Parathyroid hormone-related protein induces G1 phase growth arrest of vascular smooth muscle cells

From Good to Bad: The Opposing Effects of PTHrP on Tumor Growth, Dormancy, and Metastasis Throughout Cancer Progression

Parathyroid hormone related protein (PTHrP) is a multifaceted protein with several biologically active domains that regulate its many roles in normal physiology and human disease. PTHrP causes humoral hypercalcemia of malignancy (HHM) through its endocrine actions and tumor-induced bone destruction through its paracrine actions. PTHrP has more recently been investigated as a regulator of tumor dormancy owing to its roles in regulating tumor cell proliferation, apoptosis, and survival through autocrine/paracrine and intracrine signaling. Tumor expression of PTHrP in late stages of cancer progression has been shown to promote distant metastasis formation, especially in bone by promoting tumor-induced osteolysis and exit from dormancy. In contrast, PTHrP may protect against further tumor progression and improve patient survival in early disease stages. This review highlights current knowledge from preclinical and clinical studies examining the role of PTHrP in promoting tumor progression as well as skeletal and soft tissue metastasis, especially with regards to the protein as a regulator of tumor dormancy. The discussion will also provide perspectives on PTHrP as a prognostic factor and therapeutic target to inhibit tumor progression, prevent tumor recurrence, and improve patient survival.

Parathyroid-hormone-related protein signaling mechanisms in lung carcinoma growth inhibition

Parathyroid hormone-related protein (PTHrP) inhibits proliferation of several lung cancer cell lines, but the signaling mechanism has not been established. This study tested the hypotheses that growth inhibition is mediated through the PTHrP receptor, PTH1R, and that the process is modified by ERK activation. PTHrP-positive and negative clones of H1944 lung adenocarcinoma cells underwent stable PTH1R knockdown with lentiviral shRNA or transient transfection with ERK1 and ERK2 siRNA. Alternatively, cells were treated with 8-CPT cAMP, 8-CPT 2′-O-methyl cAMP, and N-6-phenyl cAMP analogs. H1944 cells expressing ectopic PTHrP showed 20–40% decrease in proliferation compared to the PTHrP-negative cells in the presence of normal levels of PTH1R (P < 0.01). PTH1R knockdown eliminated this difference and increased cell proliferation regardless of PTHrP status. The three cAMP analogs each inhibited proliferation over 5 days by 30–40%. ERK2 knockdown inhibited proliferation of PTHrP-positive cells alone and in combination with ERK1 knockdown. The growth inhibition mediated by cAMP analogs was unaffected by ERK1 knockdown. In conclusion, ectopic expression of PTHrP 1–87 inhibits H1944 cell proliferation. PTH1R knockdown blocks this effect and stimulates proliferation, indicating that the ligand exerts anti-mitogenic effects. cAMP, the second messenger for PTH1R also inhibits proliferation and activates ERK. PTHrP growth inhibition may be opposed by concomitant ERK activation.

EBP50 inhibits the anti-mitogenic action of the parathyroid hormone type 1 receptor in vascular smooth muscle cells

Parathyroid hormone-related protein (PTHrP) and the parathyroid hormone type 1 receptor (PTH1R) are important regulators of vascular remodeling. PTHrP expression is associated to increased proliferation of vascular smooth muscle cells (VSMC). In contrast, signaling via the PTH1R inhibits cell growth. The mechanisms regulating the dual effect of PTHrP and PTH1R on VSMC proliferation are only partially understood. In this study we examined the role of the adaptor protein ezrin-radixin-moesin-binding phosphoprotein (EBP50) on PTH1R expression, trafficking, signaling and control of A10 cell proliferation. In normal rat vascular tissues, EBP50 was restricted to the endothelium with little expression in VSMC. EBP50 expression significantly increased in VSMC following angioplasty in parallel with PTHrP. Interestingly, PTHrP was able to induce EBP50 expression. In the clonal rat aortic smooth muscle cell line A10, EBP50 increased the recruitment of PTH1R to the cell membrane and delayed its internalization in response to PTHrP(1-36). This effect required an intact C-terminal motif in the PTH1R. In naïve A10 cells, PTHrP(1-36) stimulated cAMP production but not intracellular calcium release. In contrast, PTHrP(1-36) induced both cAMP and calcium signaling in A10 cells over-expressing EBP50. Finally, EBP50 attenuated the induction of p27(kip1) and the anti-proliferative effect of PTHrP(1-36). In summary, this study demonstrates the dynamic expression of EBP50 in vessels following injury and the effects of EBP50 on PTH1R function in VSMC. These findings highlight one of the mechanisms leading to increased VSMC proliferation and have important implication in the understanding of the molecular events leading to restenosis.

Cytokine-dependent balance of mitogenic effects in primary human lung fibroblasts related to cyclic AMP signaling and phosphodiesterase 4 inhibition

Interleukin-1beta (IL-1beta) and basic fibroblast growth factor (bFGF) are important regulators of proliferation, and their expression is increased in lungs of patients with asthma, idiopathic pulmonary fibrosis (IPF), or chronic obstructive pulmonary disease (COPD). We investigated the effect of IL-1beta and bFGF on proliferation of human lung fibroblasts and the role of COX-2, PGE(2), and cAMP in this process. Furthermore, the effect of phosphodiesterase (PDE) 3 and 4 inhibition was analyzed. In primary human lung fibroblasts low concentrations of IL-1beta (<10 pg/ml) potentiated the bFGF-induced DNA synthesis, whereas higher concentrations revealed antiproliferative effects. Higher concentrations of IL-1beta-induced COX-2 mRNA and protein associated with an increase in PGE(2) and cAMP, and all of these parameters were potentiated by bFGF. The PDE4 inhibitor piclamilast concentration-dependently reduced proliferation by a partial G1 arrest. The PDE3 inhibitor motapizone was inactive by itself but enhanced the effect of the PDE4 inhibitor. This study demonstrates that bFGF and IL-1beta act in concert to fine-tune lung fibroblast proliferation resulting in amplification or reduction. The antiproliferative effect of IL-1beta is likely attributed to the induction of COX-2, which is further potentiated by bFGF, and the subsequent generation of PGE(2) and cAMP. Inhibition of PDE4 inhibition (rather than PDE3) may diminish proliferation of human lung fibroblasts and therefore could be useful in the therapy of pathological remodeling in lung diseases.

Cell cycle actions of parathyroid hormone-related protein in non-small cell lung carcinoma

Parathyroid hormone-related protein (PTHrP), a paraneoplastic protein expressed by two-thirds of human non-small cell lung cancers, has been reported to slow progression of lung carcinomas in mouse models and to lengthen survival of patients with lung cancer. This study investigated the effects of ectopic expression of PTHrP on proliferation and cell cycle progression of two human lung adenocarcinoma cell lines that are normally PTHrP negative. Stable transfection with PTHrP decreased H1944 cell DNA synthesis, measured by thymidine incorporation, bromodeoxyuridine uptake, and MTT proliferation assay. A substantial fraction of PTHrP-positive cells was arrested in or slowly progressing through G1. Cyclin D2 and cyclin A2 protein levels were 60-70% lower in PTHrP-expressing cells compared with control cells (P < 0.05, N = 3 independent clones per group), while expression of p27(Kip1), a cyclin-dependent kinase inhibitor, was increased by 35 +/- 9% (mean +/- SE, P < 0.05) in the presence of PTHrP. Expression of other cyclins, including cyclins D1 and D3, and cyclin-dependent kinases was unaffected by PTHrP. PTHrP did not alter the phosphorylation state of Rb, but decreased cyclin-dependent kinase (CDK) 2-cyclin A2 complex formation. Ectopic expression of PTHrP stimulated ERK phosphorylation. In MV522 cells, PTHrP had similar effects on DNA synthesis, cyclin A2 expression, pRb levels, CDK2-cyclin A2 association, and ERK activation. In summary, PTHrP appears to slow progression of lung cancer cells into S phase, possibly by decreasing activation of CDK2. Slower cancer cell proliferation could contribute to slower tumor progression and increased survival of patients with PTHrP-positive lung cancer.

Mutant parathyroid hormone-related protein, devoid of the nuclear localization signal, markedly inhibits arterial smooth muscle cell cycle and neointima formation by coordinate up-regulation of p15Ink4b and p27kip1

Arterial expression of PTH-related protein is markedly induced by angioplasty. PTH-related protein contains a nuclear localization signal (NLS). PTH-related protein mutants lacking the NLS (DeltaNLS-PTH-related protein) are potent inhibitors of arterial vascular smooth muscle cell (VSMC) proliferation in vitro. This is of clinical relevance because adenoviral delivery of DeltaNLS-PTH-related protein at angioplasty completely inhibits arterial restenosis in rats. In this study we explored the cellular mechanisms through which DeltaNLS-PTH-related protein arrests the cell cycle. In vivo, adenoviral delivery of DeltaNLS-PTH-related protein at angioplasty markedly inhibited VSMC proliferation as compared with angioplastied carotids infected with control adenovirus (Ad.LacZ). In vitro, DeltaNLS-PTH-related protein overexpression was associated with a decrease in phospho-pRb, and a G(0)/G(1) arrest. This pRb underphosphorylation was associated with stable levels of cdks 2, 4, and 6, the D and E cyclins, p16, p18, p19, and p21, but was associated with a dramatic decrease in cdk-2 and cdk4 kinase activities. Cyclin A was reduced, but restoring cyclin A adenovirally to normal did not promote cell cycle progression in DeltaNLS-PTH-related protein VSMC. More importantly, p15(INK4) and p27(kip1), two critical inhibitors of the G(1/S) progression, were markedly increased. Normalization of both p15(INK4b) and p27(kip1) by small interfering RNA knockdown normalized cell cycle progression. These data indicate that the changes in p15(INK4b) and p27(kip1) fully account for the marked cell cycle slowing induced by DeltaNLS-PTH-related protein in VSMCs. Finally, DeltaNLS-PTH-related protein is able to induce p15(INK4) and p27(kip1) expression when delivered adenovirally to primary murine VSMCs. These studies provide a mechanistic understanding of DeltaNLS-PTH-related protein actions, and suggest that DeltaNLS-PTH-related protein may have particular efficacy for the prevention of arterial restenosis.

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.

Dibutyryl cAMP stimulates the proliferation of SH-SY5Y human neuroblastoma cells by up-regulating Skp2 protein

PURPOSE

We previously found that the proliferation of SH-SY5Y neuroblastoma cells is stimulated when cAMP is up-regulated by stable expression of stimulatory G protein. Therefore, this study was performed to investigate the mechanism whereby cAMP stimulates the proliferation of SH-SY5Y cells.

METHODS

To investigate the effect of cAMP on cellular proliferation, SH-SY5Y neuroblastoma cells were treated with dibutyryl cAMP (dbcAMP), and then cell growth, thymidine incorporation and cell cycle phase distribution were analyzed. The expression and the activity of the molecules that regulate cell cycle progression were monitored by Western blot, RT-PCR, and kinase activity assay.

RESULTS

Treatment with dbcAMP produced a biphasic effect on cellular proliferation; especially treatment with low concentration of dbcAMP (0.5 mM) showed a higher cellular proliferation rate and promoted G1/S transition in cell cycle. The dbcAMP (0.5 mM) treatment increased CDK2 activity, and it significantly decreased p27Kip1 expression with a decreased half-life of p27Kip1 protein. Moreover, dbcAMP (0.5 mM) increased the protein level and the stability of Skp2 with a concomitant decrease in its ubiquitination.

CONCLUSIONS

cAMP up-regulates Skp2 protein by reducing its degradation probably through decreasing the ubiquitination of Skp2, which might result in accelerated degradation of p27Kip1, increase in CDK2 activity, and stimulation of SH-SY5Y cell proliferation in sequence.

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.

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.

cAMP pathway in podocytes

Podocytes possess receptors for a variety of hormones. The following receptors whose stimulation results in increased cAMP levels have been detected in podocytes: adrenergic beta(2) receptor, dopamine D(1) receptor, prostaglandin IP and EP(4) receptors, and parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor. Besides activating protein kinase A, increased levels of cAMP depolarize podocytes via opening of chloride channels. Relatively little is known about the impact of the cAMP pathway on podocyte function. Results obtained in a limited number of studies indicate that cAMP in podocytes may regulate cell morphology, actin assembly, and matrix production. In addition, cAMP seems to attenuate the action of hormones, which activate the Ca(2+)/protein kinase C pathway. Effects of the cAMP pathway on further aspects of podocyte biology, such as contractility, phosphorylation state of slit membrane-associated proteins, glomerular permeability, cell cycle control, and synthesis of reactive oxygen species can be anticipated from studies on other cell types and from studies on isolated glomeruli. In summary, the data available indicate that the cAMP pathway affects several aspects of podocyte biology in an overall glomerulo-protective manner.

Parathyroid hormone-related protein and its receptors: nuclear functions and roles in the renal and cardiovascular systems, the placental trophoblasts and the pancreatic islets

The cloning of the so-called 'parathyroid hormone-related protein' (PTHrP) in 1987 was the result of a long quest for the factor which, by mimicking the actions of PTH in bone and kidney, is responsible for the hypercalcemic paraneoplastic syndrome, humoral calcemia of malignancy. PTHrP is distinct from PTH in a number of ways. First, PTHrP is the product of a separate gene. Second, with the exception of a short N-terminal region, the structure of PTHrP is not closely related to that of PTH. Third, in contrast to PTH, PTHrP is a paracrine factor expressed throughout the body. Finally, most of the functions of PTHrP have nothing in common with those of PTH. PTHrP is a poly-hormone which comprises a family of distinct peptide hormones arising from post-translational endoproteolytic cleavage of the initial PTHrP translation products. Mature N-terminal, mid-region and C-terminal secretory forms of PTHrP are thus generated, each of them having their own physiologic functions and probably their own receptors. The type 1 PTHrP receptor, binding both PTH(1-34) and PTHrP(1-36), is the only cloned receptor so far. PTHrP is a PTH-like calciotropic hormone, a myorelaxant, a growth factor and a developmental regulatory molecule. The present review reports recent aspects of PTHrP pharmacology and physiology, including: (a) the identification of new peptides and receptors of the PTH/PTHrP system; (b) the recently discovered nuclear functions of PTHrP and the role of PTHrP as an intracrine regulator of cell growth and cell death; (c) the physiological and developmental actions of PTHrP in the cardiovascular and the renal glomerulo-vascular systems; (d) the role of PTHrP as a regulator of pancreatic beta cell growth and functions, and, (e) the interactions of PTHrP and calcium-sensing receptors for the control of the growth of placental trophoblasts. These new advances have contributed to a better understanding of the pathophysiological role of PTHrP, and will help to identify its therapeutic potential in a number of diseases.

Paradoxical actions of exogenous and endogenous parathyroid hormone-related protein on renal vascular smooth muscle cell proliferation: reversion in the SHR model of genetic hypertension

In previous studies, added parathyroid hormone-related protein (PTHrP) inhibits whereas transfected PTHrP stimulates the proliferation of A10 aortic smooth muscle cells by nuclear translocation of the peptide. In the present studies, we asked whether these paradoxical trophic actions of PTHrP occur in smooth muscle cells (SMC) cultured from small intrarenal arteries of, and whether they are altered in, 12-wk-old spontaneously hypertensive rats (SHR) as compared to normotensive Wistar-Kyoto (WKY) rats. SHR cells grew faster than WKY cells. PTHrP transcript was increased in SHR-derived cells whereas PTH1 receptor (PTH1R) transcripts were similar in both cell lines. In both strains of cells, stable transfection with human PTHrP(1-139) cDNA did not further induce proliferation, suggesting maximal effect of endogenous PTHrP in wild cells. In contrast, transfection with antisense hPTHrP(1-139) cDNA, which abolished PTHrP mRNA, decreased WKY but increased SHR cell proliferation. Added PTHrP(1-36) (1-100 pM) decreased WKY and increased SHR cell proliferation. Additional studies indicated that the preferential coupling of PTH1-R to G-protein Gi was responsible for the proliferative effect of exogenous PTHrP in SHR cells. Moreover, PTHrP was detected in the nucleolus of a fraction of WKY and SHR renal SMC, in vitro as well as in situ, suggesting that the nucleolar translocation of PTHrP might be involved in the proliferative effects of endogenous PTHrP. In renovascular SMC, added PTHrP is antimitogenic, whereas endogenously produced PTHrP is mitogenic. These paradoxical effects of PTHrP on renovascular SMC proliferation appear to be reversed in the SHR model of genetic hypertension. A new concept emerges from these results, according to which a single molecule may have opposite effects on VSMC proliferation under physiological and pathophysiological conditions.

Molecular pathways of cyclic nucleotide-induced inhibition of arterial smooth muscle cell proliferation

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a wide variety of extracellular stimuli. These signals regulate many biological processes including cell proliferation, differentiation, migration, and apoptosis. Recently, significant progress has been achieved in the molecular basis underlying cyclic nucleotide regulation of cell proliferation. This review summarizes our knowledge of the signaling pathways regulated by cyclic nucleotides in arterial smooth muscle cells.