Phospholipase A2 activating protein (PLAA) is required for 1alpha,25(OH)2D3 signaling in growth plate chondrocytes

Intestinal Ca2+ absorption revisited: A molecular and clinical approach

Ca2+ has an important role in the maintenance of the skeleton and is involved in the main physiological processes. Its homeostasis is controlled by the intestine, kidney, bone and parathyroid glands. The intestinal Ca2+ absorption occurs mainly via the paracellular and the transcellular pathways. The proteins involved in both ways are regulated by calcitriol and other hormones as well as dietary factors. Fibroblast growth factor 23 (FGF-23) is a strong antagonist of vitamin D action. Part of the intestinal Ca2+ movement seems to be vitamin D independent. Intestinal Ca2+ absorption changes according to different physiological conditions. It is promoted under high Ca2+ demands such as growth, pregnancy, lactation, dietary Ca2+ deficiency and high physical activity. In contrast, the intestinal Ca2+ transport decreases with aging. Oxidative stress inhibits the intestinal Ca2+ absorption whereas the antioxidants counteract the effects of prooxidants leading to the normalization of this physiological process. Several pathologies such as celiac disease, inflammatory bowel diseases, Turner syndrome and others occur with inhibition of intestinal Ca2+ absorption, some hypercalciurias show Ca2+ hyperabsorption, most of these alterations are related to the vitamin D endocrine system. Further research work should be accomplished in order not only to know more molecular details but also to detect possible therapeutic targets to ameliorate or avoid the consequences of altered intestinal Ca2+ absorption.

Wide-Range Effects of 1,25(OH)2D3 on Group 4A Phospholipases Is Related to Nuclear Factor κ-B and Phospholipase-A2 Activating Protein Activity in Mast Cells

INTRODUCTION

Phospholipases are enzymes that occur in many types of human cells, including mast cells, and play an important role in the molecular background of asthma pathogenesis, and the development of inflammation NF-κB activities that affect numerous biological processes has been reported in many inflammatory diseases including asthma. Vitamin D is a widely studied factor that affects many diseases, including asthma. The aim of this study is to assess the influence of 1,25-(OH)2D3 on regulation of chosen phospholipase-A2 (PLA2) expression-selected inflammation mediators.

METHODS

LUVA mast cells were stimulated with 1,25(OH)2D3, and inhibitors of NF-κB p65 and ubiquitination. Expression analysis of phospholipases (PLA2G5, PLA2G10, PLA2G12, PLA2G15, PLA2G4A, PLA2G4B, PLA2G4C, PLAA, NF-κB p65, and UBC) was done utilizing real-time PCR and Western blot. Eicosanoid (LTC4, LXA4, 15[S]-HETE, and PGE2) levels and sPLA2 were also measured.

RESULTS

We found that 1,25(OH)2D3 decreased the expression of PLA2G5, PLA2G15, PLA2G5,UBC, and NF-κB p65 but increased expression of PLAA and PLA2G4C (p < 0.05). Moreover, the expression of PLA2G5 and PLA2G15 decreased after inhibition of NF-κB p65 and UBC. Increased levels of released LXA4 and 15(S)-HETE, decreased levels of LTC4, and sPLA2s enzymatic activity in response to 1,25(OH)2D3 were also observed. Additionally, NF-κB p65 inhibition led to an increase in the LXA4 concentration.

CONCLUSION

Future investigations will be needed to further clarify the role of 1,25(OH)2D3 in the context of asthma and the inflammatory process; however, these results confirm a variety of effects which can be caused by this vitamin. 1,25(OH)2D3-mediated action may result in the development of new therapeutic strategies for asthma treatment.

Factors inhibiting intestinal calcium absorption: hormones and luminal factors that prevent excessive calcium uptake

Besides the two canonical calciotropic hormones, namely parathyroid hormone and 1,25-dihydroxyvitamin D [1,25(OH)2D3], there are several other endocrine and paracrine factors, such as prolactin, estrogen, and insulin-like growth factor that have been known to directly stimulate intestinal calcium absorption. Generally, to maintain an optimal plasma calcium level, these positive regulators enhance calcium absorption, which is indirectly counterbalanced by a long-loop negative feedback mechanism, i.e., through calcium-sensing receptor in the parathyroid chief cells. However, several lines of recent evidence have revealed the presence of calcium absorption inhibitors present in the intestinal lumen and extracellular fluid in close vicinity to enterocytes, which could also directly compromise calcium absorption. For example, luminal iron, circulating fibroblast growth factor (FGF)-23, and stanniocalcin can decrease calcium absorption, thereby preventing excessive calcium uptake under certain conditions. Interestingly, the intestinal epithelial cells themselves could lower their rate of calcium uptake after exposure to high luminal calcium concentration, suggesting a presence of an ultra-short negative feedback loop independent of systemic hormones. The existence of neural regulation is also plausible but this requires more supporting evidence. In the present review, we elaborate on the physiological significance of these negative feedback regulators of calcium absorption, and provide evidence to show how our body can efficiently restrict a flood of calcium influx in order to maintain calcium homeostasis.

Extracorporeal Shockwave Therapy Enhances Expression of Pdia-3 Which Is a Key Factor of the 1α,25-Dihydroxyvitamin D 3 Rapid Membrane Signaling Pathway in Treatment of Early Osteoarthritis of the Knee

The goal of our research was demonstrated that multiple molecules in microenvironments of the early osteoarthritis (OA) joint tissue may be actively responded to extracorporeal shockwave therapy (ESWT) treatment, which potentially regulated biological function of chondrocytes and synovial cells in early OA knee. We demonstrated that shockwave treatment induced the expression of protein-disulfide isomerase-associated 3 (Pdia-3) which was a significant mediator of the 1α,25-Dihydroxyvitamin D 3 (1α,25(OH)2D3) rapid signaling pathway, using two-dimensional electrophoresis, histological analysis and quantitative polymerase chain reaction (qPCR). We observed that the expression of Pdia-3 at 2 weeks was significantly higher than that of other group at 4, 8, and 12 weeks post-shockwave treatment in early OA rat knee model. The other factors of the rapid membrane signaling pathway, including extracellular signal-regulated protein kinases 1 (ERK1), osteopontin (OPG), alkaline phosphatase (ALP), and matrix metallopeptidase 13 (MMP13) were examined and were found to be significantly increased at 2 weeks post-shockwave treatment by qPCR in early OA of the knee. Our proteomic data revealed significant Pdia-3 expression in microenvironments of OA joint tissue that could be actively responded to ESWT, which may potentially regulate the biological functions of chondrocytes and osteoblasts in the treatment of the early OA of the knee.

A review of 1α,25(OH)2D3 dependent Pdia3 receptor complex components in Wnt5a non-canonical pathway signaling

Wnt5a and 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] regulate endochondral ossification. 1α,25(OH)2D3 initiates its calcium-dependent effects via its membrane-associated receptor, protein disulfide isomerase A3 (Pdia3). 1α,25(OH)2D3 binding to Pdia3 triggers the interaction between Pdia3 and phospholipase A2 (PLA2)-activating protein (PLAA), resulting in downstream activation of calcium/calmodulin-dependent protein kinase II (CaMKII), PLA2, and protein kinase C (PKC). Wnt5a initiates its calcium-dependent effects via binding its receptors Frizzled2 (FZD2) and Frizzled5 (FZD5) and receptor tyrosine kinase-like orphan receptor 2 (ROR2), activating intracellular calcium release and stimulating PKC and CaMKII. Recent efforts to determine the inter-relation between Wnt5a and 1α,25(OH)2D3 signaling pathways have demonstrated that Wnt5a signals through a CaMKII/PLA2/PGE2/PKC cascade in chondrocytes and osteoblasts in which the components of the Pdia3 receptor complex were required. Furthermore, ROR2, but not FZD2 or FZD5, was required to mediate the calcium-dependent actions of 1α,25(OH)2D3. This review provides evidence that 1α,25(OH)2D3 and Wnt5a mediate their calcium-dependent pathways via similar receptor components and proposes that these pathways may interact since they are competing for the same receptor complex components.

Membrane-mediated actions of 1,25-dihydroxy vitamin D3: a review of the roles of phospholipase A2 activating protein and Ca(2+)/calmodulin-dependent protein kinase II

The secosteroid 1α,25-dihydroxy vitamin D3 [1α,25(OH)2D3] acts on cells via classical steroid hormone receptor-mediated gene transcription and by initiating rapid membrane-mediated signaling pathways. In its membrane-initiated pathway, after 1α,25(OH)2D3 interacts with protein disulfide isomerase, family A, member 3 (Pdia3) in caveolae, phospholipase A2 (PLA2) and protein kinase C (PKC) are activated. Recent efforts to determine the signaling proteins involved in the 1α,25(OH)2D3 signal from Pdia3 to PLA2 have indicated that phospholipase A2 activating protein (PLAA) and Ca(2+)/calmodulin-dependent kinase II (CaMKII) are required. PLAA is located in caveolae, where it interacts with Pdia3 and caveolin-1 (Cav-1) to initiate rapid signaling via CaMKII, activating PLA2, leading to activation of protein kinase C (PKC) and PKC-dependent responses.

Membrane actions of 1α,25(OH)2D3 are mediated by Ca(2+)/calmodulin-dependent protein kinase II in bone and cartilage cells

1α,25(OH)2D3 regulates osteoblasts and chondrocytes via its membrane-associated receptor, protein disulfide isomerase A3 (Pdia3) in caveolae. 1α,25(OH)2D3 binding to Pdia3 leads to phospholipase-A2 (PLA2)-activating protein (PLAA) activation, stimulating cytosolic PLA2 and resulting in prostaglandin E2 (PGE2) release and PKCα activation, subsequently stimulating differentiation. However, how PLAA transmits the signal to cPLA2 is unknown. Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) activation is required for PLA2 activation in vascular smooth muscle cells, suggesting a similar role in 1α,25(OH)2D3-dependent signaling. The aim of the present study is to evaluate the roles of CaM and CaMKII as mediators of 1α,25(OH)2D3-stimulated PLAA-dependent activation of cPLA2 and PKCα, and downstream biological effects. The results indicated that 1α,25(OH)2D3 and PLAA-peptide increased CaMKII activity within 9 min. Silencing Cav-1, Pdia3 or Plaa in osteoblasts suppressed this effect. Similarly, antibodies against Plaa or Pdia3 blocked 1α,25(OH)2D3-dependent CaMKII. Caveolae disruption abolished activation of CaMKII by 1α,25(OH)2D3 or PLAA. CaMKII-specific and CaM-specific inhibitors reduced cPLA2 and PKC activities, PGE2 release and osteoblast maturation markers in response to 1α,25(OH)2D3. Camk2a-silenced but not Camk2b-silenced osteoblasts showed comparable effects. Immunoprecipitation showed increased interaction of CaM and PLAA in response to 1α,25(OH)2D3. The results indicate that membrane actions of 1α,25(OH)2D3 via Pdia3 triggered the interaction between PLAA and CaM, leading to dissociation of CaM from caveolae, activation of CaMKII, and downstream PLA2 activation, and suggest that CaMKII plays a major role in membrane-mediated actions of 1α,25(OH)2D3.

Rapid 1α,25(OH)₂D ₃ membrane-mediated activation of Ca²⁺/calmodulin-dependent protein kinase II in growth plate chondrocytes requires Pdia3, PLAA and caveolae

1α,25-Dihydroxy vitamin D3 [1α,25(OH)2D3] regulates growth zone chondrocytes (GC) via classical steroid hormone receptor-mediated gene transcription and by initiating rapid membrane-mediated signaling pathways. 1α,25(OH)2D3 initiates its membrane effects via its specific membrane-associated receptor (Pdia3) in caveolae. 1α,25(OH)2D3 binding to Pdia3 leads to phospholipase-A2 (PLA2)-activating protein (PLAA) activation, stimulating PLA2, resulting in prostaglandin E2 (PGE2) release and protein kinase C activation. Recently, we reported that 1α,25(OH)2D3 rapidly activates Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in GC cells. However, the roles of Pdia3, PLAA and caveolae in 1α,25(OH)2D3-dependent rapid activation of CaMKII are not clear. The aim of the present study was to evaluate the roles of Pdia3, PLAA and caveolae in 1α,25(OH)2D3 membrane-stimulated CaMKII activation. Pre-treating chondrocytes from the growth zone of the rat costochondral cartilage with antibodies against PLAA or Pdia3 blocked activation of CaMKII by 1α,25(OH)2D3. PLAA peptide rapidly activated CaMKII in GC cells. Caveolae disruption abolished CaMKII activation in response to 1α,25(OH)2D3 or PLAA peptide treatment. Immunoprecipitation studies showed increased CaM binding to PLAA in response to 1α,25(OH)2D3. The results indicated that Pdia3, PLAA and caveolae are required for rapid 1α,25(OH)2D3 membrane-mediated activation of CaMKII. 1α,25(OH)2D3 signaling via Pdia3 receptor triggered the interaction between PLAA and CaM suggesting that CaM may play a major role linking PLAA to CaMKII in membrane-mediated actions of 1α,25(OH)2D3.

Signaling components of the 1α,25(OH)2D3-dependent Pdia3 receptor complex are required for Wnt5a calcium-dependent signaling

Wnt5a and 1α,25(OH)2D3 are important regulators of endochondral ossification. In osteoblasts and growth plate chondrocytes, 1α,25(OH)2D3 initiates rapid effects via its membrane-associated receptor protein disulfide isomerase A3 (Pdia3) in caveolae, activating phospholipase A2 (PLA2)-activating protein (PLAA), calcium/calmodulin-dependent protein kinase II (CaMKII), and PLA2, resulting in protein kinase C (PKC) activation. Wnt5a initiates its calcium-dependent effects via intracellular calcium release, activating PKC and CaMKII. We investigated the requirement for components of the Pdia3 receptor complex in Wnt5a calcium-dependent signaling. We determined that Wnt5a signals through a CaMKII/PLA2/PGE2/PKC cascade. Silencing or blocking Pdia3, PLAA, or vitamin D receptor (VDR), and inhibition of calmodulin (CaM), CaMKII, or PLA2 inhibited Wnt5a-induced PKC activity. Wnt5a activated PKC in caveolin-1-silenced cells, but methyl-beta-cyclodextrin reduced its stimulatory effect. 1α,25(OH)2D3 reduced stimulatory effects of Wnt5a on PKC in a dose-dependent manner. In contrast, Wnt5a had a biphasic effect on 1α,25(OH)2D3-stimulated PKC activation; 50ng/ml Wnt5a caused a 2-fold increase in 1α,25(OH)2D3-stimulated PKC but higher Wnt5a concentrations reduced 1α,25(OH)2D3-stimulated PKC activation. Western blots showed that Wnt receptors Frizzled2 (FZD2) and Frizzled5 (FZD5), and receptor tyrosine kinase-like orphan receptor 2 (ROR2) were localized to caveolae. Blocking ROR2, but not FZD2 or FZD5, abolished the stimulatory effects of 1α,25(OH)2D3 on PKC and CaMKII. 1α,25(OH)2D3 membrane receptor complex components (Pdia3, PLAA, caveolin-1, CaM) interacted with Wnt5a receptors/co-receptors (ROR2, FZD2, FZD5) in immunoprecipitation studies, interactions that changed with either 1α,25(OH)2D3 or Wnt5a treatment. This study demonstrates that 1α,25(OH)2D3 and Wnt5a mediate their effects via similar receptor components and suggests that these pathways may interact.

New insights on membrane mediated effects of 1α,25-dihydroxy vitamin D3 signaling in the musculoskeletal system

1α,25-Dihydroxy vitamin D3 [1α,25(OH)2D3] acts on cells via classical steroid hormone receptor-mediated gene transcription and by initiating rapid membrane-mediated signaling pathways. Two receptors have been implicated to play roles in 1α,25(OH)2D3 mediated rapid signaling, the classical nuclear vitamin D receptor (VDR) and protein disulfide isomerase, family A, member 3 (Pdia3). Long term efforts to investigate the roles of these two receptors demonstrated thatPdia3 is located in caveolae, where it interacts with phospholipase A2 (PLA2) activating protein (PLAA) and caveolin-1 (Cav-1) to initiate rapid signaling via Ca(++)/calmodulin-dependent protein kinase II (CaMKII), PLA2, phospholipase C (PLC), protein kinase C (PKC), and ultimately the ERK1/2 family of mitogen activated protein kinases (MAPK). VDR is present on the plasma membrane, and it is required for 1α,25(OH)2D3 induced rapid activation of Src. PDIA3+/- mice demonstrate an impaired musculoskeletal phenotype. Moreover, our studies examining mineralization of pre-osteoblasts in 3D culture have shown the physiological importance of Pdia3 and VDR interaction: knockdown of Pdia3 or VDR is characterized by impaired mineralization of the constructs.

Vitamin D in health and disease: a literature review

Vitamin D, a fat-soluble prohormone, is synthesised in response to sunlight. Vitamin D requires two metabolic conversions, 25-hydroxylation in the liver and 1alpha-hydroxylation in the kidney, to become active hormone. The active form, 1alpha,25-(OH)2D, binds to the vitamin D receptor (VDR) to modulate gene transcription and regulate mineral ion homeostasis. Vitamin D plays several roles in the body, influencing bone health as well as serum calcium and phosphate levels. Furthermore, vitamin D may modify immune function, cell proliferation, differentiation and apoptosis. Vitamin D deficiency has been associated with numerous health outcomes, including risk of rickets in children or osteomalacia in adults, increased risk of fractures, falls, cancer, autoimmune disease, infectious disease, type 1 and type 2 diabetes, hypertension and heart disease, and other diseases such as multiple sclerosis. Here, vitamin D physiology and metabolism, its genomic action and association of polymorphisms in vitamin D pathway genes with different diseases are reviewed by focusing on new findings published in the literature.

Chaperone properties of pdia3 participate in rapid membrane actions of 1α,25-dihydroxyvitamin d3

Protein disulfide isomerase family A, member 3 (Pdia3) mediates many of the plasma membrane (PM)-associated rapid responses to 1α,25-dihydroxyvitamin D3 (1α,25[OH]2D3). It is not well understood how Pdia3, which is an endoplasmic reticulum (ER) chaperone, functions as a PM receptor for 1α,25(OH)2D3. We mutated 3 amino acids (K214 and R282 in the calreticulin interaction site and C406 in the isomerase catalytic site), which are important for Pdia3's ER chaperone function, and examined their role in responses to 1α,25(OH)2D3. Pdia3 constructs with and without the ER retention signal KDEL were used to investigate the PM requirement for Pdia3. Finally, we determined whether palmitoylation and/or myristoylation were required for Pdia3-mediated responses to 1α,25(OH)2D3. Overexpressing the Pdia3 R282A mutant in MC3T3-E1 cells increased PM phospholipase A2-activating protein, Rous sarcoma oncogene (c-Src), and caveolin-1 but blocked increases in 1α,25(OH)2D3-stimulated protein kinase C (PKC) seen in cells overexpressing wild-type Pdia3 (Pdia3Ovr cells). Cells overexpressing Pdia3 with K214A and C406S mutations had PKC activity comparable to untreated controls, indicating that the native response to 1α,25(OH)2D3 also was blocked. Overexpressing Pdia3[-KDEL] increased PM localization and augmented baseline PKC, but the stimulatory effect of 1α,25(OH)2D3 was comparable to that seen in wild-type cultures. In contrast, 1α,25(OH)2D3 increased prostaglandin E2 in Pdia3[±KDEL] cells. Although neither palmitoylation nor myristoylation was required for PM association of Pdia3, myristoylation was needed for PKC activation. These data indicate that both the chaperone functional domains and the subcellular location of Pdia3 control rapid membrane responses to 1α,25(OH)2D3.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Phospholipase A2 activating protein is required for 1α,25-dihydroxyvitamin D3 dependent rapid activation of protein kinase C via Pdia3

1α,25-Dihydroxyvitamin D(3) (1,25D3) regulates musculoskeletal cells via two different mechanisms: vitamin D receptor (VDR)-dependent gene transcription and rapid membrane-signaling via VDR as well as protein disulfide isomerase, family A, member 3 (Pdia3). In chondrocytes from the costochondral cartilage growth zone (GC), ligand binding to Pdia3 causes a rapid increase in phospholipase A(2) (PLA(2)) activity leading to release of arachidonic acid and formation of lysophospholipid (LPL). LPL activates phospholipase C (PLC), and resulting inositol trisphosphate (IP(3)) and diacylglycerol contribute to PKCα activation and downstream activation of ERK1/2. PLA(2) activating protein (PLAA) is increased in the growth zone of rat growth plates suggesting that it mediates the 1,25D3-dependent pathway. This study examined the role of PLAA in mediating 1,25D3-dependent PKC activation using GC cells and MC3T3-E1 wild-type and PLAA-silenced osteoblasts as models. PLAA, Pdia3, and caveolin-1 (Cav-1) were detected in plasma membranes and caveolae of GC and MC3T3-E1 cells. Pdia3-immunoprecipitated samples were positive for PLAA only after 1,25D3 treatment. Cav-1 was detected when immunoprecipitated with anti-Pdia3 and anti-PLAA in both vehicle and 1,25D3 treated cells. These observations were confirmed by immunohistochemistry. 1,25D3 failed to activate PLA(2) and PKC or cause PGE(2) release in PLAA-silenced cells. PLAA-antibody successfully blocked the PLAA protein and consequently suppressed PKC activity in GC and MC3T3-E1 cells. Crosslinking studies confirmed the localization of PLAA on the extracellular face on the plasma membrane in untreated MC3T3-E1 cells. Taken together, our results suggest that PLAA is an important mediator of 1α,25(OH)(2)D(3) rapid membrane mediated signaling. 1α,25(OH)(2)D(3) likely causes conformational changes bringing Pdia3 into proximity with PLAA, and aiding in transducing the signal from caveolae to the plasma membrane.

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.

Vitamin D metabolism, cartilage and bone fracture repair

The 1,25-(OH)(2)D metabolite mediates the endocrine actions of vitamin D by regulating in the small intestine the expression of target genes that play a critical role in intestinal calcium absorption. The major role of the vitamin D hormone on bone is indirect and mediated through its endocrine function on mineral homeostasis. However, genetic manipulation of the expression of Cyp27b1 or the VDR in chondrocytes strongly support a direct role for locally synthesized 1,25(OH)(2)D, acting through the VDR, in vascular invasion and osteoclastogenesis during endochondral bone development. Cells from the growth plate respond to the 24,25-(OH)(2)D and 1,25-(OH)(2)D metabolites in a cell maturation-dependent manner and the effects of 1,25-(OH)(2)D are thought to be mediated through binding to the membrane-associated receptor PDIA3 (protein disulfide isomerase associated 3). The physiological relevance of membrane-mediated 1,25-(OH)(2)D signaling is emerging and is discussed. Finally, preliminary results suggest that mice deficient for Cyp24a1 exhibit a delay in bone fracture healing and support a role for 24,25-(OH)(2)D in mammalian fracture repair.

Inorganic phosphate induces mammalian growth plate chondrocyte apoptosis in a mitochondrial pathway involving nitric oxide and JNK MAP kinase

Chondrocytes in the hypertrophic zone of the growth plate undergo apoptosis during endochondral bone development via mechanisms that involve inorganic phosphate (Pi) and nitric oxide (NO). Recent evidence suggests that Pi-dependent NO production plays a role in apoptosis of cells in the resting zone as well. This study examined the mechanism by which Pi induces NO production and the signaling pathways by which NO mediates its effects on apoptosis in these cells. Pi decreased the number of viable cells based on MTT activity; the number of TUNEL-positive cells and the level of DNA fragmentation were increased, indicating an increase in apoptosis. Blocking NO production using the NO synthase (NOS) inhibitor L: -NAME or cells from eNOS(-/-) mice blocked Pi-induced chondrocyte apoptosis, indicating that NO production is necessary. NO donors NOC-18 and SNOG both induced chondrocyte apoptosis. SNOG also upregulated p53 expression, the Bax/Bcl-2 expression ratio, and cytochrome c release from mitochondria, as well as caspase-3 activity, indicating that NO induces apoptosis via a mitochondrial pathway. Inhibition of JNK, but not of p38 or ERK1/2, MAP kinase was able to block NO-induced apoptosis, indicating that JNK is necessary in this pathway. Pi elevates NO production via eNOS in resting zone chondrocytes, which leads to a mitochondrial apoptosis pathway dependent on JNK.

Protein-disulfide isomerase-associated 3 (Pdia3) mediates the membrane response to 1,25-dihydroxyvitamin D3 in osteoblasts

Protein-disulfide isomerase-associated 3 (Pdia3) is a multifunctional protein hypothesized to be a membrane receptor for 1,25(OH)(2)D(3). In intestinal epithelium and chondrocytes, 1,25(OH)(2)D(3) stimulates rapid membrane responses that are different from genomic effects via the vitamin D receptor (VDR). In this study, we show that 1,25(OH)(2)D(3) stimulates phospholipase A(2) (PLA(2))-dependent rapid release of prostaglandin E(2) (PGE(2)), activation of protein kinase C (PKC), and regulation of bone-related gene transcription and mineralization in osteoblast-like MC3T3-E1 cells (WT) via a mechanism involving Pdia3. Pdia3 was present in caveolae based on co-localization with lipid rafts and caveolin-1. In Pdia3-silenced (Sh-Pdia3) cells, 1,25(OH)(2)D(3) failed to stimulate PKC and PGE(2) responses; in Pdia3-overexpressing cells (Ov-Pdia3), responses to 1,25(OH)(2)D(3) were augmented. Downstream mediators of Pdia3, PLA(2)-activating protein (PLAA) and arachidonic acid, stimulated similar PKC activation in wild-type, Sh-Pdia3, and Ov-Pdia3 cells supporting the hypothesis that Pdia3 mediates the membrane action of 1,25(OH)(2)D(3). Treatment of MC3T3-E1 cells with 1,25(OH)(2)D(3) for 9 min stimulated rapid phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and increased expression of alkaline phosphatase, MMP-13, and osteopontin but decreased expression of osteocalcin, osteoprotegerin (mRNA and protein), and smad2. These effects were attenuated in Sh-Pdia3 cells. Sh-Pdia3 cells produced higher numbers of von Kossa-positive nodules and alizarin red-positive nodules compared with WT cells with or without 1,25(OH)(2)D(3) treatment whereas Ov-Pdia3 did not show any mineralization. Our data suggest Pdia3 is an important initiator of 1,25(OH)(2)D(3)-stimulated membrane signaling pathways, which have both genomic and non genomic effects during osteoblast maturation.

24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] controls growth plate development by inhibiting apoptosis in the reserve zone and stimulating response to 1alpha,25(OH)2D3 in hypertrophic cells

Previously we showed that costochondral growth plate resting zone (RC) chondrocytes response primarily to 24R,25(OH)2D3 whereas prehypertrophic and hypertrophic (GC) cells respond to 1alpha,25(OH)2D3. 24R,25(OH)2D3 increases RC cell proliferation and inhibits activity of matrix processing enzymes, suggesting it stabilizes cells in the reserve zone, possibly by inhibiting the matrix degradation characteristic of apoptotic hypertrophic GC cells. To test this, apoptosis was induced in rat RC cells by treatment with exogenous inorganic phosphate (Pi). 24R,25(OH)2D3 blocked apoptotic effects in a dose-dependent manner. Similarly, apoptosis was induced in ATDC5 cell cultures and 24R,25(OH)2D3 blocked this effect. Further studies indicated that 24R,25(OH)2D3 acts via at least two independent pathways. 24R,25(OH)2D3 increases LPA receptor-1 (LPA R1) expression and production of lysophosphatidic acid (LPA), and subsequent LPA R1/3-dependent signaling, thereby decreasing p53 abundance. LPA also increases the Bcl-2/Bax ratio. In addition, 24R,25(OH)2D3 acts by increasing PKC activity. 24R,25(OH)2D3 stimulates 1-hydroxylase activity, resulting in increased levels of 1,25(OH)2D3, and it increases levels of phospholipase A2 activating protein, which is required for rapid 1alpha,25(OH)2D3-dependent activation of PKC in GC cells. These results suggest that 24R,25(OH)2D3 modulates growth plate development by controlling the rate and extent of RC chondrocyte transition to a GC chondrocyte phenotype.

Vitamin D and human health: lessons from vitamin D receptor null mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

1,25-Dihydroxy vitamin D3 is an autocrine regulator of extracellular matrix turnover and growth factor release via ERp60-activated matrix vesicle matrix metalloproteinases

As growth plate chondrocytes mature and hypertrophy, they reorganize their proteoglycan-rich type II collagen extracellular matrix (ECM), involving 1,25(OH)(2)D(3)-dependent regulation of matrix metalloproteinases (MMPs). Stromelysin-1 (MMP-3) and 72-kD gelatinase (MMP-2) are found in extracellular matrix vesicles (MVs) and release and activate ECM-bound latent TGF-beta1 and TGF-beta2, respectively. 1,25(OH)(2)D(3) regulates incorporation of MMP-2 and MMP-3 into MVs and release of these enzymes in the ECM. Plasma membranes (PMs) and MVs contain the 1alpha,25(OH)(2)D(3) membrane receptor ERp60 (protein disulfide isomerase A3), phospholipase A(2) (PLA(2)), PLA(2)-activating protein, the nuclear vitamin D receptor and caveolin-1. 1,25(OH)(2)D(3) secreted by chondrocytes binds MV ERp60, activating PLA(2). Resulting lysophospholipids destabilize MV membranes, releasing active MMPs. We examined 1,25(OH)(2)D(3)-dependent activation of latent TGF-beta1 stored in cartilage ECM. Interestingly, TGF-beta1 regulates 1,25(OH)(2)D(3) production. 1alpha,25(OH)(2)D(3) activates PM protein kinase C (PKC)-alpha via ERp60-dependent PLA(2)-signaling, lysophospholipid production and phospholipase C-gamma. It also regulates distribution of phospholipids and PKC isoforms between MVs and PMs, enriching MVs in PKC-zeta. Direct activation of MV MMP-3 requires ERp60 based on blocking antibodies and PKC based on inhibitor studies. However, treatment of MVs with 1,25(OH)(2)D(3) decreases MV PKC-zeta activity, suggesting more complex feedback mechanisms, potentially involving MV lipid signaling. Our observations indicate that one role of MVs is to provide MMPs at sites distant from the cells. Chondrocytes secrete 1,25(OH)(2)D(3), which acts directly on MV-membranes via ERp60, releasing MMPs. MMP-specific ECM components are hydrolyzed, resulting in release and activation of growth factors that can act back on the cells.

1alpha,25(OH)2D3 is an autocrine regulator of extracellular matrix turnover and growth factor release via ERp60 activated matrix vesicle metalloproteinases

Growth plate chondrocytes produce proteoglycan-rich type II collagen extracellular matrix (ECM). During cell maturation and hypertrophy, ECM is reorganized via a process regulated by 1alpha,25(OH)(2)D(3) and involving matrix metalloproteinases (MMPs), including MMP-3 and MMP-2. 1alpha,25(OH)(2)D(3) regulates MMP incorporation into matrix vesicles (MVs), where they are stored until released. Like plasma membranes (PM), MVs contain the 1alpha,25(OH)(2)D(3)-binding protein ERp60, phospholipase A(2) (PLA(2)), and caveolin-1, but appear to lack nuclear Vitamin D receptors (VDRs). Chondrocytes produce 1alpha,25(OH)(2)D(3) (10(-8)M), which binds ERp60, activating PLA(2), and resulting lysophospholipids lead to MV membrane disorganization, releasing active MMPs. MV MMP-3 activates TGF-beta1 stored in the ECM as large latent TGF-beta1 complexes, consisting of latent TGF-beta1 binding protein, latency associated peptide, and latent TGF-beta1. Others have shown that MMP-2 specifically activates TGF-beta2. TGF-beta1 regulates 1alpha,25(OH)(2)D(3)-production, providing a mechanism for local control of growth factor activation. 1alpha,25(OH)(2)D(3) activates PKCalpha in the PM via ERp60-signaling through PLA(2), lysophospholipid production, and PLCbeta. It also regulates distribution of phospholipids and PKC isoforms between MVs and PMs, enriching the MVs in PKCzeta. Direct activation of MMP-3 in MVs requires ERp60. However, when MVs are treated with 1alpha,25(OH)(2)D(3), PKCzeta activity is decreased and PKCalpha is unaffected, suggesting a more complex feedback mechanism, potentially involving MV lipid signaling.

Regulation of growth plate chondrocytes by 1,25-dihydroxyvitamin D3 requires caveolae and caveolin-1

We examined the role of caveolae and caveolin-1 in the mechanism of 1alpha,25(OH)(2)D(3) action in growth plate chondrocytes. We found that caveolae are required for rapid 1alpha,25(OH)(2)D(3)-dependent PKC signaling, and caveolin-1 must be present based on studies using chondrocytes from Cav-1(-/-) mice.

INTRODUCTION

1,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] regulates endochondral ossification in part through membrane-associated mechanisms, including protein kinase C (PKC) signaling activated by a membrane-associated 1alpha,25(OH)(2)D(3)-binding protein, ERp60. We tested the hypothesis that caveolae are required for 1alpha,25(OH)(2)D(3) action and play an important role in regulating chondrocyte biology and growth plate physiology.

MATERIALS AND METHODS

Rat costochondral chondrocytes were examined for caveolae by transmission electron microscopy of cultured cells and of cells in situ. Western blots and confocal microscopy were used to detect caveolae proteins including caveolin-1 (Cav-1) and 1alpha,25(OH)(2)D(3) receptors. Caveolae cholesterol was depleted with beta-cyclodextrin (CD) and effects of 1alpha,25(OH)(2)D(3) on PKC, DNA synthesis, alkaline phosphatase, and proteoglycan production determined. Chondrocytes from Cav-1(-/-) and C57BL/6 wildtype mice were also treated with 1alpha,25(OH)(2)D(3). Epiphyses and costochondral junctions of 8-week-old male Cav-1(-/-) and wildtype mice (N = 8) were compared by histomorphometry and microCT. Data were analyzed by ANOVA and Bonferroni for posthoc comparisons.

RESULTS

Growth zone chondrocytes had caveolae and Cav-1, -2, and -3. Resting zone chondrocytes, which do not exhibit a rapid 1alpha,25(OH)(2)D(3)-dependent increase in PKC activity, also had these caveolins, but caveolae were larger and fewer in number. ERp60 but not VDR co-localized with Cav-1 in plasma membranes and in lipid rafts. CD-treatment blocked 1alpha,25(OH)(2)D(3) effects on all parameters tested. The Cav-1(-/-) cells did not respond to 1alpha,25(OH)(2)D(3), although 1alpha,25(OH)(2)D(3) increased PKC, alkaline phosphatase, and [(35)S]-sulfate incorporation in wildtype C57BL/6 cells. Histology and microCT showed that Cav-1(-/-) growth plates were longer and had more hypertrophic cells in each column. Growth plate changes were reflected in the metaphysis.

CONCLUSIONS

The membrane-mediated effects of 1alpha,25(OH)(2)D(3) require caveolae and Cav-1, and Cav-1 deficiency results in altered growth plate physiology.

Sex-specific regulation of growth plate chondrocytes by estrogen is via multiple MAP kinase signaling pathways

Both male and female rat growth plate cartilage cells possess estrogen receptors (ERs), but 17beta-estradiol (E(2)) activates protein kinase C (PKC) and PKC-dependent biological responses to E(2) only in cells from female animals. PKC signaling can elicit genomic responses via mitogen activated protein kinase (MAPK) and E(2) has been shown to activate ERK MAPK in many cells, suggesting that MAPK may play a role in growth plate chondrocytes as well. We tested if E(2) increases MAPK activity and if so, whether the response is limited to female cells, if it is PKC-dependent, and if the mechanism involves traditional ER pathways. We also determined the contribution of MAPK to the biological response of growth plate chondrocytes and assessed the relative contributions of ERK, p38 and JNK MAPKs. Female rat costochondral cartilage cells were treated with E(2) and MAPK-specific activity determined in cell layer lysates. The mechanism of MAPK activation was determined by treating the cells with E(2) conjugated to bovine serum albumin (E(2)-BSA) to assess if membrane receptors were involved; stereospecificity was determined using 17alpha-estradiol; PKC and phospholipase C (PLC) dependence was determined using specific inhibitors; and the ER agonist diethylstilbestrol, the ER antagonist ICI 182780, and tamoxifen were used to assess the role of traditional ER pathways. E(2) regulation of ERK1/2 MAPK was assessed and the relative roles of ERK1/2, p38 and JNK MAPKs determined using specific inhibitors. E(2) caused a rapid dose-dependent activation of MAPK that was greatest in cells treated for 9 min with 10(-9) M hormone; activity remained elevated for 3 h. E(2)'s effect on MAPK was stereospecific and comparable to that of E(2)-BSA. It was insensitive to DES and ICI 182780, dependent on PKC and PLC, blocked by tamoxifen and it did not require gene transcription or translation. E(2) had no effect on ERK1 or ERK2 mRNA or protein but it caused a rapid phosphorylation of ERK1/2 at 9 min. Inhibition of ERK1/2 and p38 MAPK reduced the stimulatory effects of E(2) on alkaline phosphatase activity and [(35)S]-sulfate incorporation. These results suggest that E(2) regulates MAPK through a sex-specific membrane-mediated mechanism that does not involve cytosolic ERs in a traditional sense and that ERK1/2 and p38 mediate the downstream biological effects of the hormone.

Does calcitriol have actions independent from the vitamin D receptor in maintaining skeletal and mineral homeostasis?

PURPOSE OF REVIEW

Although the active metabolite of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D), is classically appreciated to exert its calcemic and other actions via interaction with the vitamin D receptor, thereby modulating gene transcription, some of its actions cannot be explained in this way when examined in vitro.

RECENT FINDINGS

Comparison of mouse models deleted for either the 25-hydroxyvitamin D-1alpha-hydroxylase enzyme (deficient in 1,25(OH)2D) or the vitamin D receptor or both has allowed an assessment of whether 1,25(OH)2D can function in the absence of the vitamin D receptor in vivo. The data indicated that calcium absorption required both the ligand and the receptor as did bone and cartilage remodeling. However, with respect to parathyroid gland function and development of the cartilaginous growth plate, calcium and 1,25(OH)2D acted cooperatively and there was evidence that 1,25(OH)2D could act independently of the vitamin D receptor.

SUMMARY

Results from the genetic models are consistent with recent reports that rapid actions of vitamin D metabolites occur in chondrocytes through a membrane receptor distinct from the vitamin D receptor. In addition, in osteoblasts it has been proposed that the vitamin D receptor localized in plasma membrane caveolae signals the rapid effects of the active vitamin D secosterol.