrine-stimulated PLA(2) activity. Metabolism of arachidonic acid to leukotrienes is not involved in the response to 24R, 25-(OH)(2)D(3) because inhibition of lipoxygenase had no effect. The effect of 24R,25-(OH)(2)D(3) was specific because 24S,25-(OH)(2)D(3), the biologically inactive stereoisomer, failed to elicit a response from the cells. These results support the hypothesis that 24R, 25-(OH)(2)D(3) exerts its effects via more than one signaling pathway and that these pathways are interrelated via the modulation of PLA(2). PKC regulation may occur at multiple stages in the signal transduction cascade.

Physiological importance of the 1,25(OH)2D3 membrane receptor and evidence for a membrane receptor specific for 24,25(OH)2D3

We have recently identified a membrane vitamin D receptor (mVDR) specific for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and shown that it mediates the rapid activation of protein kinase C (PKC) in growth zone chondrocytes (GCs). In this study, we examine the role of the 1, 25(OH)2D3-mVDR in chondrocyte physiology and provide evidence for the existence of a specific membrane receptor for 24, 25-dihydroxyvitamin D3 (24,25(OH)2D3-mVDR). Fourth-passage cultures of growth plate chondrocytes at two distinct stages of endochondral development, resting zone (RC) and growth zone (GC) cells, were used to assess the role of the mVDR in cell proliferation, PKC activation, and proteoglycan sulfation. To preclude the involvement of the nuclear vitamin D receptor (nVDR), we used hybrid analogs of 1, 25(OH)2D3 with <0.1% affinity for the nVDR (2a, 1alpha-CH2OH-3beta-25D3; 3a, 1alpha-CH2OH-3beta-20-epi-22-oxa-25D3; and 3b, 1beta-CH2OH-3alpha-20-epi-22-oxa-25D3). To determine the involvement of the mVDR, we used an antibody generated against the highly purified 1,25(OH)2D3 binding protein from chick intestinal basolateral membranes (Ab99). Analog binding to the mVDR was demonstrated by competition with [3H]1,25(OH)2D3 using matrix vesicles (MVs) isolated from cultures of RC and GC cells. Specific recognition sites for 24,25(OH)2D3 in RC MVs were demonstrated by saturation binding analysis. Specific binding of 24,25(OH)2D3 was also investigated in plasma membranes (PMs) from RC and GC cells and GC MVs. In addition, we examined the ability of Ab99 to block the stimulation of PKC by analog 2a in isolated RC PMs as well as the inhibition of PKC by analog 2a in GC MVs. Like 1,25(OH)2D3, analogs 2a, 3a, and 3b inhibit RC and GC cell proliferation. The effect was dose dependent and could be blocked by Ab99. In GC cells, PKC activity was stimulated maximally by analogs 2a and 3a and very modestly by 3b. The effect of 2a and 3a was similar to that of 1, 25(OH)2D3 and was blocked by Ab99, whereas the effect of 3b was unaffected by antibody. In contrast, 2a was the only analog that increased PKC activity in RC cells, and this effect was unaffected by Ab99. Analog 2a had no effect on proteoglycan sulfation in RC cells, whereas analogs 3a and 3b stimulated it and this was not blocked by Ab99. Binding of [3H]1,25(OH)2D3 to GC MVs was displaced completely with 1,25(OH)2D3 and analogs 2a, 3a, and 3b, but 24, 25(OH)2D3 only displaced 51% of the bound ligand. 24,25(OH)2D3 displaced 50% of [3H]1,25(OH)2D3 bound to RC MVs, but 2a, 3a, and 3b displaced <50%. Scatchard analysis indicated specific binding of 24, 25(OH)2D3 to recognition sites in RC MVs with a Kd of 69.2 fmol/ml and a Bmax of 52.6 fmol/mg of protein. Specific binding for 24, 25(OH)2D3 was also found in RC and GC PMs and GC MVs. GC membranes exhibited lower specific binding than RC membranes; MVs had greater specific binding than PMs in both cell types. 2a caused a dose-dependent increase in PKC activity of RC PMs that was unaffected by Ab99; it inhibited PKC activity in GC MVs, and this effect was blocked by Ab99. The results indicate that the 1, 25(OH)2D3 mVDR mediates the antiproliferative effect of 1,25(OH)2D3 on chondrocytes. It also mediates the 1,25(OH)2D3-dependent stimulation of PKC in GC cells, but not the 2a-dependent increase in RC PKC activity, indicating that 24,25(OH)2D3 mediates its effects through a separate receptor. This is supported by the failure of Ab99 to block 2a-dependent stimulation of PKC in isolated PMs. The data demonstrate for the first time the presence of a specific 24, 25(OH)2D3 mVDR in endochondral chondrocytes and show that, although both cell types express mVDRs for 1,25(OH)2D3 and 24,25(OH)2D3, their relative distribution is cell maturation-dependent.

Studies on 24R,25-dihydroxyvitamin D3: evidence for a nonnuclear membrane receptor in the chick tibial fracture-healing callus

The effect(s) of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] on fracture healing was studied in a vitamin D-depleted chick model. 24R,25(OH)2D3, together with another hormonally active vitamin D metabolite, 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], improved bone mechanical strength parameters (torsional strength, angular deformation, and stiffness) and the ash content. The synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25(OH)2D3] was not as potent as the natural 24R,25(OH)2D3. In light of the ability of the fracture-healing callus to discriminate between 24R,25(OH)2D3 and 24S,25(OH)2D3, a search was initiated in fracture-healing callus tissue for the presence of a specific 24R,25(OH)2D3 receptor. No evidence was obtained for a classical nuclear/cytosol receptor for 24R,25(OH)2D3 in the fracture-healing callus. A specific receptor/binding protein for 24R,25(OH)2D3 was found in the callus membrane fraction, which showed different ligand binding affinities [KD = 18.3 +/- 1.9 nmol/L, Bmax = 43.9 +/- 6.0 fmol/mg; relative competitive index (RCI) for 24R,25(OH)2D3/24S,25(OH)2D3/25(OH)D3/1alpha,25(OH)2D3 = 100/37/401/2.0] compared with the ubiquitous serum vitamin D-binding protein (RCI = 100/99/219/5). Also, a callus membrane-binding protein/receptor for 1alpha,25(OH)2D3 was detected with a KD = 0.83 +/- 0.35 nmol/L and a Bmax = 35.5 +/- 5.2 fmol/mg. Thus, we have demonstrated a biological role for 24R,25(OH)2D3 in fracture healing and described the presence of its receptor/binding protein in a callus membrane fraction.