The mitogenic effect of parathyroid hormone is associated with E2F-dependent activation of cyclin-dependent kinase 1 (cdc2) in osteoblast precursors

Intermittent parathyroid hormone enhances the healing of medication-related osteonecrosis of the jaw lesions in rice rats

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe adverse event in patients treated with antiresorptives. Management of MRONJ is challenging, and no non-antibiotic, established medical treatment exists. Intermittent parathyroid hormone (iPTH) has been used off-label to treat MRONJ with favorable results. However, its medical efficacy has rarely been substantiated in clinical or preclinical experiments. Using a validated rice rat, infection-based model of MRONJ, we evaluated the effects of iPTH on established MRONJ. We hypothesize that iPTH contributes to MRONJ resolution by enhancing alveolar bone turnover and healing oral soft tissues. Eighty-four rice rats began a standard rodent chow diet at age 4 weeks to induce localized periodontitis. Rats were simultaneously randomized to receive saline (vehicle, VEH) or zoledronic acid (ZOL, 80 μg/kg IV) every 4 weeks. Oral exams were conducted bi-weekly to assign a gross quadrant grade (GQG, 0-4) to evaluate any lesion at the lingual aspect of the interdental space between maxillary molar (M2) and M3. 14 of 20 VEH-treated rice rats (70%) developed maxillary localized periodontitis with GQG 2-3 after 30 ± 10 weeks of saline. Additionally, 40 of 64 ZOL-treated rice rats with periodontitis developed MRONJ-like lesions after 30 ± 10 weeks of ZOL treatment. Rice rats with localized periodontitis or MRONJ-like lesions were treated with saline or iPTH (40 μg/kg) subcutaneously (SC) 3 times/week For 6 weeks until euthanasia. We found that iPTH -treated ZOL rats had a lower prevalence of MRONJ (p < 0.001), with lower severity extent of oral lesions (p = 0.003) and percentage of empty osteocyte lacunae (p < 0.001). ZOL rats treated with iPTH displayed a higher osteoblast surface (p < 0.001), more osteoblasts (p < 0.001), higher osteoclast surface (p < 0.001) and more osteoclasts (p = 0.002) at alveolar bone surfaces than ZOL/VEH rats. Greater gingival epithelial thickness and epithelial cell proliferation rate was found in the oral mucosa and gingiva of ZOL/PTH rats than in ZOL/VEH rats (p < 0.001). Our data suggest that iPTH is an efficacious non-operative medicinal therapy that accelerates oral healing and enhances the resolution of MRONJ lesions in ZOL-treated rice rats.

Undifferentiated Pleomorphic Sarcoma and Hyperparathyroidism in an Adolescent Male: A Case Report and Review of Hyperparathyroidism-associated Sarcomas

The association between hyperparathyroidism and sarcoma is extremely rare with other reported cases describing the development of osteosarcoma and chondrosarcomas in middle-aged adults. This case describes an adolescent male with hyperparathyroidism and a pathologic fracture of a biopsy-proven brown tumor in the distal right femur. The fracture healed but later developed an undifferentiated pleomorphic sarcoma of the bone at the site of the known brown tumor. Although in vitro and in vivo studies have demonstrated the risks of elevated parathyroid hormone with development of sarcomas, there is limited evidence of a human association. The effects of elevated parathyroid hormone on the skeletally immature bone in the setting of sarcoma formation are currently not well understood without current description of adolescent hyperparathyroidism-associated sarcomas. This case highlights a sarcoma originating at a pathologically proven brown tumor within an adolescent male, discusses the association of sarcoma with hyperparathyroidism, and reviews the other nine reported cases in the literature.

Loss of cyclin-dependent kinase 1 impairs bone formation, but does not affect the bone-anabolic effects of parathyroid hormone

Bone mass is maintained by a balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. Although recent genetic studies have uncovered various mechanisms that regulate osteoblast differentiation, the molecular basis of osteoblast proliferation remains unclear. Here, using an osteoblast-specific loss-of-function mouse model, we demonstrate that cyclin-dependent kinase 1 (Cdk1) regulates osteoblast proliferation and differentiation. Quantitative RT-PCR analyses revealed that Cdk1 is highly expressed in bone and is down-regulated upon osteoblast differentiation. We also noted that Cdk1 is dispensable for the bone-anabolic effects of parathyroid hormone (PTH). Cdk1 deletion in osteoblasts led to osteoporosis in adult mice due to low bone formation, but did not affect osteoclast formation in vivo Cdk1 overexpression in osteoblasts promoted proliferation, and conversely, Cdk1 knockdown inhibited osteoblast proliferation and promoted differentiation. Of note, we provide direct evidence that PTH's bone-anabolic effects occur without enhancing osteoblast proliferation in vivo Furthermore, we found that Cdk1 expression in osteoblasts is essential for bone fracture repair. These findings may help reduce the risk of nonunion after bone fracture and identify patients at higher risk for nonresponse to PTH treatment. Collectively, our results indicate that Cdk1 is essential for osteoblast proliferation and that it functions as a molecular switch that shifts osteoblast proliferation to maturation. We therefore conclude that Cdk1 plays an important role in bone formation.

Human cytomegalovirus UL97 kinase alters the accumulation of CDK1

The UL97 protein kinase is a serine/threonine kinase expressed by human cytomegalovirus (CMV) that phosphorylates ganciclovir. An investigation of the subcellular localization of pUL97 in infected cells indicated that, early in infection, pUL97 localized to focal sites in the nucleus that transitioned to subnuclear compartments and eventually throughout the entire nucleus. When UL97 kinase activity was eliminated with a K355M mutation or pharmacologically inhibited with maribavir, the expansion and redistribution of pUL97 foci within the nucleus was delayed, nuclear reorganization did not occur and assembly complexes in the cytoplasm failed to form normally. As UL97 kinase and its homologues appear to be functionally related to CDK1, a known regulator of nuclear structural organization, the effects of the UL97 kinase on CDK1 were investigated. Expression of CDK1 in infected cells appeared to be induced by UL97 kinase activity at the level of transcription and was not tied to other virus life-cycle events, such as viral DNA replication or virion assembly. These results suggest that, in addition to phosphorylating CDK1 targets, the UL97 kinase modifies G₂/M cell-cycle checkpoint regulators, specifically CDK1, to promote virus replication.

Role of PDE3A in regulation of cell cycle progression in mouse vascular smooth muscle cells and oocytes: implications in cardiovascular diseases and infertility

Phosphodiesterase-3 (PDE3) is a major cAMP-hydrolyzing PDE in vascular smooth muscle cells (VSMCs) and oocytes. The exact role and contribution of the two PDE3 isoforms, PDE3A and PDE3B, in VSMC growth regulation and oocyte maturation was examined using PDE3A (3A) and PDE3B (3B) knockout (KO) mouse models. PDE3A-deficient VSMCs exhibit marked reduction in mitogen-induced cell growth due to cell cycle arrest at G₀-G₁ phase, which resulted from dysregulation of cAMP/protein kinase A (PKA)-activated and mitogen-activated protein kinase (MAPK)-signaling pathways, as well as from alterations in key cell cycle regulatory proteins. Similarly, PDE3A-deficient oocytes exhibit cell cycle arrest at G₂/M phase because increased cAMP/PKA signaling in KO oocytes most likely inhibits Cdc25B-catalyzed dephosphorylation/activation of Cdc2 (maturation promoting factor (MPF)), a key regulator of G₂/M transition.

Osteoporotic fracture and parathyroid hormone

Osteoporosis and age-related bone loss is associated with changes in bone remodeling characterized by decreased bone formation relative to bone resorption, resulting in bone fragility and increased risk of fractures. Stimulating the function of bone-forming osteoblasts, is the preferred pharmacological intervention for osteoporosis. Recombinant parathyroid hormone (PTH), PTH(1-34), is an anabolic agent with proven benefits to bone strength and has been characterized as a potential therapy for skeletal repair. In spite of PTH's clinical use, safety is a major consideration for long-term treatment. Studies have demonstrated that intermittent PTH treatment enhances and accelerates the skeletal repair process via a number of mechanisms. Recent research into the molecular mechanism of PTH action on bone tissue has led to the development of PTH analogs to control osteoporotic fractures. This review summarizes a number of advances made in the field of PTH and bone fracture to combat these injuries in humans and in animal models. The ultimate goal of providing an alternative to PTH, currently the sole anabolic therapy in clinical use, to promote bone formation and improve bone strength in the aging population is yet to be achieved.

Phosphodiesterase 3A (PDE3A) deletion suppresses proliferation of cultured murine vascular smooth muscle cells (VSMCs) via inhibition of mitogen-activated protein kinase (MAPK) signaling and alterations in critical cell cycle regulatory proteins

Cyclic nucleotide phosphodiesterase 3 (PDE3) is an important regulator of cyclic adenosine monophosphate (cAMP) signaling within the cardiovascular system. In this study, we examined the role of PDE3A and PDE3B isoforms in regulation of growth of cultured vascular smooth muscle cells (VSMCs) and the mechanisms by which they may affect signaling pathways that mediate mitogen-induced VSMC proliferation. Serum- and PDGF-induced DNA synthesis in VSMCs grown from aortas of PDE3A-deficient (3A-KO) mice was markedly less than that in VSMCs from PDE3A wild type (3A-WT) and PDE3B-deficient (3B-KO) mice. The reduced growth response was accompanied by significantly less phosphorylation of extracellular signal-regulated kinase (ERK) in 3A-KO VSMCs, most likely due to a combination of greater site-specific inhibitory phosphorylation of Raf-1(Ser-²⁵⁹) by protein kinase A (PKA) and enhanced dephosphorylation of ERKs due to elevated mitogen-activated protein kinase phosphatase 1 (MKP-1). Furthermore, 3A-KO VSMCs, compared with 3A-WT, exhibited higher basal PKA activity and cAMP response element-binding protein (CREB) phosphorylation, higher levels of p53 and p53 phosphorylation, and elevated p21 protein together with lower levels of Cyclin-D1 and retinoblastoma (Rb) protein and Rb phosphorylation. Adenoviral overexpression of inactive CREB partially restored growth effects of serum in 3A-KO VSMCs. In contrast, exposure of 3A-WT VSMCs to VP16 CREB (active CREB) was associated with inhibition of serum-induced DNA synthesis similar to that in untreated 3A-KO VSMCs. Transfection of 3A-KO VSMCs with p53 siRNA reduced p21 and MKP-1 levels and completely restored growth without affecting amounts of Cyclin-D1 and Rb phosphorylation. We conclude that PDE3A regulates VSMC growth via two complementary pathways, i.e. PKA-catalyzed inhibitory phosphorylation of Raf-1 with resulting inhibition of MAPK signaling and PKA/CREB-mediated induction of p21, leading to G₀/G₁ cell cycle arrest, as well as by increased accumulation of p53, which induces MKP-1, p21, and WIP1, leading to inhibition of G₁ to S cell cycle progression.

Cytokine-induced tumor suppressors: a GRIM story

Cytokines belonging to the IFN family are potent growth suppressors. In a number of clinical and preclinical studies, vitamin A and its derivatives like retinoic acid (RA) have been shown to exert synergistic growth-suppressive effects on several tumor cells. We have employed a genome-wide expression-knockout approach to identify the genes critical for IFN/RA-induced growth suppression. A number of novel genes associated with Retinoid-Interferon-induced Mortality (GRIM) were isolated. In this review, we will describe the molecular mechanisms of actions of one, GRIM-19, which participates in multiple pathways for exerting growth control and/or cell death. This protein is emerging as a new tumor suppressor. In addition, GRIM-19 appears to participate in innate immune responses as its activity is modulated by several viruses and bacteria. Thus, GRIMs seem to couple with multiple biological responses by acting at critical nodes.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Lipoxygenase metabolites are mediators of PTH-dependent human osteoblast growth

PTH-induced osteoblast proliferation may contribute to its anabolic effects in bone. Since PTH-dependent osteoblast-like cell (Ob) growth is mediated via protein kinase C (PKC) and MAP kinase-kinase (MEK) and since lipoxygenase (LO) products activate PKC in a number of cell types, we assessed the expression of LO pathways in primary human cultured Ob. Ob from pre- or post-menopausal women were cultured and were treated with PTH and assayed for the expression of 12-LO and both type I and type II 15-LO mRNA and for the release their enzymatic products, 12- and 15-hydroxyeicosatetraenoic acid (HETE). Cells were also treated with PTH for stimulation DNA synthesis. First, Ob express platelet type- 12-LO and both type I and type II 15-LO mRNA and release their enzymatic products, 12- and 15-hydroxyeicosatetraenoic acid (HETE). Second, in female Ob, PTH induced a rapid increase in 12-HETE (50 fold increase) and 15-HETE (80 fold increase) and increased the expression of 12-LO mRNA but not of the two isoforms of 15-LO. PTH as well as 12 and 15-HETE stimulated DNA synthesis in Ob. The LO inhibitor baicalein inhibited PTH-stimulated DNA synthesis, which was reversed in the presence of either 12- or 15-HETE. A PKC inhibitor (bisindolylmaleimide I) as well as a MEK inhibitor (PD 98059) completely inhibited the stimulation of DNA synthesis by PTH, 12-HETE and the combination of PTH and 12-HETE. In contrast, 15-HETE-induced DNA synthesis was not abolished by these inhibitors. Further, 15-HETE partially restored the stimulatory effect of PTH on DNA synthesis in cells treated with PKC or MEK inhibitors. Finally, PTH- induced ERK1/2 phosphorylation, was blocked by a MEK inhibitor. These results demonstrate a novel mechanism of PTH-induced human bone cell proliferation operating through LO enzymes.

Tumor suppressive protein gene associated with retinoid-interferon-induced mortality (GRIM)-19 inhibits src-induced oncogenic transformation at multiple levels

Interferons (IFNs) inhibit the growth of infectious pathogens and tumor development. Although IFNs are potent tumor suppressors, they modestly inhibit the growth of some human solid tumors. Their weak activity against such tumors is augmented by co-treatment with differentiation-inducing agents such as retinoids. Previous studies from our laboratory identified a novel gene product, gene associated with retinoid-interferon-induced mortality (GRIM)-19, as an IFN/all-trans retinoic acid-induced growth suppressor. However, the mechanisms of its growth suppressive actions are unclear. The src-family of tyrosine kinases is important regulators of various cell growth responses. Mutational activation of src causes cellular transformation by altering transcription and cytoskeletal properties. In this study, we show that GRIM-19 suppresses src-induced cellular transformation in vitro and in vivo by down-regulating the expression of a number of signal transducer and activator of transcription-3 (STAT3)-dependent cellular genes. In addition, GRIM-19 inhibited the src-induced cell motility and metastasis by suppressing the tyrosyl phosphorylation of focal adhesion kinase, paxillin, E-cadherin, and gamma-catenin. Effects of GRIM-19 on src-induced cellular transformation are reversible in the presence of specific short hairpin RNA, indicating its direct effect on transformation. GRIM-19-mediated inhibition of the src-induced tyrosyl phosphorylation of cellular proteins, such as focal adhesion kinase and paxillin, seems to occur independently of the STAT3 protein. GRIM-19 had no significant effect on the cellular transformation induced by other oncogenes such as myc and Ha-ras. Thus, GRIM-19 not only blocks src-induced gene expression through STAT3 but also the activation of cell adhesion molecules.

Evidences for a role of p38 MAP kinase in the stimulation of alkaline phosphatase and matrix mineralization induced by parathyroid hormone in osteoblastic cells

Increased bone formation by PTH mainly results from activation of osteoblasts, an effect largely mediated by the cAMP-PKA pathway. Other pathways, however, are likely to be involved in this process. In this study we investigated whether PTH can activate p38 MAPK and the role of this kinase in osteoblastic cells. Bovine PTH(1-34) and forskolin markedly increased alkaline phosphatase (ALP) activity and doubled osteocalcin (Oc) expression in early differentiating MC3T3-E1 cells. These effects were associated with increase in cellular cAMP and activation of the MAP kinases ERK and p38. Activation of these MAP kinases was detectable after 1 h incubation with 10(-7) M PTH and lasted 1-2 h. Activation of p38 was mimicked by 10 microM forskolin and prevented by H89 suggesting a cAMP-PKA-dependent mechanism of p38 activation. Interestingly, PTH-induced ALP stimulation was dose-dependently inhibited by a specific p38 inhibitor with no change in the generation of cAMP and the production of osteocalcin. Similar inhibitory effect was obtained in cells stably expressing a dominant-negative p38 molecule. Finally, treatment of MC3T3-E1 cells with PTH for 3 weeks significantly enhanced matrix mineralization and this effect was markedly reduced by a selective p38 but not a specific MEK inhibitor. In conclusion, data presented in this study indicate that PTH can activate p38 in early differentiating osteoblastic cells. Activation of p38 is cAMP-PKA-dependent and mediates PTH-induced stimulation of ALP which plays a critical role for the calcification of the bone matrix.

Cyclin D1 as a target for the proliferative effects of PTH and PTHrP in early osteoblastic cells

PTHrP induced a proliferative cyclin D1 activation in low-density osteoblastic cells. The process was PKA and MAPK dependent and involved both AP-1 and CRE sites. In ectopic ossicles generated from implanted bone marrow stromal cells, PTH upregulated cyclin D1 after acute or intermittent anabolic treatment. These data suggest a positive role of PTH and PTHrP in the cell cycle of early osteoblasts.

INTRODUCTION

The mechanisms underlying the actions of PTH and its related protein (PTHrP) in osteoblast proliferation, differentiation, and bone remodeling remain unclear. The action of PTH or PTHrP on the cell cycle during osteoblast proliferation was studied.

MATERIALS AND METHODS

Mouse calvarial MC3T3-E1 clone 4 cells were synchronized by serum starvation and induced with 100 nM PTHrP for 2-24 h under defined low serum conditions. Western blot, real-time PCR, EMSAs, and promoter/luciferase assays were performed to evaluate cyclin D1 expression. Pharmacological inhibitors were used to determine the relevant signaling pathways. Ectopic ossicles generated from implanted bone marrow stromal cells were treated with acute (a single 8- or 12-h injection) or intermittent anabolic PTH treatment for 7 days, and RNA and histologic analysis were performed.

RESULTS

PTHrP upregulated cyclin D1 and CDK1 and decreased p27 expression. Cyclin D1 promoter/luciferase assays showed that the PTHrP regulation involved both activator protein-1 (AP-1) and cyclic AMP response element binding protein (CRE) sites. AP-1 and CRE double mutants completely abolished the PTHrP effect of cyclin D1 transcription. Upregulation of cyclin D1 was found to be protein kinase A (PKA) and mitogen-activated protein kinase (MAPK) dependent in proliferating MC3T3-E1 cells. In vivo expression of cyclin D1 in ectopic ossicles was upregulated after a single 12-h PTH injection or intermittent anabolic PTH treatment for 7 days in early developing ossicles.

CONCLUSIONS

These data indicate that PTH and PTHrP induce cyclin D1 expression in early osteoblastic cells and their action is developmental stage specific.

Effects of transient PTH on early proliferation, apoptosis, and subsequent differentiation of osteoblast in primary osteoblast cultures

In primary calvarial osteoblast cultures derived from transgenic mice expressing green fluorescent protein (GFP) under the control of 3.6-kb Col1a1 promoter, the emergence of GFP signal marks the transition of multipotential osteoprogenitors into preosteoblasts. Early transient treatment (days 1-7) of these cultures with parathyroid hormone (PTH) has an anabolic effect that is not associated with an increase in total DNA content or cell number in day 21 cultures. In the present study, the effect of early PTH treatment on cell proliferation and apoptosis was examined in greater detail in GFP(+) and GFP(-) cells using flow cytometry. In preconfluent cultures, PTH significantly reduced the proportion of cells in S phase but increased those in G(0)/G(1) and G(2)+M phases in both GFP(+) and GFP(-) subpopulations. PTH decreased apoptosis only in GFP(-) but not GFP(+) cells, indicating an increased survival of GFP(-) cells. In contrast, PTH did not change the amounts of cell proliferation and apoptosis seen in either compartment after these cultures reached confluence. To further assess the effect of early PTH treatment on osteogenic differentiation, secondary cultures of sorted GFP(+) or GFP(-) cells were obtained from day 7 primary cultures that had been treated for 1 wk with PTH. This treatment resulted in larger areas of GFP expression accompanied by increased xylenol orange/von Kossa staining in the secondary cultures of GFP fractions. Early transient PTH treatment appears to enhance the commitment of progenitor cells to an osteogenic fate and results in a higher proportion of cells that achieve full osteoblast differentiation.

Effects of arachidonic acid, docosahexaenoic acid and prostaglandin E(2) on cell proliferation and morphology of MG-63 and MC3T3-E1 osteoblast-like cells

During bone remodelling bone is resorbed by osteoclasts and replaced again by osteoblasts through the process of bone formation. Clinical trials and in vivo animal studies suggest that specific polyunsaturated fatty acids (PUFAs) might benefit bone health. As the number of functional osteoblasts is important for bone formation the effects of specific PUFAs on in vitro osteoblastic cell proliferation were investigated. Morphological studies were conducted to determine whether exposure of the cells to these agents caused structural damage to the cells thereby yielding invalid results. Results from this study showed that arachidonic acid (AA) and docosahexaenoic acid (DHA) both inhibit cell growth significantly at high concentrations. The anti-mitotic effect of AA is possibly independent of PGE(2) production, as PGE(2) per se had little effect on proliferation. Further study is required to determine whether reduced proliferation due to fatty acids could be due to increased differentiation of osteoblasts to the mature mineralising osteoblastic phenotype.

Primary hyperparathyroidism and malignancy: "studies by nature"

The American Food and Drug Administration approval of parathyroid hormone (PTH) administration for osteoporosis as well as the possibility for its future therapeutic applications requires an examination of the suggested association between PTH and cancer, particularly osteosarcomas. The objective was to evaluate such a connection by collecting observational data from two groups of patients, designated as "studies by nature". Cohort 1: Medical records of all patients with primary hyperparathyroidism that were treated in a referral center during a 12-year period were retrospectively reviewed for malignancy before, at the time or after diagnosis. Cohort 2: Records of patients with osteosarcomas that were treated in referral centers during 15 years were retrospectively reviewed for hyperparathyroidism, as indicated by history or laboratory results. There were 582 patients with primary hyperparathyroidism. While 56 (9.6%) had malignancy, 47 (8%) developed cancer after diagnosis with hyperparathyroidism during 6.1 years of documentation. This rate did not exceed the incidence of developing cancer among the general population. Although thyroid cancer was about 4 times the incidence in the general population, this may be attributed to a high level of detection while work-up, treating and following the parathyroid disease. None had osteosarcoma. None of the 126 patients with osteosarcoma had documentation of primary hyperparathyroidism or had biochemical evidence of hyperparathyroidism. No obvious association was found between primary hyperparathyroidism and cancer. Similarly, there was no demonstrable relationship between osteosarcomas and hyperthyroidism biochemical stigmata. Since PTH may contribute to tumor invasiveness, screening for existing neoplasms, especially prostate and breast, before PTH treatment may be of importance.

Alterations in CDK1 expression and nuclear/nucleolar localization following induction in a spontaneous canine mammary cancer model

Transcription of CDK1 is induced as cells re-enter the cell cycle from quiescence and these early cell cycle re-entry events have been modeled by okadaic acid treatment due to its activity on specific enhancer sequences in the human CDK1 promoter. To investigate heterogeneity of control of this mechanism in the context of neoplastic transformation, a cellular model derived from spontaneous canine mammary cancer (CMT) was developed that includes six cell lines derived from different animals. Notable heterogeneity in response to okadaic acid was observed in expression of CDK1 mRNA and protein. In response to okadaic acid treatment, two CMT cell lines exhibited a CDK1 mRNA induction while one cell line exhibited CDK1 mRNA suppression, and three remained unchanged. Despite this variability, three CMT cell lines arrested in S or G2/M phase and five exhibited marked increases in apoptosis. Moderation of some of these differences were observed at the level of CDK1 protein as three of six CMT cell lines exhibited only moderate enhancement in CDK1 protein levels while three remained essentially unchanged. Some additional differences in distribution of CDK1 protein, favoring enhanced nuclear over cytoplasmic CDK1 localization, were observed in treated cells in the form of concentrated nuclear CDK1 labeled foci. Confocal microscopy revealed the presence of brightly labeled punctate foci containing CDK1 protein within nuclei as well as nucleoli in okadaic acid treated non-mitotic cells suggesting a role for this kinase outside the normal G2/mitotic phase of the cell cycle and suggesting a possible new function within the nucleolus.

PTHrP signaling targets cyclin D1 and induces osteoblastic cell growth arrest

PTHrP control of the MC3T3-E1 cell cycle machinery showed that, during differentiation, PTHrP induced G1 growth arrest. Cyclin D1 was a critical mediator as a downstream effector of cAMP, PKC, and MAPK signaling, and the process was PKA-independent. The involvement of JunB has been found critical for PTHrP effects.

INTRODUCTION

PTH-related protein (PTHrP) has been implicated in the control of bone cell turnover, but the mechanisms underlying its effect on osteoblast proliferation and differentiation have not been clearly defined. The mechanisms by which PTHrP impacts cell cycle proteins and the role of signaling pathways in differentiated osteoblasts were studied.

MATERIALS AND METHODS

To elucidate the role of PTHrP, flow cytometric analyses were performed using MC3T3-E1 and primary mouse calvarial cells. Relative protein abundance (Western blot), physical association of partners (immunoprecipitation), and kinase activities (in vitro kinase assays using either GST-Rb or H1-histone as substrates) of cell cycle-associated proteins in vehicle and PTHrP-treated 7-day differentiated cells were determined. ELISA and/or Northern blot analyses were done to evaluate JunB and cyclin D1 expression. SiRNA-mediated gene silencing experiments were performed to silence JunB protein. Finally, inhibitors of cAMP, protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK) were used to determine involvement of different signaling pathways.

RESULTS

PTHrP inhibited cyclin D1 protein expression 7-fold in a dose- and time-dependent manner and increased the level of p16 protein in differentiated osteoblasts. Additionally, PTHrP reduced cyclin D1-CDK4/CDK6 and CDK1 kinase activities. Forskolin, a cAMP agonist, mimicked PTHrP action, and the PKC inhibitor, GF109203X, slightly blocked downregulation of cyclin D1, implying involvement of both cAMP and PKC. U0126, a MAPK inhibitor, alone decreased cyclin D1 protein, suggesting that the basal cyclin D1 protein is MAPK dependent. H-89, a PKA inhibitor, did not alter the effect of PTHrP on cyclin D1, suggesting a PKA-independent mechanism. Finally, expression of JunB, an activating protein-1 transcription factor, was significantly upregulated, and silencing JunB (siRNA) partially reversed the cyclin D1 response, implying involvement of JunB in the PTHrP-mediated growth arrest of MC3T3-E1 cells.

CONCLUSION

PTHrP upregulates JunB and reduces cyclin D1 expression while inducing G1 cell cycle arrest in differentiated osteoblasts. Such regulation could be an important determinant of the life span and bone-forming activity of osteoblasts.

Insulin receptor substrate-1 is required for bone anabolic function of parathyroid hormone in mice

Bone anabolic action of PTH has been suggested to be mediated by induction of IGF-I in osteoblasts; however, little is known about the molecular mechanism by which IGF-I leads to bone formation under the PTH stimulation. This study initially confirmed in mouse osteoblast cultures that PTH treatment increased IGF-I mRNA and protein levels and alkaline phosphatase activity, which were accompanied by phosphorylations of IGF-I receptor, insulin receptor substrate (IRS)-1 and IRS-2, essential adaptor molecules for the IGF-I signaling. To learn the involvement of IRS-1 and IRS-2 in the bone anabolic action of PTH in vivo, IRS-1-/- and IRS-2-/- mice and their respective wild-type littermates were given daily injections of PTH (80 mug/kg) or vehicle for 4 wk. In the wild-type mice, the PTH injection increased bone mineral densities of the femur, tibia, and vertebrae by 10-20% without altering the serum IGF-I level. These stimulations were similarly seen in IRS-2-/- mice; however, they were markedly suppressed in IRS-1-/- mice. Although the PTH anabolic effects were stronger on trabecular bones than on cortical bones, the stimulations on both bones were blocked in IRS-1-/- mice but not in IRS-2-/- mice. Histomorphometric and biochemical analyses showed an increased bone turnover by PTH, which was also blunted by the IRS-1 deficiency, though not by the IRS-2 deficiency. These results indicate that the PTH bone anabolic action is mediated by the activation of IRS-1, but not IRS-2, as a downstream signaling of IGF-I that acts locally as an autocrine/paracrine factor.

Novel Roles of Unphosphorylated STAT3 in Oncogenesis and Transcriptional Regulation

Signal transducer and activator of transcription 3 (STAT3) is phosphorylated on tyrosine residue 705 in response to growth factors or cytokines to form activated homodimers that drive gene expression. Because the stat3 promoter has a binding site for STAT3 dimers, the amount of STAT3 protein increases when STAT3 is activated (e.g., in response to interleukin 6). Unphosphorylated STAT1 is known to drive the expression of certain genes. To explore the possibility of a similar role for the induced expression of unphosphorylated STAT3, we overexpressed either Y705F STAT3, which can not be phosphorylated on residue 705, or wild-type STAT3 in normal human mammary epithelial cells or STAT3-null mouse cells. The levels of many mRNAs were affected strongly by high levels of either form of STAT3. Some genes whose expression was increased by overexpressed STAT3, but not by activated STAT3 dimers, encode well-known oncoproteins (e.g., MRAS and MET). In many tumors, STAT3 is activated constitutively, and thus the unphosphorylated form is likely to be expressed highly, driving oncogene expression by a novel mechanism. In addition, expression of the stat3 gene is increased strongly in response to interleukin 6, and the high levels of unphosphorylated STAT3 that result drive a substantial late phase of gene expression in response to this cytokine. Thus, unphosphorylated STAT3, which activates gene expression by a novel mechanism distinct from that used by STAT3 dimers, is very likely to be an important transcription factor both in cancer and in responses to cytokines.

Lifetime high calcium intake increases osteoporotic fracture risk in old age

Caloric restriction prolongs life span. Calcium restriction may preserve bone health. In osteoporosis, bone mineral density (BMD) has significantly decreased, due to a lack of osteoblast bone formation. Traditional osteoporosis prevention is aimed at maximizing BMD, but the lifetime effects of continuously maintaining a high BMD on eventual bone health in old age, have not been studied. Strikingly, in countries with a high mean BMD, fracture rates in the elderly are significantly higher than in countries with a low mean BMD. Studies show that this is not based on genetic differences. Also, in primary hyperparathyroidism, on the brink of osteoporosis, BMD levels may be significantly higher than normal. Maybe, BMD does not represent long term bone health, but merely momentary bone strength. And maybe, maintaining a high BMD might actually wear out bone health. Since osteoporosis particularly occurs in the elderly, and because in osteoporotic bone less osteoblasts are available, the underlying process may have to do with ageing of osteoblastic cells. In healthy subjects, osteoblastic bone cells respond to the influx of calcium by composing a matrix upon which calcium precipitates. In the process of creating this matrix, 50-70% of the involved osteoblasts die. The greater the influx of calcium, the greater osteoblast activity, and the greater osteoblast apoptosis rate. An increased osteoblast apoptosis rate leads to a decrease in the age-related osteoblast replicative capacity (ARORC). In comparison to healthy bone, in osteoporotic bone the decrease in the replicative capacity of osteoblastic cells is greater. Due to the eventual resulting lack of osteoblast activity, micro-fractures cannot be repaired. Continuously maintaining a high BMD comes with continuously high bone remodeling rates, which regionally exhaust the ARORC, eventually leading to irreparable microfractures. Regarding long time influences on bone health, adequate estrogen levels are known to be protective against osteoporosis. This is generally attributed to its inhibiting influence on osteoclast activity. Instead, its net effects on osteoblast metabolism may be the key to osteoporosis prevention. Adequate estrogen levels inhibit osteoblast activity, calcium apposition and osteoblast apoptosis rate, preserving the ARORC.

CONCLUSION

Regarding osteoporosis prevention, ARORC better than BMD represents bone health. Regarding ARORC, adequate estrogen levels are protective, opposing the similar effects of hyperparathyroidism and a high calcium diet. Tests need to be performed in mice to assess the lifetime effects of a high versus a low calcium diet, on eventual bone fracture toughness.

Parathyroid hormone uses multiple mechanisms to arrest the cell cycle progression of osteoblastic cells from G1 to S phase

Parathyroid hormone (PTH) plays a major role in bone remodeling and has the ability to increase bone mass if administered daily. In vitro, PTH inhibits the growth of osteoblastic cell lines, arresting them in G(1) phase. Here, we demonstrate that PTH regulates the expression of at least three genes to achieve the following: inducing expression of MAPK phosphatase 1 (MKP-1) and p21(Cip1) and decreasing expression of cyclin D1 at both mRNA and protein levels. The induction of MKP-1 causes the dephosphorylation of extracellular signal-regulated kinase and therefore the decrease in cyclin D1. Overexpression of MKP-1 arrests UMR cells in G(1) phase. The mechanisms involved in PTH regulation of these genes were studied. Most importantly, PTH administration produces similar effects on expression of these genes in rat femoral metaphyseal primary spongiosa. Analyses of p21(Cip1) expression levels in bone indicate that repeated daily PTH injections make the osteoblast more sensitive to successive PTH treatments, and this might be an important feature for the anabolic functions of PTH. In summary, our data suggest that one mechanism for PTH to exert its anabolic effect is to arrest the cell cycle progression of the osteoblast and hence increase its differentiation.

Effect of Duration of Alcohol Consumption on Calcium and Bone Metabolism During Pregnancy in the Rat

BACKGROUND

Little is known about the consequences of drinking during pregnancy for the long-term health of the mother. Alcohol (ethanol) has been shown to disrupt calcium (Ca) homeostasis and is known to have deleterious effects on bone. During pregnancy, bone turnover is increased to maintain Ca homeostasis; therefore, pregnancy may be a time of life when maternal bone is particularly susceptible to the effects of ethanol. This study investigated the effect of duration of ethanol consumption on Ca homeostasis and bone during pregnancy in the rat.

METHODS

Rats were fed ethanol (36% ethanol-derived calories) in liquid diets for 3 (21 days gestation only) or 6 (3 weeks before and throughout 21 days gestation) weeks. Maternal blood was analyzed for Ca (total and ionized Ca [iCa]), the Ca-regulating hormones (parathyroid hormone [PTH], 1,25(OH)2D, calcitonin), and osteocalcin (a marker for bone formation). Bone was analyzed for ash (mineral) content.

RESULTS

Dams consuming ethanol (E dams) had decreased blood Ca levels (total and iCa) at both 3 and 6 weeks, but iCa was lower in E dams after 6 compared with 3 weeks. Importantly, ethanol seemed to interfere with the normal compensatory response to these decreased Ca levels. In contrast to pair-fed controls, serum PTH levels actually were decreased, 1,25(OH)2D levels failed to increase, and calcitonin levels were increased in ethanol-consuming dams, regardless of duration. Moreover, ethanol decreased bone formation, as indicated by serum osteocalcin levels, after both 3 and 6 weeks consumption, and after 6 weeks, the ash content of bone also was decreased. In addition, a relationship was found between the blood alcohol concentration (BAC) and some measures of Ca and bone metabolism. Serum 1,25(OH)2D and osteocalcin levels varied inversely, whereas serum calcitonin varied directly with BAC, suggesting that time of sampling after drinking may be an important variable for interpreting ethanol's effects on Ca and bone metabolism. In all rats, serum osteocalcin levels varied directly with PTH and 1,25(OH)2D levels.

CONCLUSIONS

Ethanol consumption during pregnancy impaired Ca homeostasis in the dam, regardless of duration of consumption, and resulted in decreased bone formation and ash content of bone. Significant relationships among the Ca-regulating hormones, BAC, and osteocalcin support the hypothesis that ethanol's effects on the Ca-regulating hormones may mediate some of its effects on bone.

Effects of combined treatment of insulin and human parathyroid hormone(1-34) on cancellous bone mass and structure in streptozotocin-induced diabetic rats

The purpose of this study was to test the hypothesis that combined treatment with insulin and human parathyroid hormone (hPTH) is more effective than treatment with insulin or hPTH alone in improving cancellous bone mass, connectivity, and strength in insulin-dependent diabetic rats. Diabetes was induced by intraperitoneal injection of streptozotocin (STZ) in 7-month-old female Wistar rats. The diabetic rats received insulin, hPTH, insulin and hPTH, or hPTH vehicle for 4 weeks, starting 8 weeks after STZ injection. They were compared with baseline controls and normal controls that received STZ alone and STZ vehicle alone, respectively. The rats' proximal right tibias were processed to serve as undecalcified Villanueva-stained bone sections for histomorphometry. Changes in trabecular connectivity were determined through node-strut analysis. The decreased cancellous bone volume (BV/TV) and bone formation in diabetic rats improved in all the drug-treated groups compared with baseline controls. Furthermore, recovery of BV/TV was greater in rats that received the combination of insulin and hPTH than in those that received insulin or hPTH alone. In node-strut analysis, the node-related parameter (N.Nd/TV) significantly increased in rats that received the combination of insulin and hPTH, but did not increase in those that received insulin or hPTH alone. In addition to these results, the combination treatment significantly increased bone mineral density of the femur and bone strength in the femoral metaphysis compared with treatment with insulin or hPTH alone. These results indicate that the doses of insulin and hPTH employed in the combination treatment were more effective in improving not only bone mass but also trabecular connectivity and bone strength than treatment with insulin or hPTH alone in insulin-dependent diabetic rats.

Insulin-like growth factor I is required for the anabolic actions of parathyroid hormone on mouse bone

Parathyroid hormone (PTH) is a potent anabolic agent for bone, but the mechanism(s) by which it works remains imperfectly understood. Previous studies have indicated that PTH stimulates insulin-like growth factor (IGF) I production, but it remains uncertain whether IGF-I mediates some or all of the skeletal actions of PTH. To address this question, we examined the skeletal response to PTH in IGF-I-deficient (knockout [k/o]) mice. These mice and their normal littermates (NLMs) were given daily injections of PTH (80 microg/kg) or vehicle for 2 weeks after which their tibias were examined for fat-free weight (FFW), bone mineral content, bone structure, and bone formation rate (BFR), and their femurs were assessed for mRNA levels of osteoblast differentiation markers. In wild-type mice, PTH increased FFW, periosteal BFR, and cortical thickness (C.Th) of the proximal tibia while reducing trabecular bone volume (BV); these responses were not seen in the k/o mice. The k/o mice had normal mRNA levels of the PTH receptor and increased mRNA levels of the IGF-I receptor but markedly reduced basal mRNA levels of the osteoblast markers. Surprisingly, these mRNAs in the k/o bones increased several-fold more in response to PTH than the mRNAs in the bones from their wild-type littermates. These results indicate that IGF-I is required for the anabolic actions of PTH on bone formation, but the defect lies distal to the initial response of the osteoblast to PTH.

T antigens of simian virus 40: molecular chaperones for viral replication and tumorigenesis

Simian virus 40 (SV40) is a small DNA tumor virus that has been extensively characterized due to its relatively simple genetic organization and the ease with which its genome is manipulated. The large and small tumor antigens (T antigens) are the major regulatory proteins encoded by SV40. Large T antigen is responsible for both viral and cellular transcriptional regulation, virion assembly, viral DNA replication, and alteration of the cell cycle. Deciphering how a single protein can perform such numerous and diverse functions has remained elusive. Recently it was established that the SV40 T antigens, including large T antigen, are molecular chaperones, each with a functioning DnaJ domain. The molecular chaperones were originally identified as bacterial genes essential for bacteriophage growth and have since been shown to be conserved in eukaryotes, participating in an array of both viral and cellular processes. This review discusses the mechanisms of DnaJ/Hsc70 interactions and how they are used by T antigen to control viral replication and tumorigenesis. The use of the DnaJ/Hsc70 system by SV40 and other viruses suggests an important role for these molecular chaperones in the regulation of the mammalian cell cycle and sheds light on the enigmatic SV40 T antigen-a most amazing molecule.

Treatment of a murine model of high-turnover renal osteodystrophy by exogenous BMP-7

BACKGROUND

The secondary hyperparathyroidism of chronic kidney disease (CKD) produces a high turnover osteodystrophy that is associated with peritrabecular fibrosis. The nature of the cells involved in the development of peritrabecular fibrosis may represent osteoprogenitors expressing a fibroblastic phenotype that are retarded from progressing through osteoblast differentiation.

METHODS

To test the hypothesis that osteoblast differentiation is retarded in secondary hyperparathyroidism due to CKD producing bone marrow fibrosis, we administered bone morphogenetic protein 7 (BMP-7), a physiologic regulator of osteoblast regulation, to C57BL6 mice that had CKD produced by electrocautery of one kidney followed by contralateral nephrectomy two weeks later. Following the second surgical procedure, a subgroup of mice received daily intraperitoneal injections of BMP-7 (10 microg/kg). Three to six weeks later, the animals were sacrificed, blood was obtained for measurements of blood urea nitrogen (BUN) and parathyroid hormone (PTH) levels, and the femora and tibiae were processed for histomorphometric analysis.

RESULTS

The animals had significant renal insufficiency with BUN values of 77.79 +/- 22.68 mg/dL, and the level of renal impairment between the CKD untreated mice and the CKD mice treated with BMP-7 was the same in the two groups. PTH levels averaged 81.13 +/- 51.36 and 75.4 +/- 43.61 pg/mL in the CKD and BMP-7 treated groups, respectively. The animals with CKD developed significant peritrabecular fibrosis. In addition, there was an increase in osteoblast surface and osteoid accumulation as well as increased activation frequency and increased osteoclast surface consistent with high turnover renal osteodystrophy. Treatment with BMP-7 eliminated peritrabecular fibrosis, increased osteoblast number, osteoblast surface, mineralizing surface and single labeled surface. There was also a significant decrease in the eroded surface induced by treatment with BMP-7.

CONCLUSIONS

These findings indicate that BMP-7 treatment in the setting of high turnover renal osteodystrophy prevents the development of peritrabecular fibrosis, affects the osteoblast phenotype and mineralizing surfaces, and decreases bone resorption. This is compatible with a role of osteoblast differentiation in the pathophysiology of osteitis fibrosa.

Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. PTH and PTH-related protein (PTHrP) indirectly activate osteoclasts resulting in increased bone resorption. During this process, PTH changes the phenotype of the osteoblast from a cell involved in bone formation to one directing bone resorption. In addition to these catabolic effects, PTH has been demonstrated to be an anabolic factor in skeletal tissue and in vitro. As a result, PTH has potential medical application to the treatment of osteoporosis, since intermittent administration of PTH stimulates bone formation. Activation of osteoblasts by PTH results in expression of genes important for the degradation of the extracellular matrix, production of growth factors, and stimulation and recruitment of osteoclasts. The ability of PTH to drive changes in gene expression is dependent upon activation of transcription factors such as the activator protein-1 family, RUNX2, and cAMP response element binding protein (CREB). Much of the regulation of these processes by PTH is protein kinase A (PKA)-dependent. However, while PKA is linked to many of the changes in gene expression directed by PTH, PKA activation has been shown to inhibit mitogen-activated protein kinase (MAPK) and proliferation of osteoblasts. It is now known that stimulation of MAPK and proliferation by PTH at low concentrations is protein kinase C (PKC)-dependent in both osteoblastic and kidney cells. Furthermore, PTH has been demonstrated to regulate components of the cell cycle. However, whether this regulation requires PKC and/or extracellular signal-regulated kinases or whether PTH is able to stimulate other components of the cell cycle is unknown. It is possible that stimulation of this signaling pathway by PTH mediates a unique pattern of gene expression resulting in proliferation in osteoblastic and kidney cells; however, specific examples of this are still unknown. This review will focus on what is known about PTH-mediated cell signaling, and discuss the established or putative PTH-regulated pattern of gene expression in osteoblastic cells following treatment with catabolic (high) or anabolic (low) concentrations of the hormone.

Evidence that anabolic effects of PTH on bone require IGF-I in growing mice

Although it has been established that PTH exerts potent anabolic effects on bone in animals and humans, the mechanism of PTH action on bone remains controversial. Based on the previous findings that PTH treatment increased production of IGF-I in bone cells and that PTH effects on bone cells in vitro were blocked by IGF-I-blocking antibodies, we proposed that IGF-I action is required for the stimulatory effects of PTH on bone formation. To test this hypothesis, we evaluated the effects of PTH on bone formation parameters in growing mice lacking functional IGF-I genes. Five-week-old IGF-I(-/-) mice and wild-type littermates were given daily sc injections of 160 microg/kg body weight of PTH (1-34) or vehicle for 10 d. In wild-type animals, PTH caused a significant increase in serum osteocalcin levels (113%), serum alkaline phosphatase activity (48%), and alkaline phosphatase activity in femoral bone extracts (>80%), compared with the vehicle-treated control group. In contrast, in IGF-I(-/-) mice, there was no significant effect of PTH on any bone formation parameters. PTH treatment increased total bone mineral density, as evaluated by peripheral quantitative computer tomography, at the distal metaphysis of the femur by 40% in wild-type mice, but it had no effect on bone mineral density in mice lacking functional IGF-I genes. In vitro studies using osteoblasts derived from control and IGF-I(-/-) mice revealed that PTH treatment increased cell number in osteoblasts derived from IGF-I knockout mice in the presence of exogenously added IGF-I but not without IGF-I. These data to our knowledge provide the first direct evidence that the anabolic effects of PTH on bone formation in vivo require IGF-I action in growing mice.

Comparison of bone formation responses to parathyroid hormone(1-34), (1-31), and (2-34) in mice

In this study we used a mouse model system to compare the in vivo effects of parathyroid hormone(1-34) [PTH(1-34)] with that of PTH(1-31) or PTH(2-34) analogs. Daily subcutaneous administration of PTH(1-34) for 15 days caused a dose-dependent increase in the serum osteocalcin level and bone extract alkaline phosphatase activity, markers of bone formation. PTH(2-34) was much less potent, whereas PTH(1-31) was equipotent in stimulating bone formation parameters in mice. PTH(1-34) caused significant increases in serum calcium (after 4 h) and tartrate-resistant acid phosphatase activity in bone extract (after 4 h), whereas PTH(2-34) and PTH(1-31) were less potent. Because PTH(1-31) caused a smaller increase in bone resorption parameters compared to PTH(1-34), despite similar effects on bone formation parameters, we evaluated the long-term anabolic effects of PTH(1-31) and PTH(1-34) in mice. Weekly evaluations of serum osteocalcin levels demonstrated that daily injections of PTH(1-34) and PTH(1-31) at 80 microg/kg body weight increased serum osteocalcin levels within 1 week of the start of treatment, which were maintained during the entire 22 week treatment. Assessment of bone density at the end of the treatment period with peripheral quantitated computed tomography (pQCT) revealed that PTH(1-34) caused a significantly greater increase in femoral bone density compared to PTH(1-31) at the middiaphysis (18% vs. 9% over vehicle control; p < 0.001). Both PTH(1-34) and PTH(1-31) increased periosteal circumference compared to vehicle (p < 0.01) without a significant difference between the two treatments. In contrast, PTH(1-34) caused a significantly greater reduction in endosteal circumference than PTH(1-31) (p < 0.001). Both analogs significantly increased maximum load and area of moment of inertia over the vehicle group. In conclusion, our findings suggest that PTH(1-34) and PTH(1-31) may exhibit different anabolic effects at the periosteum vs. endosteum in the long bones of mice.

Failure of cdc2 promoter activation and G(2)/M transition by ANG II and AVP in vascular smooth muscle cells

The physiological role of the vasoconstrictive hormones arginine vasopressin (AVP) and angiotensin II (ANG II) in the development of vascular hyperplasia is still unclear. We examined the effects of these hormones on cell cycle regulation of cultured rat vascular smooth muscle cells (VSMC). AVP and ANG II were able to induce G(1)/S transition and DNA synthesis in serum-starved quiescent VSMC but failed to promote further progression into G(2)/M phases. AVP and ANG II enhanced the expression and activity of cdk2, cyclin E, and proliferating cell nuclear antigen but did not induce expression of cdc2/cyclin B complex, a critical regulator of G(2)/M transition. The failure of cdc2 mRNA induction was found to be caused by a defect in cdc2 promoter activation. Binding of free E2F-1 to the cdc2 promoter did not occur in hormone-treated VSMC, which may account for the defective induction of cdc2. The absence of cdc2 promoter activation and G(2)/M transition may be important for the prevention of hyperplasia under physiological conditions but underlies the hypertrophy of VSMC.

Increased bone formation by prevention of osteoblast apoptosis with parathyroid hormone

The mass of regenerating tissues, such as bone, is critically dependent on the number of executive cells, which in turn is determined by the rate of replication of progenitors and the life-span of mature cells, reflecting the timing of death by apoptosis. Bone mass can be increased by intermittent parathyroid hormone (PTH) administration, but the mechanism of this phenomenon has remained unknown. We report that daily PTH injections in mice with either normal bone mass or osteopenia due to defective osteoblastogenesis increased bone formation without affecting the generation of new osteoblasts. Instead, PTH increased the life-span of mature osteoblasts by preventing their apoptosis - the fate of the majority of these cells under normal conditions. The antiapoptotic effect of PTH was sufficient to account for the increase in bone mass, and was confirmed in vitro using rodent and human osteoblasts and osteocytes. This evidence provides proof of the basic principle that the work performed by a cell population can be increased by suppression of apoptosis. Moreover, it suggests novel pharmacotherapeutic strategies for osteoporosis and, perhaps, other pathologic conditions in which tissue mass diminution has compromised functional integrity.

Induction of p21(CIP1/Waf1) and activation of p34(cdc2) involved in retinoic acid-induced apoptosis in human hepatoma Hep3B cells

The biological activity of retinoic acid (RA) was examined in human hepatoma Hep3B cells. Under serum-deprived conditions, RA induced S/M-phase elevation and mitotic index increase within 24 h, followed by apoptosis. This RA-induced apoptosis was accompanied by p53-independent up-regulation of endogenous p21(CIPI/Waf1) and Bax proteins, as well as activation of p34(cdc2) kinase, and increase of Rb2 protein level and phosphorylation pattern. In addition, RA had no effect on the levels of Bcl-XL; Bcl-XS; cyclins A, B, D1, D3, or E; or Rb1 expression but markedly down-modulated Cdk2 kinase activity and reduced Cdk4 expression. RA also slightly delayed p27(Kip1) expression. Olomoucine, a potent p34(cdc2) and Cdk2 inhibitor, effectively blocked RA-mediated p34(cdc2) kinase activation and prevented RA-induced apoptosis. Furthermore, antisense oligonucleotide complementary to p21(CIP2/Waf1) and p34(cdc2) mRNA significantly rescued RA-induced apoptosis. Our data indicate that p21(CIP2/Waf1) overexpression may not be the only regulatory factor necessary for RA-induced apoptosis in human hepatoma Hep3B cells. RA treatment leads to Rb2 hyperphosphorylation, and p34(cdc2) kinase activation is coincident with an aberrant mitotic progression, followed by appearance of abnormal nucleus. This aberrant cell cycle progression appeared requisite for RA-induced cell death. These findings suggest that inappropriate regulation of the cell cycle regulators p21(CIP2/Waf1) and p34(cdc2) is coupled with induction of Bax and involved in cell death with apoptosis when Hep3B cells are exposed to RA.