Parathyroid hormone-related protein promotes quiescence and survival of serum-deprived chondrocytes by inhibiting rRNA synthesis

Parathyroid Hormone-Related Protein/Parathyroid Hormone Receptor 1 Signaling in Cancer and Metastasis

PTHrP exerts its effects by binding to its receptor, PTH1R, a G protein-coupled receptor (GPCR), activating the downstream cAMP signaling pathway. As an autocrine, paracrine, or intracrine factor, PTHrP has been found to stimulate cancer cell proliferation, inhibit apoptosis, and promote tumor-induced osteolysis of bone. Despite these findings, attempts to develop PTHrP and PTH1R as drug targets have not produced successful results in the clinic. Nevertheless, the efficacy of blocking PTHrP and PTH1R has been shown in various types of cancer, suggesting its potential for therapeutic applications. In light of these conflicting data, we conducted a comprehensive review of the studies of PTHrP/PTH1R in cancer progression and metastasis and highlighted the strengths and limitations of targeting PTHrP or PTH1R in cancer therapy. This review also offers our perspectives for future research in this field.

BMP7 increases protein synthesis in SW1353 cells and determines rRNA levels in a NKX3-2-dependent manner

BMP7 is a morphogen capable of counteracting the OA chondrocyte hypertrophic phenotype via NKX3-2. NKX3-2 represses expression of RUNX2, an important transcription factor for chondrocyte hypertrophy. Since RUNX2 has previously been described as an inhibitor for 47S pre-rRNA transcription, we hypothesized that BMP7 positively influences 47S pre-rRNA transcription through NKX3-2, resulting in increased protein translational capacity. Therefor SW1353 cells and human primary chondrocytes were exposed to BMP7 and rRNA (18S, 5.8S, 28S) expression was determined by RT-qPCR. NKX3-2 knockdown was achieved via transfection of a NKX3-2-specific siRNA duplex. Translational capacity was assessed by the SUNsET assay, and 47S pre-rRNA transcription was determined by transfection of a 47S gene promoter-reporter plasmid. BMP7 treatment increased protein translational capacity. This was associated by increased 18S and 5.8S rRNA and NKX3-2 mRNA expression, as well as increased 47S gene promotor activity. Knockdown of NKX3-2 led to increased expression of RUNX2, accompanied by decreased 47S gene promotor activity and rRNA expression, an effect BMP7 was unable to restore. Our data demonstrate that BMP7 positively influences protein translation capacity of SW1353 cells and chondrocytes. This is likely caused by an NKX3-2-dependent activation of 47S gene promotor activity. This finding connects morphogen-mediated changes in cellular differentiation to an aspect of ribosome biogenesis via key transcription factors central to determining the chondrocyte phenotype.

Localization and ER membrane insertion of parathyroid hormone-related protein analyzed without effects of reporter proteins

Parathyroid hormone-related protein (PTHrP) is transported to both the secretory pathway and the nucleus/nucleolus by its dual targeting signals, that is, an N-terminal signal peptide and nuclear targeting signal. Curiously, reporter proteins such as enhanced green fluorescent protein strongly affect the localization of the fusion protein. Here, we report a novel methionine tag for ³⁵ S-labelling added to the C-terminus of its prepro-form, which has no methionine and cysteine residue other than the initiation methionine that enables analyses of the molecular mechanism of its dual localization without the effects of the reporter proteins. Mutational analyses including insertion of a glycosylation site for the tagged PTHrP revealed that the evolutionarily conserved regions in the signal peptide and the pro-region facilitate the redirection of ppPTHrP from the secretory pathway to the nuclear targeting pathway.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

PTHrP is a novel mediator for TGF-β-induced apoptosis

Parathyroid hormone-related protein (PTHrP) is a polyhormone secretory protein that plays fundamental roles in the development and function of various tissues. Transforming growth factor (TGF)-β is an important tumor suppressor that induces cell cycle arrest and apoptosis. Increased PTHrP expression has been implicated in TGF-β-induced growth inhibition in human hepatocellular carcinoma cells. However, whether PTHrP is involved in TGF-β-induced apoptosis remains unknown. Using Hep3B and HuH-7, two human hepatocellular carcinoma cell lines, the current study examined the hypothesis that TGF-β-induced apoptosis is mediated by the induction of PTHrP expression. We found that (1) TGF-β induces PTHrP mRNA expression, protein expression and secretion in a time-dependent fashion; (2) knockdown of PTHrP gene expression or neutralization of secreted PTHrP isoforms blocks TGF-β-induced apoptosis; and (3) TGF-β-induced PTHrP expression is Smad3-dependent. Thus, we have identified PTHrP as a novel mediator for TGF-β-induced apoptosis in Hep3B cells. Our findings provide further insights into the mechanisms through which TGF-β conveys tumor suppression activity.

Twenty-five years of PTHrP progress: from cancer hormone to multifunctional cytokine

Twenty-five years ago a "new" protein was identified from cancers that caused hypercalcemia. It was credited for its ability to mimic parathyroid hormone (PTH), and hence was termed parathyroid hormone-related protein (PTHrP). Today it is recognized for its widespread distribution, its endocrine, paracrine, and intracrine modes of action driving numerous physiologic and pathologic conditions, and its central role in organogenesis. The multiple biological activities within a complex molecule with paracrine modulation of adjacent target cells present boundless possibilities. The protein structure of PTHrP has been traced, dissected, and deleted comprehensively and conditionally, yet numerous questions lurk in its past that will carry into the future. Issues of the variable segments of the protein, including the enigmatic nuclear localization sequence, are only recently being clarified. Aspects of PTHrP production and action in the menacing condition of cancer are emerging as dichotomies that may represent intended temporal actions of PTHrP. Relative to PTH, the hormone regulating calcium homeostasis, PTHrP "controls the show" locally at the PTH/PTHrP receptor throughout the body. Great strides have been made in our understanding of PTHrP actions, yet years of exciting investigation and discovery are imminent. © 2012 American Society for Bone and Mineral Research.

PTHrP overexpression partially inhibits a mechanical strain-induced arthritic phenotype in chondrocytes

OBJECTIVE

Cell-based tissue engineering strategies are currently in clinical use and continue to be developed at a rapid pace for the repair of cartilage defects. Regardless of the repair methodology, chondrocytes within newly regenerated cartilage remain susceptible to the abnormal inflammatory and mechanical environments that underlie osteoarthritic disease, likely compromising the implant's integration, function, and longevity. The present study investigates the use of parathyroid hormone-related peptide (PTHrP) overexpression for chondroprotection.

DESIGN

Bovine articular chondrocytes were transfected with human PTHrP (hPTHrP) constructs (1-141 or 1-173) and subjected to injurious cyclic tensile strain (CTS; 0.5 Hz and 16% elongation) for 48 h. mRNA expression of matrix remodeling, inflammatory signaling, hypertrophic, and apoptotic genes were examined with real-time reverse transcription polymerase chain reaction. Nitric oxide (NO) and prostaglandin E₂ (PGE₂) production were measured using the Griess assay and enzyme immunoassay (EIA), respectively.

RESULTS

CTS-induced an arthritic phenotype in articular chondrocytes as indicated by increased gene expression of collagenases and aggrecanases and increased production of NO and PGE₂. Additionally, CTS increased collagen type X (Col10a1) mRNA expression, whereas overexpression of either hPTHrP isoform inhibited CTS-induced Col10a1 gene expression. However, hPTHrP 1-141 augmented CTS-induced NO and PGE₂ production, and neither hPTHrP isoform had any significant effect on apoptotic genes.

CONCLUSIONS

Our results suggest that chondrocytes overexpressing PTHrP resist mechanical strain-induced hypertrophic-like changes. Therapeutic PTHrP gene transfer may be considered for chondroprotection applications in newly regenerated cartilage.

Parathyroid hormone related protein (PTHrP) in tumor progression

Parathyroid hormone-related protein (PTHrP) is widely expressed in fetal and adult tissues and is a key regulator for cellular calcium transport and smooth muscle cell contractility, as well as a crucial control factor in cell proliferation, development and differentiation. PTHrP stimulates or inhibits apoptosis in an autocrine/paracrine and intracrine fashion, and is particularly important for hair follicle and bone development, mammary epithelial development and tooth eruption. PTHrP's dysregulated expression has traditionally been associated with oncogenic pathologies as the major causative agent of malignancy-associated hypercalcemia, but recent evidence revealed a driving role in skeletal metastasis progression. Here, we demonstrate that PTHrP is also closely involved in breast cancer initiation, growth and metastasis through mechanisms separate from its bone turnover action, and we suggest that PTHrP as a facilitator of oncogenes would be a novel target for therapeutic purposes.

FGFR3 down-regulates PTH/PTHrP receptor gene expression by mediating JAK/STAT signaling in chondrocytic cell line

The signaling axis comprising the parathyroid hormone (PTH)-related peptide (PTHrP), the PTH/PTHrP receptor and the fibroblast growth factor receptor 3 (FGFR3) plays a central role in chondrocyte proliferation. The Indian hedgehog (IHH) gene is normally expressed in early hypertrophic chondrocytes, and its negative feedback loop was shown to regulate PTH/PTHrP receptor signaling. In this study, we examined the regulation of PTH/PTHrP receptor gene expression in a FGFR3-transfected chondrocytic cell line, CFK2. Expression of IHH could not be verified on these cells, with consequent absence of hypertrophic differentiation. Also, expression of the PTH/PTHrP receptor (75% reduction of total mRNA) and the PTHrP (50% reduction) genes was reduced in CFK2 cells transfected with FGFR3 cDNA. Interestingly, we verified significant reduction in cell growth and increased apoptosis in the transfected cells. STAT1 was detected in the nuclei of the CFK2 cells transfected with FGFR3 cDNA, indicating predominance of the JAK/STAT signaling pathway. The reduction in PTH/PTHrP receptor gene in CFK2 cells overexpressing FGFR3 was partially blocked by treatment with an inhibitor of JAK3 (WHI-P131), but not with an inhibitor of MAPK (SB203580) or JAK2 (AG490). Altogether, these findings suggest that FGFR3 down-regulates PTH/PTHrP receptor gene expression via the JAK/STAT signaling in chondrocytic cells.

The midregion, nuclear localization sequence, and C terminus of PTHrP regulate skeletal development, hematopoiesis, and survival in mice

The functions of parathyroid hormone-related protein (PTHrP) on morphogenesis, cell proliferation, apoptosis, and calcium homeostasis have been attributed to its N terminus. Evidence suggests that many of these effects are not mediated by the N terminus but by the midregion, a nuclear localization sequence (NLS), and C terminus of the protein. A knock-in mouse lacking the midregion, NLS, and C terminus of PTHrP (Pthrp(Delta/Delta)) was developed. Pthrp(Delta/Delta) mice had craniofacial dysplasia, chondrodysplasia, and kyphosis, with most mice dying by d 5 of age. In bone, there were fewer chondrocytes and osteoblasts per area, bone mass was decreased, and the marrow was less cellular, with erythroid hypoplasia. Cellular proliferation was impaired, and apoptosis was increased. Runx2, Ocn, Sox9, Crtl1, beta-catenin, Runx1, ephrin B2, cyclin D1, and Gata1 were underexpressed while P16/Ink4a, P21, GSK-3beta, Il-6, Ffg3, and Ihh were overexpressed. Mammary gland development was aberrant, and energy metabolism was deregulated. These results establish that the midregion, NLS, and C terminus of PTHrP are crucial for the commitment of osteogenic and hematopoietic precursors to their lineages, and for survival, and many of the effects of PTHrP on development are not mediated by its N terminus. The down-regulation of Runx1, Runx2, and Sox9 indicates that PTHrP is a modulator of transcriptional activation during stem cell commitment.

Parathyroid hormone-related protein regulates cell survival pathways via integrin alpha6beta4-mediated activation of phosphatidylinositol 3-kinase/Akt signaling

Parathyroid hormone-related protein (PTHrP) is expressed by human prostatic tissues and cancer cell lines. PTHrP enhances tumor cell growth and metastasis in vivo and up-regulates proinvasive integrin alpha6beta4 expression in vitro. Hallmarks of malignant tumor cells include resistance to apoptosis and anchorage-independent cell growth. In this study, we used the human prostate cancer cell lines C4-2 and PC-3 as model systems to study the effects of PTHrP on these processes. We report that PTHrP protects these cells from doxorubicin-induced apoptosis and promotes anchorage-independent cell growth via an intracrine pathway. Conversely, autocrine/paracrine PTHrP action increases apoptosis in C4-2 cells and has no effect on apoptosis in PC-3 cells. The intracrine effects of PTHrP on apoptosis are mediated via activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. PTHrP also affects the phosphorylation state of Akt substrates implicated in apoptosis suppression, including glycogen synthase kinase-3 and Bad. The prosurvival effects of PTHrP are accompanied by increases in the ratio of antiapoptotic to proapoptotic members of the Bcl-2 family and in levels of c-myc. PTHrP also increases nuclear factor-kappaB activity via a PI3K-dependent pathway. Integrin alpha6beta4 is known to activate PI3K. Here, we also show that knockdown of integrin alpha6beta4 negates the PTHrP-mediated activation of the PI3K/Akt pathway. Taken together, these observations provide evidence of a link between PTHrP and the PI3K/Akt signaling pathway through integrin alpha6beta4, resulting in the activation of survival pathways. Targeting PTHrP production in prostate cancer may thus prove therapeutically beneficial.

Severe growth retardation and early lethality in mice lacking the nuclear localization sequence and C-terminus of PTH-related protein

Parathyroid hormone (PTH) plays a central role in the regulation of serum calcium and phosphorus homeostasis, while parathyroid hormone-related protein (PTHrP) has important developmental roles. Both peptides signal through the same G protein-coupled receptor, the PTH/PTHrP or PTH type 1 receptor (PTH1R). PTHrP, normally a secreted protein, also contains a nuclear localization signal (NLS) that in vitro imparts functionality to the protein at the level of the nucleus. We investigated this functionality in vivo by introducing a premature termination codon in Pthrp in ES cells and generating mice that express PTHrP (1-84), a truncated form of the protein that is missing the NLS and the C-terminal region of the protein but can still signal through its cell surface receptor. Mice homozygous for the knock-in mutation (Pthrp KI) displayed retarded growth, early senescence, and malnutrition leading postnatally to their rapid demise. Decreased cellular proliferative capacity and increased apoptosis in multiple tissues including bone and bone marrow cells were associated with altered expression and subcellular distribution of the senescence-associated tumor suppressor proteins p16(INK4a) and p21 and the oncogenes Cyclin D, pRb, and Bmi-1. These findings provide in vivo experimental proof that substantiates the biologic relevance of the NLS and C-terminal portion of PTHrP, a polypeptide ligand that signals mainly via a cell surface G protein-coupled receptor.

Rat wild-type parathyroid hormone receptor (PTH-R) and mutant PTH-R(P132L) show the different intracellular localization in vitro

A replacement of proline with leucine at position 132 of the receptor for parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP), i.e., PTH-R, has been discovered in human Blomstrand's lethal chondrodysplasia. As skeletal deformities in this type of chondrodysplasia appear to compromise the receptor binding to its ligands, we examined the possibility that rat PTH-R carrying P132L mutation (PTH-R(P132L)) would result in abnormal intracellular localization. Osteoblastic MC3T3-E1 cells were transfected with expression vectors containing cDNAs encoding either wild-type PTH-R or mutant PTH-R(P132L). The cells expressing the wild-type PTH-R produced a receptor protein with a molecular mass of 66.3 kDa, which localized its immunoreactivity mainly on the cell surfaces. In contrast, the PTH-R(P132L) was hardly detected on the cell surfaces, but accumulated within the rough-surfaced endoplasmic reticulum. Consistent with this localization, the cells expressing the mutant receptor failed to generate cyclic AMP in response to PTH. Furthermore, a remarkably weaker intensity of the 66.3 kDa band compared with the wild-type counterpart suggests that PTH-R(P132L) is prone to degradation in the transfected cells. In summary, these findings indicate that defective transport of PTH-R(P132L) to the cell surface would be a molecular basis for Blomstrand's chondrodysplasia.

PTHrP increases transcriptional activity of the integrin subunit alpha5

Increasing evidence is emerging highlighting the role of parathyroid hormone-related protein (PTHrP) during metastasis by regulating cell adhesion. The current study demonstrated that modulation of PTHrP expression by PTHrP overexpression and small interfering RNA-induced silencing resulted in changes in cell adhesion and integrin expression. RNA interference of endogenous PTHrP caused a significant reduction in cell adhesion of a breast cancer cell line to collagen type I, fibronectin and laminin (P<0.05) and of a colon cancer cell to collagen type I and fibronectin (P<0.05). Overexpression of PTHrP induced a significant increase in cell adhesion of colon (P<0.0001) and breast (P<0.05) cancer cells to the same extracellular matrix proteins. These PTHrP-mediated effects were attributed to changes in integrin expression as the differences in adhesion profile correlated with the integrin expression profile. In an attempt to elucidate the mechanism whereby PTHrP regulates integrin expression, promoter activity of the integrin α₅ subunit was analysed and significant increases in transcriptional activity were observed in PTHrP overexpressing cells (P<0.0001), which was dependent on nuclear localisation. These results indicate that modulation of cell adhesion is a normal physiological action of PTHrP, mediated by increasing integrin gene transcription.

Parathyroid hormone related protein (PTHrP) inhibits TNFalpha-induced apoptosis by blocking the extrinsic and intrinsic pathways

Parathyroid hormone related protein (PTHrP) is expressed at low levels in many fetal and adult tissues where it plays a central role in regulating cell proliferation, cell death, and tissue homeostasis. In vivo and in vitro, PTHrP has been shown to promote the survival of a variety of cells by regulating expression of the anti-apoptotic protein Bcl2. Additional work has shown that intra-nuclear accumulation of PTHrP in CFK2 (PTH1R positive) and 27m21 (PTH1R negative) condrogenic cells promotes their survival by closing down ribosome biogenesis and promoting quiescence. The current studies were undertaken to examine the role of wild-type PTHrP and a mutant form that cannot translocate to the nucleus in protecting cells from TNFalpha-induced apoptosis. Both forms of the protein were equally effective in blocking the extrinsic pathway by inhibiting expression of the TNF receptor death domain, activating Bid, and promoting cleavage of caspase 8. These observations suggest a direct mechanism of PTHrP action on components of the extrinsic pathway, involving a region of the protein outside of the NTS. PTHrP and M1PTHrP also inhibited the intrinsic pathway by preventing the exchange of anti-apoptotic for pro-apoptotic proteins at the mitochondrial membrane, thus maintaining its integrity and preventing the release of caspase-activating factors into the cytosol. In general, this mitochondrial-related activity was somewhat delayed and was mediated more effectively by PTHrP than by M1PTHrP, suggesting an indirect mechanism of action that might require the presence of an intact NTS.

Parathyroid hormone-related protein (PTHrP) inhibits mitochondrial-dependent apoptosis through CK2

Over the past decade, parathyroid hormone-related protein (PTHrP) has been identified as a key survival factor for cells subjected to apoptotic stimuli. Its anti-apoptotic activity has been attributed to nuclear accumulation of the intact protein, or a synthetic peptide corresponding to its nuclear targeting sequence (NTS), which promotes rapid exit of nutrient deprived cells from the cell cycle. Intracellular PTHrP also inhibited apoptosis by blocking tumor necrosis factor alpha (TNFalpha)-induced apoptosis by blocking signaling from the "death receptor" and preventing damage to the mitochondrial membrane. In both cases, the anti-apoptotic activity was significantly reduced in the presence of a nuclear deficient form of PTHrP with a (88)K/E K/E.K/I(91) mutation in the NTS. The current work was undertaken to determine the mechanism by which nuclear PTHrP blocked mitochondrial-mediated apoptosis. Using sub-cellular fractionation and functional assays we showed that pre-treatment of HEK293 cells with exogenous NTS peptide before inducing apoptosis with TNFalpha was as effective as expression of the full-length protein in inhibiting apoptosis. Inhibition of apoptosis was associated with increased expression of protein kinase casein kinase 2 (CK2) and in sustained CK2 accumulation and activity in the nuclear fraction. In primary chondrogenic cells harvested from the limb buds of PTHrP(+/-) and PTHrP(-/-) embryonic mice, there was a dose-dependent decrease in CK2 expression and activity that correlated with increased susceptibility to apoptosis. Taken together the results indicate that nuclear accumulation of PTHrP effectively inhibits mitochondrial-mediated apoptosis through regulation of the expression, activity, and sub-cellular trafficking of CK2.

Mid-region parathyroid hormone-related protein (PTHrP) binds chromatin of MDA-MB231 breast cancer cells and isolated oligonucleotides "in vitro"

We have previously shown that PTHrP(38-94)-amide restrains growth and invasion "in vitro", causes striking toxicity and accelerates death of some breast cancer cell lines, the most responsive being MDA-MB231 whose tumorigenesis was also attenuated "in vivo". PTHrP(38-94)-amide contains the domain implicated in the nuclear import of PTHrP. Although the nucleus was identified as a destination for mid-region PTHrP, evidence for direct DNA-binding capability is lacking to date. Here, we examined the localization of PTHrP(38-94)-amide within MDA-MB231 cells and within metaphase spread preparations and characterized its DNA-binding properties, employing a combination of immunocytochemical, cytogenetic, "whole genome"/conventional PCR, EMSA and DNase footprinting techniques. The results obtained: (i) show that PTHrP(38-94)-amide gains access to the nuclear compartment of MDA-MB231 cell; (ii) demonstrate that PTHrP(38-94)-amide is a DNA-binding peptide; and, (iii) represent the first data to date on the potential molecular targets in both cellular chromatin and isolated oligonucleotides "in vitro".

Cellular mechanism through which parathyroid hormone-related protein induces proliferation in arterial smooth muscle cells: definition of an arterial smooth muscle PTHrP/p27kip1 pathway

Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27(kip1). Adenoviral restoration of p27(kip1) fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27(kip1). In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27(kip1) expression and accelerates cell cycle progression. p27(kip1) is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27(kip1) pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury.

Fibroblast Growth Factor (FGF) 18 Signals through FGF Receptor 3 to Promote Chondrogenesis

Signaling by fibroblast growth factor (FGF) 18 and FGF receptor 3 (FGFR3) have been shown to regulate proliferation, differentiation, and matrix production of articular and growth plate chondrocytes in vivo and in vitro. Notably, the congenital absence of either FGF18 or FGFR3 resulted in similar expansion of the growth plates of fetal mice and the addition of FGF18 to human articular chondrocytes in culture enhanced proliferation and matrix production. Based on these and other experiments it has been proposed that FGF18 signals through FGFR3 to promote cartilage production by chondrocytes. Its role in chondrogenesis remains to be defined. In the current work we used the limb buds of FGFR3(+/+) and FGFR3(-/-) embryonic mice as a source of mesenchymal cells to determine how FGF18 signaling affects chondrogenesis. Confocal laser-scanning microscopy demonstrated impaired cartilage nodule formation in the FGFR3(-/-) cultures. Potential contributing factors to the phenotype were identified as impaired mitogenic response to FGF18, decreased production of type II collagen and proteoglycan in response to FGF18 stimulation, impaired interactions with the extracellular matrix resulting from altered integrin receptor expression, and altered expression of FGFR1 and FGFR2. The data identified FGF18 as a selective ligand for FGFR3 in limb bud mesenchymal cells, which suppressed proliferation and promoted their differentiation and production of cartilage matrix. This work, thus, identifies FGF18 and FGFR3 as potential molecular targets for intervention in tissue engineering aimed at cartilage repair and regeneration of damaged cartilage.

Regulation of Indian hedgehog mRNA levels in chondrocytic cells by ERK1/2 and p38 mitogen-activated protein kinases

Indian hedgehog (Ihh) is produced by growth plate pre-hypertrophic chondrocytes, and is an important regulator of endochondral ossification. However, little is known about the regulation of Ihh in chondrocytes. We have examined the role of integrins and mitogen-activated protein (MAP) kinases in Ihh mRNA regulation in CFK-2 chondrocytic cells. Cells incubated with the beta1-integrin blocking antibody had decreased Ihh mRNA levels, which was accompanied by decreases of activated extracellular signal-regulated kinases (ERK1/2) and activated p38 MAPK. Ihh mRNA levels were also inhibited by U0126, a specific MEK1/2 inhibitor, or SB203580, a specific p38 MAPK inhibitor. Cells transfected with constitutively active MEK1 or MKK3 had increased Ihh mRNA levels, which were diminished by dominant-negative MEK1, p38alpha or p38beta. Stimulation of the PTH1R with 10(-8) M rPTH (1-34) resulted in dephosphorylation of ERK1/2 that was evident within 15 min and sustained for 1 h, as well as transient dephosphorylation of p38 MAPK that was maximal after 25 min. PTH stimulation decreased Ihh mRNA levels, and this effect was blocked by transfecting the cells with constitutively active MEK1 but not by MKK3. These studies demonstrated that activation of ERK1/2 or p38 MAPK increased Ihh mRNA levels. Stimulation of the PTH1R or blocking of beta1-integrin resulted in inhibition of ERK1/2 and p38 MAPK and decreased levels of Ihh mRNA. Our data demonstrate the central role of MAPK in the regulation of Ihh in CFK-2 cells.

Parathyroid Hormone-Related Protein in Preeclampsia: A Linkage Between Maternal and Fetal Failures

Preeclampsia is a disorder associated with pregnancy that affects both the mother and the fetus. Typical features of the disease are maternal hypertension, proteinuria, and edema as well as fetal growth retardation. Although the etiological details are still being debated, a consensus exists that the starting point is deficient placentation in the first half of pregnancy. The crucial early steps are reduced trophoblast invasiveness and enhanced apoptotic death. In the present review, we demonstrate that parathyroid hormone-related protein is involved not only in the maternal and fetal failures but also in the etiological aspects of the disease. We hypothesize that reduced local production of the peptide is a major causative event.

Parathyroid hormone-related protein and lung biology

Parathyroid hormone-related protein (PTHrP) is expressed in normal and malignant lung and has roles in development, homeostasis, and pathophysiology of injury and cancer. Its effects in developing lung include regulation of branching morphogenesis and type II cell maturation. In adult lung, PTHrP stimulates disaturated phosphatidylcholine secretion, inhibits type II cell growth, and sensitizes them to apoptosis. In lung cancer, PTHrP may play a role in carcinoma progression, or metastasis. The protein could be a useful marker for assessing lung maturity or type II cell function, predicting risk of injury, and detecting lung cancer. PTHrP-based therapies could also prove useful in lung injury and lung cancer.

The inhibition of tumor growth by triplex-forming oligonucleotides

We have previously shown that oligonucleotides designed to bind in triplex fashion to a specific p53 binding site homology inhibit the proliferation of colon cancer cells in vitro. The present study was designed to extend these observations in an in vivo model. HCT 116 human colon carcinoma cells were injected subcutaneously into Ncr nude mice and tumors formed at one to two weeks. Tumors were injected daily for 14 days with either triplex forming oligonucleotide (Hoog 1), a scrambled Hoog 1 oligonucleotide (Hoog3) as control, or vehicle. Tumor size was measured twice weekly. Active triplex forming oligonucleotide (Hoog1) reduced tumor size in comparison to either control oligonucleotide (Hoog3) or vehicle. Tumor sizes in the three groups were significantly different (P < 0.001). Student Newman Keuls test shows statistically significant differences between the experimental group and each of the control and vehicle groups (P < 0.05). A triplex forming oligonucleotide directed at a p53 consensus binding site reduces tumor growth suggesting a novel method of tumor inhibition.

Parathyroid-hormone-related protein as a regulator of pRb and the cell cycle in arterial smooth muscle

BACKGROUND

Parathyroid hormone-related protein (PTHrP), a normal product of arterial vascular smooth muscle (VSM), contains a nuclear localization signal (NLS) and at least 2 translational initiation sites, one that generates a conventional signal peptide and one that disrupts the signal peptide. These unusual features allow PTHrP either to be secreted in a paracrine/autocrine fashion, and thereby to inhibit arterial smooth muscle proliferation, or to be retained within the cytosol and to translocate into the nucleus, thereby serving as an intracrine stimulator of smooth muscle proliferation.

METHODS AND RESULTS

Here, we demonstrate 2 important findings. First, PTHrP dramatically increases the percentage of VSM cells in the S and in G2/M phases of the cell cycle. These effects require critical serine and threonine residues at positions Ser119, Ser130, Thr132, and Ser138 in the carboxy-terminus of PTHrP and are associated with the phosphorylation of the key cell cycle checkpoint regulator retinoblastoma protein, pRb. Second, because PTHrP devoid of the NLS serves as an inhibitor of VSM proliferation, we hypothesized that local delivery of NLS-deleted PTHrP to the arterial wall at the time of angioplasty might prevent neointimal hyperplasia. As hypothesized, using a rat carotid angioplasty model, adenoviral delivery of NLS-deleted PTHrP completely abolished the development of the neointima after angioplasty.

CONCLUSIONS

PTHrP interacts with key cell cycle regulatory pathways within the arterial wall. Moreover, NLS-deleted PTHrP delivered to the arterial wall at the time of angioplasty seems to have promise as an agent that could reduce or eliminate the neointimal response to angioplasty.

Signalling by fibroblast growth factor receptor 3 and parathyroid hormone-related peptide coordinate cartilage and bone development

Bone development is regulated by conserved signalling pathways that are linked to multifunctional growth factors and their high affinity receptors. Parathyroid hormone-related peptide (PTHrP) and fibroblast growth factor receptor 3 (FGFR3) have been shown to play pivotal, and sometimes complementary, roles in the replication, maturation and death of chondrocytes during endochondral bone formation. To gain further insight into how these pathways coordinate cartilage and bone development, we generated mice lacking expression of both PTHrP and FGFR3. The phenotype of compound mutant mice resembled that of their PTHrP-deficient littermates with respect to neonatal lethality, facial dysmorphism and foreshortening of the limbs. The absence of PTHrP in the developing epiphyseal cartilage of PTHrP-/- and PTHrP-/-/FGFR3-/- mice resulted in a dominant hypo-proliferative phenotype. However, abnormalities such as the presence of nonhypertrophic cells among hypertrophic chondrocytes and excessive apoptosis seen in the hypertrophic zone of PTHrP-/- mice were absent in the PTHrP-/-/FGFR3-/- mice. Furthermore, the absence of FGFR3 in single and compound mutant mice led to decreased expression of vascular endothelial growth factor (VEGF) and an increase in depth of hypertrophic chondrocytes. These observations indicate that FGFR3 deficiency can rescue some of the defects seen in PTHrP-deficient mice and that it plays an important role in the regulation of chondrocyte differentiation and hypertrophy. These studies support a dominant role for PTHrP in regulating the pool of proliferating cells during limb development and suggest that signalling by FGFR3 plays a more prominent role in cartilage maturation and vascular invasion at the chondro-osseous junction.

Parathyroid hormone-related protein (PTHrP): a nucleocytoplasmic shuttling protein with distinct paracrine and intracrine roles

Parathyroid hormone-related protein (PTHrP) was first discovered as a circulating factor secreted by certain cancers responsible for the syndrome of humoral hypercalcemia of malignancy. PTHrP possesses distinct paracrine and intracrine signaling roles. The similarity of its N-terminus to that of parathyroid hormone (PTH) enables it to share PTH's paracrine signaling properties, whereas the rest of the molecule possesses other functions, largely relating to an intracrine signaling role in the nucleus/nucleolus in regulating apoptosis and cell proliferation. Recent advances have shown that intracellularly expressed PTHrP is able to shuttle in cell-cycle- and signal-dependent fashion between nucleus and cytoplasm through the action of the distinct intracellular transport receptors importin beta 1 and exportin 1 (Crm1) mediating nuclear import and export of PTHrP, respectively. Together, the import and export pathways constitute an integrated system for PTHrP subcellular localization. Intriguingly, PTHrP nuclear/nucleolar import is dependent on microtubule integrity, transport to the nucleus appearing to occur in vectorial fashion along microtubules, mediated in part by the action of importin beta 1. PTHrP has recently been shown to be able to bind to RNA, meaning that PTHrP's nucleocytoplasmic shuttling ability may relate to a specific role within the nucleus/nucleolus to regulate RNA synthesis and/or transport.

PTHrP [67-86] regulates the expression of stress proteins in breast cancer cells inducing modifications in urokinase-plasminogen activator and MMP-1 expression

It was previously reported that a midregion domain of parathyroid hormone-related protein (PTHrP), that is, [67-86]-amide, is able to restrain growth and promote matrigel penetration by the 8701-BC cell line, derived from a biopsy fragment of a primary ductal infiltrating carcinoma of the human breast, and that cell invasion in vitro is drastically impaired by inactivation of urokinase-plasminogen activator (uPa). In this study we started a more detailed investigation of the possible effects on gene expression arising from the interaction between PTHrP [67-86]-amide and 8701-BC breast cancer cells by a combination of conventional-, differential display-and semi-quantitative multiplex-polymerase chain reaction (PCR) assays. We present here the first evidence that the upregulation of some stress-related genes, most noticeably heat shock factor binding protein-1 (hsbp1) and heat shock protein 90 (hsp-90), is involved in the acquisition of an in vitro more invasive phenotype by cells treated with midregion PTHrP. This is conceivably accomplished by sequestering and inactivating heat shock factor-1 (hsf1) which is able to recognize Ets transcription-factor-binding sites present in some gene promoters, such as those of uPa and matrix metalloprotease-1 (MMP-1). In fact, our data show that incubation of PTHrP [67-86]-amide-treated cells with either antisense hsbp1-oligonucleotide or geldanamycin, an hsp90-inactivating antibiotic, results in downregulation of uPa and upregulation of MMP-1, and in a prominent inhibition of cell invasion in matrigel-containing Transwell chambers. Alternatively, incubation of untreated 8701-BC cells with quercetin, a flavonoid known to decrease the amount of free hsf1, is found to induce upregulation of uPa and downregulation of MMP-1, and an increase of matrigel invasion by cells, thus providing further supporting data of the involvement of hsf unavailability on the modulation of uPa and MMP-1 expression and on cell invasive behaviour. These studies confirm a previous postulate that over-secretion of uPa, rather than of other extracellular proteases, is a primary condition for the increase of invasive activity triggered by PTHrP [67-86]-amide in vitro, and support a role for midregion forms of PTHrP in potentially affecting pathological mammary growth and differentiation. They also identify two new key protagonists in the complex scenario of breast tumor cell invasiveness in vitro, that is, hsbp1 and hsp90, which deserve further and more extensive studies as potential and attractive molecular targets for anti-breast cancer treatments.

Hair-cycle-dependent expression of parathyroid hormone-related protein and its type I receptor: evidence for regulation at the anagen to catagen transition

The humoral hypercalcemia factor parathyroid hormone-related protein is a paracrine-signaling molecule that regulates the development of several organ systems, including the skin. In pathologic circumstances such as hypercalcemia and in development, parathyroid hormone-related protein signaling appears to be mediated by the type I parathyroid hormone/parathyroid hormone-related protein receptor. In order to clarify the role of the ligand and receptor pair in cutaneous biology, gene expression was monitored in a series of murine skin samples ranging from embryonic day 14 to 2 y with in situ hybridization and RNase protection. In all samples, high levels of parathyroid hormone-related protein transcripts were exclusively expressed in the developing and adult hair follicle but were not observed in the interfollicular epidermis. In the adult, parathyroid hormone-related protein mRNA expression was dynamically regulated as a function of the murine hair cycle in a way similar to other signaling molecules that regulate the anagen to catagen transition. PTH receptor transcripts were abundantly expressed in the developing dermis. In the adult skin, PTH receptor mRNA was markedly reduced, but again demonstrated hair-cycle-dependent expression. The dorsal skin of the keratin 14-parathyroid hormone-related protein mouse was used to evaluate the impact of overexpression of the peptide on the murine hair cycle. All types of hair were 30-40% shorter in adult keratin 14-parathyroid hormone-related protein mice as compared with wild-type littermates. This appeared to result from a premature entry into the catagen phase of the hair cycle. Finally, the relationship between parathyroid hormone-related protein signaling and other growth factors that regulate the hair cycle was examined by cross-breeding experiments employing keratin 14-parathyroid hormone-related protein mice and fibroblast growth factor-5-knockout mice. It appears that parathyroid hormone-related protein and fibroblast growth factor-5 regulate the anagen to catagen transition by independent pathways.

The intracrine hypothesis and intracellular peptide hormone action

There is evidence that many peptide growth factors and hormones act in the intracellular space after either internalization or retention in their cells of synthesis. These factors, commonly called intracrines, are structurally diverse while sharing some common functional features. Reports of intracellular peptide hormone binding and action are reviewed here. Also, this laboratory has made proposals regarding the origin and actions of intracrines and these areas are further explored. Intracrine interactions and the relationship of intracrines to transcription factors are discussed. The intracellular/intracrine renin-angiotensin system (iRAS) is reviewed to illustrate the intracrine analogue of a well-established physiological system. The role of intracrine action in metazoan development is also considered.

The parathyroid hormone-related protein system: more data but more unsolved questions

PURPOSE OF REVIEW

The present review focuses on recent studies that might be considered as the most relevant advances in the parathyroid hormone-related protein field, with special emphasis on proven functions in renovascular and cardiovascular systems, in physiological as well as pathological conditions. Thus, the questions as to whether and how parathyroid hormone-related protein intervenes in vascular development and homeostasis and in vascular diseases such as hypertension, atherosclerosis, restenosis and heart failure have begun to be unraveled.

RECENT FINDINGS

Since its discovery from hypercalcemia-associated tumors in 1987, it has become clear that parathyroid hormone-related protein is a ubiquitously expressed poly-hormone and plays crucial roles in normal life. The early lethality to parathyroid hormone-related protein knockout mice emphasizes the crucial roles of the protein in development but has limited the use of these models. However, data accumulated from transgenic animals overexpressing the protein in particular cells have provided considerable support to its physiological and pathological relevance. The recent demonstration that nascent parathyroid hormone-related protein not only follows the secretory pathways, but also directly translocates to the nucleus, is beginning to uncover new actions for the protein in a number of physiological systems such as bone, mammary gland and vascular smooth muscle, as well as in pathological situations, such as cancer, osteoporosis, sepsis, atherosclerosis and hypertension.

SUMMARY

The development of mice with conditionally deleted parathyroid hormone-related protein or parathyroid hormone-1 receptor alleles will allow the creation of cell- or tissue-specific parathyroid hormone-related protein knockout mice which will greatly facilitate the determination of the biological relevance of this protein in a specific cell or tissue type, particularly in the cardiovascular system.

Biological action of parathyroid hormone (PTH)-related peptide (PTHrP) mediated either by the PTH/PTHrP receptor or the nucleolar translocation in chondrocytes

Parathyroid hormone (PTH)-related peptide (PTHrP) has been believed to act by binding the common receptor to PTH (PTH/PTHrP receptor). However, PTHrP is localized not only in the secretory pathway, but also in nucleoli by virtue of its nucleolar targeting signal (NTS). This review demonstrates the bipartite action of PTHrP on chondrocytes, the receptor-mediated and -independent signaling pathway. Mice with deletion of the PTHrP gene were characterized by a chondrodysplasia due to markedly reduced proliferation of epiphyseal chondrocytes. The PTH/PTHrP receptor was localized mainly in proliferative chondrocytes in the epiphyseal cartilage, indicating that PTHrP modulates normal proliferation via the receptor. In contrast to the receptor-mediated action, the mid-region of the amino acid sequence of PTHrP contains an NTS. The PTHrP-translation was found to initiate from both methionine-coding AUG and downstream leucine-coding CUGs in its signal sequence. When translated from CUGs, PTHrP accumulated in the nucleoli, and the translation from AUG localized PTHrP in both the Golgi apparatus and nucleoli. Therefore, nucleolar PTHrP appears to be derived from the translation initiating from both AUG and CUGs. A chondrocytic cell line expressing a full-length PTHrP, but not PTHrP lacking NTS, were resistant to apoptosis caused by serum depletion, suggesting that the nucleolar PTHrP in chondrocytes serves as a survival factor against apoptosis. Thus, PTHrP regulates chondrocyte proliferation, differentiation and apoptosis by mediating its receptor or acting directly on the nucleolus.

Toward a theory of intracrine hormone action

A growing body of evidence indicates that in some cases, peptide hormones can function in the intracellular space. These findings are reviewed. In addition, this laboratory has made proposals regarding the origin, nature and function of intracrines--that is, intracellularly acting peptide hormones that also function in an autocrine, paracrine or endocrine manner. Here, these hypotheses are developed, and potential implications/applications of this point of view are discussed. Possible implications for cellular differentiation, cellular memory and hormonal responsiveness, as well as for the assumption of novel functions by intracellular regulatory proteins are discussed.

Parathyroid Hormone, Its Fragments and Their Analogs for the Treatment of Osteoporosis

The susceptibility to traumatic fracturing of osteopenic bones, and the spontaneous fracturing of osteoporotic bones by normal body movements caused by the microstructural deterioration and loss of bone, are currently treated with antiresorptive drugs, such as the bisphosphonates, calcitonin, estrogens, and selective estrogen receptor modulators. These antiresorptive agents target osteoclasts and, as their name indicates, reduce or stop bone resorption. They cannot directly stimulate bone formation, increase bone mass above normal values in ovariectomized rat models, or improve microstructure. However, there is a family of agents - the parathyroid hormone (PTH) and some of its fragments and their analogs - which directly stimulate bone growth and improve microstructure independently from impairing osteoclasts. These drugs are about to make their clinical debut in treating patients with osteoporosis and, probably not too far in the future, for accelerating fracture healing. They stimulate osteoblast accumulation and bone formation in three ways via signals from the type 1 PTH/PTH-related protein (PTHR1) receptors on proliferatively inactive preosteoblasts, osteoblasts, osteocytes and bone-lining cells. The receptor signals shut down the proliferative machinery in preosteoblasts and push their maturation to osteoblasts, cause the osteoblastic cells to make and secrete several factors that stimulate the extensive proliferation of osteoprogenitors without PTHRI receptors, stimulate the reversion of bone-lining cells to osteoblasts, and extend osteoblast lifespan and productivity by preventing them from suicidally initiating apoptosis. The first of the PTHs to reach the clinic will be teriparatide [recombinant human (h)PTH-(1-34)], which was recommended for approval in 2001 by the US Food and Drug Administration Endocrinology and Metabolic Drugs Advisory Committee for the treatment of postmenopausal osteoporosis. Teriparatide has been shown to considerably increase cancellous and cortical bone mass, improve bone microstructure, prevent fractures and thus provide benefits that cannot be provided by current antiresorptive drugs, when administered subcutaneously at a daily dose of 20 microg for no longer than 2 years to patients with osteoporosis.