Effects of age on parathyroid hormone signaling in human marrow stromal cells

PTH and the Regulation of Mesenchymal Cells within the Bone Marrow Niche

Parathyroid hormone (PTH) plays a pivotal role in maintaining calcium homeostasis, largely by modulating bone remodeling processes. Its effects on bone are notably dependent on the duration and frequency of exposure. Specifically, PTH can initiate both bone formation and resorption, with the outcome being influenced by the manner of PTH administration: continuous or intermittent. In continuous administration, PTH tends to promote bone resorption, possibly by regulating certain genes within bone cells. Conversely, intermittent exposure generally favors bone formation, possibly through transient gene activation. PTH's role extends to various aspects of bone cell activity. It directly influences skeletal stem cells, osteoblastic lineage cells, osteocytes, and T cells, playing a critical role in bone generation. Simultaneously, it indirectly affects osteoclast precursor cells and osteoclasts, and has a direct impact on T cells, contributing to its role in bone resorption. Despite these insights, the intricate mechanisms through which PTH acts within the bone marrow niche are not entirely understood. This article reviews the dual roles of PTH-catabolic and anabolic-on bone cells, highlighting the cellular and molecular pathways involved in these processes. The complex interplay of these factors in bone remodeling underscores the need for further investigation to fully comprehend PTH's multifaceted influence on bone health.

Age-related alterations and senescence of mesenchymal stromal cells: Implications for regenerative treatments of bones and joints

The most common clinical manifestations of age-related musculoskeletal degeneration are osteoarthritis and osteoporosis, and these represent an enormous burden on modern society. Mesenchymal stromal cells (MSCs) have pivotal roles in musculoskeletal tissue development. In adult organisms, MSCs retain their ability to regenerate tissues following bone fractures, articular cartilage injuries, and other traumatic injuries of connective tissue. However, their remarkable regenerative ability appears to be impaired through aging, and in particular in age-related diseases of bones and joints. Here, we review age-related alterations of MSCs in musculoskeletal tissues, and address the underlying mechanisms of aging and senescence of MSCs. Furthermore, we focus on the properties of MSCs in osteoarthritis and osteoporosis, and how their changes contribute to onset and progression of these disorders. Finally, we consider current treatments that exploit the enormous potential of MSCs for tissue regeneration, as well as for innovative cell-free extracellular-vesicle-based and anti-aging treatment approaches.

Fibroblast growth factor 23 counters vitamin D metabolism and action in human mesenchymal stem cells

Chronic kidney disease (CKD) is associated with elevated circulating fibroblast growth factor 23 (FGF23), impaired renal biosynthesis of 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), low bone mass, and increased fracture risk. Our previous data with human mesenchymal stem cells (hMSCs) indicated that vitamin D metabolism in hMSCs is regulated as it is in the kidney and promotes osteoblastogenesis in an autocrine/paracrine manner. In this study, we tested the hypothesis that FGF23 inhibits vitamin D metabolism and action in hMSCs. hMSCs were isolated from discarded marrow during hip arthroplasty, including two subjects receiving hemodialysis and a series of 20 subjects (aged 49-83 years) with estimated glomerular filtration rate (eGFR) data. The direct in vitro effects of rhFGF23 on hMSCs were analyzed by RT-PCR, Western immunoblot, and biochemical assays. Ex vivo analyses showed positive correlations for both secreted and membrane-bound αKlotho gene expression in hMSCs with eGFR of the subjects from whom hMSCs were isolated. There was downregulated constitutive expression of αKlotho, but not FGFR1 in hMSCs obtained from two hemodialysis subjects. In vitro, rhFGF23 countered vitamin D-stimulated osteoblast differentiation of hMSCs by reducing the vitamin D receptor, CYP27B1/1α-hydroxylase, biosynthesis of 1α,25(OH)₂D₃, and signaling through BMP-7. These data demonstrate that dysregulated vitamin D metabolism in hMSCs may contribute to impaired osteoblastogenesis and altered bone and mineral metabolism in CKD subjects due to elevated FGF23. This supports the importance of intracellular vitamin D metabolism in autocrine/paracrine regulation of osteoblast differentiation in hMSCs.

Changes in bone mineral density and bone turnover markers during treatment with teriparatide in pregnancy- and lactation-associated osteoporosis

CONTEXT

Teriparatide (TPTD) therapy has been proposed as a potential treatment strategy in severe cases of pregnancy- and lactation-associated osteoporosis (PLO) characterized by the occurrence of fragility fractures in the third trimester or early postpartum.

OBJECTIVE

To investigate the changes in bone mineral density (BMD) and bone turnover markers in patients with PLO with and without TPTD treatment.

DESIGN

Retrospective cohort study.

PATIENTS

Thirty-two patients with PLO who presented with multiple vertebral fractures to a tertiary institution between 2007 and 2015 were included.

MEASUREMENTS

Changes in BMD at the lumbar spine (LSBMD) and proximal femur after 12 months of daily subcutaneous injections of 20 μg TPTD (n = 27) were assessed. Subjects who rejected the TPTD treatment were used as controls (n = 5).

RESULTS

LSBMD increased in both subjects treated with TPTD and controls, with greater increases in the TPTD group (15.5 ± 6.6% vs 7.5 ± 7.1%, P = .020) after adjustment for age and baseline LSBMD. During follow-up, serum levels of osteocalcin (OCN) and C-telopeptide of type I collagen (CTX) increased significantly in the TPTD group. In multivariate linear regression models, TPTD treatment (adjusted β = 7.92, P = .032) and younger age (adjusted β = 1.06, P = .046), but not baseline LSBMD, body mass index, serum OCN level and CTX level, were independently associated with greater increases in LSBMD.

CONCLUSIONS

In patients with PLO, LSBMD at 12 months increased in both the TPTD-treated and control groups. TPTD treatment and younger age were associated with greater increases in LSMBD irrespective of baseline LSBMD.

Intermittent Administration of Parathyroid Hormone 1-34 Enhances Osteogenesis of Human Mesenchymal Stem Cells by Regulating Protein Kinase Cδ

Human mesenchymal stem cells (hMSCs) can differentiate into osteoblasts and are regulated by chemical cues. The recombinant N-terminal (1–34 amino acids) fragment of the parathyroid hormone (PTH (1–34)) is identified to promote osteogenesis. The osteoanabolic effects of intermittent PTH (1–34) treatment are linked to a complex consisting of signaling pathways; additionally, protein kinase C (PKC) act as mediators of multifunctional signaling transduction pathways, but the role of PKC δ (PKCδ), a downstream target in regulating osteoblast differentiation during intermittent administration of PTH (1–34) is less studied and still remains elusive. The purpose of this study is to examine the role of PKCδ during intermittent and continuous PTH (1–34) administration using osteoblast-lineage-committed hMSCs. Relative gene expression of osteoblast-specific genes demonstrated significant upregulation of RUNX2, type I Collagen, ALP, and Osterix and increased alkaline phosphatase activity in the presence of PTH (1–34). Intermittent PTH (1–34) administration increased PKC activity at day 7 of osteogenic differentiation, whereas inhibition of PKC activity attenuated these effects. In addition, the specific isoform PKCδ was activated upon treatment. These findings demonstrate that intermittent PTH (1–34) treatment enhances the osteogenesis of hMSCs by upregulating osteoblast-specific genes via PKCδ activation.

Chronic kidney disease and vitamin D metabolism in human bone marrow-derived MSCs

Vitamin D that is synthesized in the skin or is ingested undergoes sequential steps of metabolic activation via a cascade of cytochrome P450 enzymatic hydroxylations in the liver and kidney to produce 1α,25-dihydroxyvitamin D (1α,25(OH)2 D). There are many tissues that are able to synthesize 1α,25(OH)2 D, but the biological significance of extrarenal hydroxylases is unresolved. Human marrow-derived mesenchymal stem cells (marrow stromal cells, hMSCs) give rise to osteoblasts, and their differentiation is stimulated by 1α,25(OH)2 D. In addition to being targets of 1α,25(OH)2 D, hMSCs can synthesize it; on the basis of those observations, we further examined the local autocrine/paracrine role of vitamin D metabolism in osteoblast differentiation. Research with hMSCs from well-characterized subjects provides an innovative opportunity to evaluate the effects of clinical attributes on the regulation of hMSCs. Like the renal 1α-hydroxylase, the enzyme in hMSCs is constitutively decreased with age and chronic kidney disease (CKD); both are regulated by PTH1-34, insulin-like growth factor 1, calcium, 1α,25(OH)2 D, 25(OH)D, and fibroblast growth factor 23. CKD is associated with impaired renal biosynthesis of 1α,25(OH)2 D, low bone mass, and increased fracture risk. Studies with hMSCs from CKD patients or aged subjects indicate that circulating 25(OH)D may have an important role in osteoblast differentiation on vitamin D metabolism and action in hMSCs.

Glycinol enhances osteogenic differentiation and attenuates the effects of age on mesenchymal stem cells

AIM

Phytoestrogens, such as glycinol, have recently gained significant attention as an alternative therapy for osteoporosis due to their structural similarity to estradiol and their bone-generating potential.

METHODS

The osteogenic effects of glycinol were investigated in human bone marrow mesenchymal stem cells (BMSCs) derived from older (>50 years old) and younger subjects (<25 years old).

RESULTS

BMSCs isolated from older donors demonstrated reduced osteogenesis. 17β-estradiol and glycinol exposure rescued the age-related reduction in osteogenic differentiation of BMSCs. These results correlated with the induction of osteogenic genes and estrogen receptor-α (ER-α) following glycinol treatment. ER antagonist studies further support that glycinol promotes osteogenesis through ER signaling.

CONCLUSION

The results from these studies support investigating glycinol as a potential preventive or treatment for osteoporosis.

Role of Osteoblast Gi Signaling in Age-Related Bone Loss in Female Mice

Age-related bone loss is an important risk factor for fractures in the elderly; it results from an imbalance in bone remodeling mainly due to decreased bone formation. We have previously demonstrated that endogenous G protein-coupled receptor (GPCR)-driven Gi signaling in osteoblasts (Obs) restrains bone formation in mice during growth. Here, we launched a longitudinal study to test the hypothesis that Gi signaling in Obs restrains bone formation in aging mice, thereby promoting bone loss. Our approach was to block Gi signaling in maturing Obs by the induced expression of the catalytic subunit of pertussis toxin (PTX) after the achievement of peak bone mass. In contrast to the progressive cancellous bone loss seen in aging sex-matched littermate control mice, aging female Col1(2.3)+/PTX+ mice showed an age-related increase in bone volume. Increased bone volume was associated with increased bone formation at both trabecular and endocortical surfaces as well as increased bending strength of the femoral middiaphyses. In contrast, male Col1(2.3)+/PTX+ mice were not protected from age-related bone loss. Our results indicate that Gi signaling markedly restrains bone formation at cancellous and endosteal bone surfaces in female mice during aging. Blockade of the relevant Gi-coupled GPCRs represents an approach for the development of osteoporosis therapies-at least in the long bones of aging women.

Regenerative capacity of autologous stem cell transplantation in elderly: a report of biomedical outcomes

The occurrence of chronic diseases such as neurological, metabolic and cardiovascular degenerative disorders increases with age. Cell therapy is an emerging approach to the treatment of these conditions. Of particular interest is the application of autologous stem cells because it eliminates post-transplantation immune rejection and there are less ethical concerns associated with their use. The regenerative capacity of stem cells harvested from elderly people is however controversial. In this review, we analyze if self-renewal potential, differentiation capability and expression of stemness genes in stem cells collected from elderly patients validate their application in clinical trials and examine the results.

Vitamin D metabolism and regulation in pediatric MSCs

Vitamin D is crucial for mineral homeostasis and contributes to bone metabolism by inducing osteoblast differentiation of marrow stromal cells (MSCs). We recently reported that MSCs from adults demonstrate 1α-hydroxylase activity in vitro and express vitamin D-related genes; this raises a possible autocrine/paracrine role for D activation in pre-osteoblasts. In this studies, we tested the hypotheses that pediatric MSCs have 1α-hydroxylase activity and express vitamin D-related genes. With IRB approval, we isolated MSCs from discarded excess iliac marrow graft from 6 male and 6 female subjects (age 8-12 years) undergoing alveolar cleft repair. 1α-hydroxylation of substrate 25(OH)D₃ was measured by ELISA for 1α,25(OH)₂D. RT-PCR was used for gene expression. Pediatric MSCs showed a range of 1α-hydroxylase activity in vitro. There was constitutive expression of vitamin D receptor (VDR), megalin, d-hydroxylases (CYP27B1, CYP27A1, CYP2R1, and CYP24A1), and estrogen receptor (ER). There was 2.6-fold greater expression of CYP27B1 and 3.5-fold greater expression of CYP24A1 in MSCs from boys compared with girls. There was 2.4-fold greater expression of ERα and 3.2-fold greater expression of megalin in MSCs from boys. In preliminary studies, treatment of female pediatric MSCs with 10nM 17β-estradiol resulted in upregulation of CYP27B1 and CYP24A1, as well as VDR, megalin, ERα, and ERβ. Treatment with 25(OH)D₃ upregulated CYP27B1, VDR, and ERα. Expression and regulation of vitamin D related genes in pediatric hMSCs reinforces an autocrine/paracrine role for vitamin D in hMSCs. Finding striking gender differences in MSCs from children was not seen with MSCs from adults and adds insight to the metabolic environment of bone and presents a research approach for investigating and optimizing pediatric bone health.

Paracrine effects of haematopoietic cells on human mesenchymal stem cells

Stem cell function decline during ageing can involve both cell intrinsic and extrinsic mechanisms. Bone and blood formation are intertwined in bone marrow, therefore haematopoietic cells and bone cells could be extrinsic factors for each other. In this study, we assessed the paracrine effects of extrinsic factors from haematopoietic cells on human mesenchymal stem cells (MSCs). Our data showed that haematopoietic cells stimulate proliferation, osteoblast differentiation and inhibit senescence of MSCs; TNF-α, PDGF-β, Wnt1, 4, 6, 7a and 10a, sFRP-3 and sFRP-5 are dominantly expressed in haematopoietic cells; the age-related increase of TNF-α in haematopoietic cells may perform as a negative factor in the interactions of haematopoietic cells on MSCs via TNF-α receptors and then activating NF-κB signaling or Wnt/β-catenin signaling to induce senescence and reduce osteoblast differentiation in MSCs. In conclusion, our data demonstrated that there are paracrine interactions of haematopoietic cells on human MSCs; immunosenescence may be one of the extrinsic mechanisms by which skeletal stem cell function decline during human skeletal ageing.

Effect of age on regulation of human osteoclast differentiation

Human skeletal aging is characterized as a gradual loss of bone mass due to an excess of bone resorption not balanced by new bone formation. Using human marrow cells, we tested the hypothesis that there is an age-dependent increase in osteoclastogenesis due to intrinsic changes in regulatory factors [macrophage-colony stimulating factor (M-CSF), receptor activator of NF-κB ligand (RANKL), and osteoprotegerin (OPG)] and their receptors [c-fms and RANK]. In bone marrow cells (BMCs), c-fms (r = 0.61, P = 0.006) and RANK expression (r = 0.59, P = 0.008) were increased with age (27-82 years, n = 19). In vitro generation of osteoclasts was increased with age (r = 0.89, P = 0.007). In enriched marrow stromal cells (MSCs), constitutive expression of RANKL was increased with age (r = 0.41, P = 0.049) and expression of OPG was inversely correlated with age (r = -0.43, P = 0.039). Accordingly, there was an age-related increase in RANKL/OPG (r = 0.56, P = 0.005). These data indicate an age-related increase in human osteoclastogenesis that is associated with an intrinsic increase in expression of c-fms and RANK in osteoclast progenitors, and, in the supporting MSCs, an increase in pro-osteoclastogenic RANKL expression and a decrease in anti-osteoclastogenic OPG. These findings support the hypothesis that human marrow cells and their products can contribute to skeletal aging by increasing the generation of bone-resorbing osteoclasts. These findings help to explain underlying molecular mechanisms of progressive bone loss with advancing age in humans.

Ankle-brachial index and bone turnover in patients on dialysis

An association between atherosclerosis and osteoporosis has been reported in several studies. This association could result from local intraosseous atherosclerosis and ischemia, which is shown by limb osteoporosis in patients with peripheral artery disease (PAD), but also could result from bidirectional communication between the skeleton and blood vessels. Systemic bone disorders and PAD are frequent in ESRD. Here, we investigated the possible interaction of these disorders. For 65 prevalent nondiabetic patients on hemodialysis, we measured ankle-brachial pressure index (ABix) and evaluated mineral and bone disorders with bone histomorphometry. In prevalent patients on hemodialysis, PAD (ABix<0.9 or >1.4/incompressible) was associated with low bone turnover and pronounced osteoblast resistance to parathyroid hormone (PTH), which is indicated by decreased double-labeled surface and osteoblast surface (P<0.001). Higher osteoblast resistance to PTH in patients with PAD was characterized by weaker correlation coefficients (slopes) between serum PTH and double-labeled surface (P=0.02) or osteoblast surface (P=0.03). The correlations between osteoclast number or eroded surface and serum mineral parameters, including PTH, did not differ for subjects with normal ABix and PAD. Common vascular risk factors (dyslipidemia, smoking, and sex) were similar for normal, low, and incompressible ABix. Patients with PAD were older and had high C-reactive protein levels and longer hemodialysis vintage. These results indicate that, in prevalent nondiabetic patients with ESRD, PAD associates with low bone turnover and pronounced osteoblast resistance to PTH.

Enhancement of Flow-Induced AP-1 Gene Expression by Cyclosporin A Requires NFAT-Independent Signaling in Bone Cells

Growing evidence suggests that aging compromises the ability of the skeleton to respond to anabolic mechanical stimuli. Recently, we reported that treating senescent mice with Cyclosporin A (CsA) rescued aging-related deficits in loading-induced bone formation. Given that the actions of CsA are often attributed to inhibition of the calcineurin/NFAT axis, we hypothesized that CsA enhances gene expression in bone cells exposed to fluid flow, by inhibiting nuclear NFATc1 accumulation. When exposed to flow, MC3T3-E1 osteoblastic cells exhibited rapid nuclear accumulation of NFATc1 that was abolished by CsA treatment. Under differentiation conditions, intermittent CsA treatment enhanced gene expression of late osteoblastic differentiation markers and activator protein 1 (AP-1) family members. Superimposing flow upon CsA further enhanced expression of the AP-1 members Fra-1 and c-Jun. To delineate the contribution of NFAT in this response, cells were treated with VIVIT, a specific inhibitor of the calcineurin/NFAT interaction. Treatment with VIVIT blocked flow-induced nuclear NFATc1 accumulation but did not recapitulate the CsA-mediated enhancement of flow-induced AP-1 component gene expression. Taken together, our study is the first to demonstrate that CsA enhances mechanically-induced gene expression of AP-1 components in bone cells, and suggests that this response requires calcineurin-dependent mechanisms that are independent of inhibiting NFATc1 nuclear accumulation.

Impact of age on survival predictability of bone turnover markers in hemodialysis patients

BACKGROUND

Abnormalities in serum alkaline phosphatase (ALP) and intact parathyroid hormone (PTH) concentrations, as biochemical markers of bone turnover in dialysis patients, correlate with increased mortality in maintenance hemodialysis (MHD) patients. Changes in bone turnover rate vary with age. The mortality predictability of serum ALP and PTH levels in MHD patients may be different across ages.

METHODS

We examined differences across four age groups (18 to <45, 45 to <65, 65 to <75 and ≥ 75 years) in the mortality predictability of serum ALP and PTH in 102 149 MHD patients using Cox models.

RESULTS

Higher serum ALP levels were associated with higher mortality across all ages; however, the ALP-mortality association was much stronger in young patients (<45 years) compared with older patients. The association between higher serum PTH levels and mortality was stronger in older patients compared with the younger groups. Serum PTH levels were incrementally associated with mortality only in middle-aged and elderly patients (≥ 45 years). Compared with patients with serum PTH 150 to <300 pg/mL, the death risks were higher in patients with serum PTH 300 to <600 pg/mL [HRs (95% CI): 1.05 (1.01-1.10), 1.15 (1.10-1.21) and 1.25 (1.19-1.31) for patients 45 to <65, 65 to <75 and ≥ 75 years, respectively], and ≥ 600 pg/mL [HRs(95% CI): 1.07 (1.01-1.14), 1.31(1.21-1.42) and 1.45(1.33-1.59) for age categories 45 to <65, 65 to <75 and ≥ 75 years, respectively]. However, no significant association between higher serum PTH levels and mortality was observed in patients <45 years.

CONCLUSIONS

There are important differences in mortality-predictability of serum ALP and PTH in older MHD patients compared with their younger counterparts. The effect of age needs to be considered when interpreting the prognostic implications of serum ALP and PTH levels.

Vitamin D metabolism and action in human marrow stromal cells: effects of chronic kidney disease

Human marrow stromal cells (hMSCs) are targets of 1α,25-dihydroxyvitamin D [1α,25(OH)2D3] action to promote their differentiation to osteoblasts, but they also participate in vitamin D metabolism by converting 25-dihydroxyvitamin D3 [25(OH)D3] to 1α,25(OH)2D3 by 1α-hydroxylase (CYP27B1). Chronic kidney disease (CKD) is associated with impaired renal biosynthesis of 1α,25(OH)2D, low bone mass, and increased fracture risk. We tested whether CKD influences hMSCs' responses to vitamin D3 metabolites. The hMSCs were obtained from tissues discarded during arthroplasty for hip osteoarthrosis, including a subject who had been undergoing hemodialysis for 2+ years. There was a significant positive correlation between in vitro stimulation of osteoblastogenesis (alkaline phosphatase activity) by 1α,25(OH)2D3 and subjects' estimated glomerular filtration rate (eGFR, r=0.47, p=0.015, n=26, 56-83 years of age). Osteoblastogenesis was stimulated in hMSCs from both the hemodialysis and control subjects by 1α,25(OH)2D3 (10μM), 25(OH)D3 (100μM), or D3 (1000μM). Thus, vitamin D metabolism may play an autocrine/paracrine role in osteoblast differentiation of hMSCs. These findings suggest that in CKD patients 25(OH)D-sufficiency may play an important role in skeletal health; osteoblastic bone formation in CKD patients may not be optimal unless there is sufficient serum 25(OH)D substrate for the MSCs to synthesize and respond to local 1α,25(OH)2D. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Histone deacetylation mediates the rejuvenation of osteoblastogenesis by the combination of 25(OH)D3 and parathyroid hormone in MSCs from elders

Vitamin D metabolites are important effectors of bone and mineral homeostasis. Human bone marrow stromal cells (hMSCs) are targets of 1α,25-dihydroxyvitamin D [1α,25(OH)2D] action to promote their differentiation to osteoblasts. Osteoblastogenesis is also stimulated by 25-hydroxyvitamin D [25(OH)D], an effect that requires conversion to 1α,25(OH)2D3 by 25-hydroxyvitamin D3 1α-hydroxylase (CYP27B1). These findings support an autocrine/paracrine role of vitamin D metabolism in osteoblastogenesis of hMSCs. In this study, we assessed whether and by what mechanisms osteoblastogenesis could be rejuvenated with hMSCs from elders. First, knockdown studies with VDR-siRNA showed that both the pro-differentiation and anti-proliferative effects of 1α,25(OH)2D3 required VDR. Second, 100nM 25(OH)D3 (p<0.01 vs. control, ANOVA) and 100nM PTH1-34 (p<0.05) significantly stimulated alkaline phosphatase (ALP) activity (a measure of osteoblastogenesis), with a synergistic effect when combined (p<0.001). Scriptaid, an inhibitor of histone deacetylase, blocked the effect of 25(OH)D3 and PTH on osteoblastogenesis. Scriptaid alone downregulated VDR in hMSCs. These data demonstrate that histone deacetylation is required for the synergistic effect of 25(OH)D3 and PTH on osteoblastogenesis in hMSCs. Both VDR siRNA and Scriptaid dowregulated VDR mRNA and inhibited osteoblastogenesis. Thus, epigenetic regulation of the VDR may be central to rejuvenating osteoblastogenesis in hMSCs from elders. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Vitamin D metabolism in human bone marrow stromal (mesenchymal stem) cells

There are many human extra-renal tissues and cells that biosynthesize 1α,25-dihydroxyvitamin D (1α,25(OH)(2)D) by the action of CYP27B1/1α-hydroxylase. Human marrow stromal cells (hMSCs), also known as mesenchymal stem cells, were isolated from marrow discarded from well-characterized, consented subjects during common orthopedic procedures. Human MSCs can give rise to osteoblasts, chondrocytes, adipocytes, and other lineages. Their in vitro differentiation to osteoblasts is stimulated by 1α,25(OH)(2)D, and recent evidence indicates that they have the capacity to metabolize vitamin D in a regulated manner. Human MSCs express the vitamin D receptor, 25-hydroxylases, 1α-hydroxylase, and 24-hydroxylase; stimulation of in vitro osteoblastogenesis by 25(OH)D depends on the activity of CYP27B1/1α-hydroxylase. The finding that hMSCs are a both a producer and target of 1α,25(OH)(2)D suggests a potential autocrine/paracrine role of vitamin D metabolism in osteoblast differentiation. Expression and enzyme activity of CYP27B1/1α-hydroxylase are upregulated by substrate 25(OH)D and Parathyroid Hormone (PTH) and are downregulated by 1α,25(OH)(2)D. With subject age, there are decreases in basal osteoblast potential and in stimulation of osteoblastogenesis by 1α,25(OH)(2)D, 25(OH)D, and PTH. In vitro treatment with a combination of 25(OH)D and PTH rejuvenated osteoblastogenesis with hMSCs from elders; this was attributable to increases in CYP27B1/1α-hydroxylase and in receptor for each hormone by the reciprocal factor. Other clinical variables beside age, i.e. low serum 25(OH)D or low estimated glomerular filtration rate, are correlated with reduced osteoblastogenesis. These studies suggest that osteoblastogenesis may not be optimal unless there is sufficient serum 25(OH)D substrate for hMSCs to synthesize and respond to local 1α,25(OH)(2)D.

Functional impairment of bone formation in the pathogenesis of osteoporosis: the bone marrow regenerative competence

The skeleton is a high-renewal organ that undergoes ongoing cycles of remodeling. The regenerative bone formation arm ultimately declines in the aging, postmenopausal skeleton, but current therapies do not adequately address this deficit. Bone marrow is the primary source of the skeletal anabolic response and the mesenchymal stem cells (MSCs), which give rise to bone matrix-producing osteoblasts. The identity of these stem cells is emerging, but it now appears that the term 'MSC' has often been misapplied to the bone marrow stromal cell (BMSC), a progeny of the MSC. Nevertheless, the changes in BMSC phenotype associated with age and estrogen depletion likely contribute to the attenuated regenerative competence of the marrow and may reflect alterations in MSC phenotype. Here we summarize current concepts in bone marrow MSC identity, and within this context, review recent observations on changes in bone marrow population dynamics associated with aging and menopause.

Clinical characteristics influence in vitro action of 1,25-dihydroxyvitamin D(3) in human marrow stromal cells

Vitamin D is important for bone health, with low vitamin D levels being associated with skeletal fragility and fractures. Among its other biological activities, 1,25-dihydroxyvitamin D (1,25(OH)(2) D), stimulates the in vitro differentiation of human marrow stromal cells (hMSCs) to osteoblasts, which can be monitored by increases in alkaline phosphatase enzyme activity or osteocalcin gene expression. In this study, we tested the hypotheses that age and clinical attributes of subjects influence in vitro responsiveness of hMSCs to 1,25(OH)(2) D(3) . In a cohort of subjects whose hMSCs were isolated from bone marrow discarded during hip replacement surgery for osteoarthritis, there were significant inverse correlations with age for bone mineral density, renal function, body mass index, fat mass index, and lean mass index (n = 36-53). There were significant correlations with serum 25(OH)D for serum parathyroid hormone (PTH), body mass index, fat mass index, and lean mass index (n = 47-50). In vivo-in vitro correlation analyses indicated that there were significantly greater in vitro effects of 1,25(OH)(2) D(3) to stimulate osteoblast differentiation in hMSCs obtained from subjects who were younger than 65 years of age, or who had serum 25(OH)D ≤ 20 ng/mL, elevated serum PTH, or better renal function, assessed by estimated glomerular filtration rate. The greater in vitro stimulation of osteoblast differentiation by 1,25(OH)(2) D(3) in hMSCs from vitamin D-deficient subjects suggests that vitamin D replenishment may lead to more vigorous bone formation in subjects at risk.

Age-related decline in osteoblastogenesis and 1α-hydroxylase/CYP27B1 in human mesenchymal stem cells: stimulation by parathyroid hormone

With aging, there is a decline in bone mass and in osteoblast differentiation of human mesenchymal stem cells (hMSCs) in vitro. Osteoblastogenesis can be stimulated with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ] and, in some hMSCs, by the precursor 25-hydroxyvitamin D(3) (25OHD(3) ). CYP27B1/1α-hydroxylase activates 25OHD(3) and, to a variable degree, hMSCs express CYP27B1. In this study, we tested the hypotheses (i) that age affects responsiveness to 25OHD(3) and expression/activity of CYP27B1 in hMSCs and (ii) that parathyroid hormone (PTH) upregulates CYP27B1 in hMSCs, as it does in renal cells. There were age-related declines in osteoblastogenesis (n=8, P=0.0286) and in CYP27B1 gene expression (n=27, r= -0.498; P=0.008) in hMSCs. Unlike hMSCs from young subjects (≤50 years), hMSCs from older subjects (≥55 years) were resistant to 25OHD(3) stimulation of osteoblastogenesis. PTH1-34 (100 nm) provided hMSCs with responsiveness to 25OHD(3) (P=0.0313, Wilcoxon matched pairs test) and with two episodes of increased 1,25(OH)(2) D(3) synthesis, of cAMP response element binding protein (CREB) activation, and of CYP27B1 upregulation. Both increases in CYP27B1 expression by PTH were obliterated by CREB-siRNA or KG-501 (which specifically inhibits the downstream binding of activated CREB). Only the second period of CREB signaling was diminished by AG1024, an inhibitor of insulin-like growth factor-I receptor kinase. Thus, PTH stimulated hMSCs from elders with responsiveness to 25OHD(3) by upregulating expression/activity of CYP27B1 and did so through CREB and IGF-I pathways.