Expression and regulation of parathyroid hormone-related peptide in normal and malignant melanocytes

We examined parathyroid hormone-related peptide (PTHrP) production and regulation in both normal human melanocytes and in a human amelanotic melanoma cell line (A375). Northern blot and immunocytochemical analysis demonstrated that both cultured A375 cells and normal human melanocytes express PTHrP, but A375 cells expressed much higher levels of the peptide. PTHrP secretory rate increased at least 10-fold after treatment with 10% fetal bovine serum (100.2 +/- 2.8 pmol/10(6) cells vs. basal <15 pmol/10(6) cells) in proliferating A375 cells but only twofold in confluent cells. Treatment of A375 cells with increasing concentrations of 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] or its low-calcemic analog EB-1089 revealed that EB-1089 was 10-fold more potent than 1, 25-(OH)(2)D(3) on inhibition of both cell proliferation and PTHrP expression. Furthermore, inoculation of A375 cells into the mammary fat pad of female severe combined immunodeficiency mice resulted in the development of hypercalcemia and elevated concentrations of plasma immunoreactive PTHrP in the absence of detectable skeletal metastases. Our study, therefore, demonstrates a stepwise increase in PTHrP expression when cells progress from normal to malignant phenotype and suggests that EB-1089 should be further evaluated as a therapeutic agent in human melanoma.

The vitamin D analogue EB 1089 prevents skeletal metastasis and prolongs survival time in nude mice transplanted with human breast cancer cells

1,25-Dihydroxyvitamin D has potent antiproliferative and anti-invasive properties in vitro in cancer cells. However, its calcemic effect in vivo limits its therapeutic applications. Here, we report the efficacy of EB 1089, a low calcemic analogue of vitamin D, on the development of osteolytic bone metastases after intracardiac injection of the human breast cancer cell line MDA-MB-231 in nude mice. Animals injected with tumor cells were implanted simultaneously with osmotic minipumps containing either EB 1089 or vehicle. Both groups remained normocalcemic for the duration of the experiment. The total number of bone metastases, the mean surface area of osteolytic lesions, and tumor burden within bone per animal were markedly decreased in EB1089-treated mice. Furthermore, longitudinal analysis revealed that mice treated with EB1089 displayed a marked increase in survival and developed fewer bone lesions and less hind limb paralysis over time as compared with untreated animals. These results suggest that EB1089 may be beneficial in the prevention of metastatic bone lesions associated with human breast cancer.

Molecular basis of the spectrum of skeletal complications of neoplasia

BACKGROUND

Neoplasia may produce a spectrum of dysregulatory effects on bone and mineral metabolism. The range of these effects and the known molecular mechanisms causing them are reviewed.

METHODS

The current review is mainly based on previously published scientific reports from North America, Europe, and Japan that were identified from references in the literature.

RESULTS

Osteolysis is the most common skeletal manifestation of neoplasia and may be focal or generalized. When tumors release abundant parathyroid hormone-related peptide (PTHrP) into the circulation, this may act as an endocrine substance to produce generalized osteopenia and, ultimately, hypercalcemia. PTHrP also may act in a paracrine manner to enhance focal osteolysis associated with metastasis and to generate hypercalcemia. The increased circulating PTHrP in tumor states also can augment serum calcium by renal mechanisms. PTHrP may contribute to focal osteolysis by tumor metastases, even in the absence of hypercalcemia. Strategies to reduce PTHrP production or PTHrP signaling, therefore, may be useful to treat the tumor-induced bone resorption induced both in hypercalcemic and nonhypercalcemic states. The most commonly used intervention, bisphosphonates, targets the osteoclast directly. Although osteolytic lesions generally occur with some degree of reactive new bone formation, osteoblastic lesions may be particularly abundant in association with certain tumors, such as prostate carcinoma. The mechanisms underlying these lesions remain unknown; however, a variety of osteoblast growth factors may contribute. These include the urokinase system, which may have growth factor activity as well as enzymatic activity. Finally, osteomalacia may be a manifestation of tumors either through accelerated bone formation with insufficient mineralization or through the production of a phosphaturic substance.

CONCLUSIONS

Elucidation of the mechanisms underlying the spectrum of skeletal manifestations of neoplasia is yielding important insights into both tumor diagnosis and patient management.

Characterization of 3-epi-1alpha,25-dihydroxyvitamin D3 involved in 1alpha,25-dihydroxyvitamin D3 metabolic pathway in cultured cell lines

Using six different cultured cell models representing osteoblast, intestine, kidney and keratinocyte, we have demonstrated that 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) is metabolized into 3-epi-1alpha,25(OH)2D3 in vitamin D-target cells. Although differences existed in the amount of 3-epi-1alpha,25(OH)2D3 formed with different cell types, it was apparent that 1alpha,25(OH)2D3 was subjected to metabolism both through the C24-oxidation and 3-epimerization pathways. Time course and dose response studies showed that the production of 3-epi-1alpha,25(OH)2D3 was enzymatic. It is interesting to note that this epimerization proceeded from 3beta towards 3alpha unidirectionally, and this conversion was not inhibited by ketoconazole. These data suggest that cytochrome P450 related enzymes including the 24-hydroxylase would not affect this reaction. The biological activity of 3-epi-1alpha,25(OH)2D3 was found to be lower than the native 1alpha,25(OH)2D3 in suppressing of proliferation of HL-60 cells, while the affinity of 3-epi-1alpha,25(OH)2D3 for vitamin D-binding protein was 2.5-fold higher than that of 1alpha,25(OH)2D3. The results indicate that 3-epimerization may change the pharmacokinetics and catabolism of 1alpha,25(OH)2D3 in vitamin D-target cells.

Reversal of hypercalcemia with the vitamin D analogue EB1089 in a human model of squamous cancer

EB1089, an analogue of 1,25 dihydroxyvitamin D with low calcemic activity is a potent inhibitor of parathyroid hormone-related peptide (PTHRP) production in vitro. The purpose of the present study was to determine whether EB1089 could reverse established hypercalcemia in BALB C nude mice implanted s.c. with a human epithelial cancer previously shown to produce high levels of PTHRP in vitro. Total plasma calcium was monitored before and after tumor development and increased steadily when the tumor reached > or =0.5 cm3. When total calcium was 22.85 mmol/liter, animals were treated with a constant infusion of EB1089 or vehicle alone for a period of 2 weeks. A significant and sustained reduction of plasma calcium from 3.2+/-0.1 to 2.7+/-0.08 (P < 0.01) mmol/liter was observed during infusion with EB1089. In contrast, calcium levels in vehicle-treated animals continued to rise during the infusion period. Tumor growth velocity also slowed significantly after the administration of EB1089 as compared with vehicle-treated animals. Plasma PTHRP levels measured at the end of the 2 weeks' infusion period were significantly lower in animals treated with EB1089 as compared with animals treated with vehicle alone (44+/-8 pg/ml versus 194+/-35 pg/ml, P < 0.001). These results, therefore, demonstrate that EB1089 can reverse established hypercalcemia in a human model of squamous cancer.

Mitogen-activated protein kinase inhibits 1,25-dihydroxyvitamin D3-dependent signal transduction by phosphorylating human retinoid X receptor alpha

Human retinoid X receptor alpha (hRXR alpha) is a member of the nuclear receptor family of transcriptional regulators. It regulates transcription through its association with several heterodimeric partners, including the vitamin D3 receptor (VDR). Signaling through the VDR is essential for normal calcium homeostasis and has been shown to inhibit the proliferation of cancer cells derived from a number of tissues. Here we show that phosphorylation of hRXR alpha in ras-transformed human keratinocytes through the activated Ras-Raf-mitogen-activated protein kinase (Ras-Raf-MAP kinase) pathway results in attenuated transactivation by the VDR and resistance to the growth inhibitory action of 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] and RXR-specific agonist LG1069 (4-[1-(5,6,7, 8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphthalenyl) ethenyl]-benzoic acid). Phosphorylation of hRXR alpha occurs at serine 260, a consensus MAP kinase site. Inhibition of MAP kinase activity or point mutagenesis of serine 260 of hRXR alpha reverses the observed resistance to 1,25(OH)2D3 and LG1069. Thus, hRXR alpha is a downstream target of MAP kinase, and its phosphorylation may play an important role in malignant transformation.

Disruption of vitamin D receptor-retinoid X receptor heterodimer formation following ras transformation of human keratinocytes

A partial resistance to the growth inhibitory influence of 1, 25-dihydroxyvitamin D3 is apparent when immortalized keratinocytes are transformed by the ras oncogene. The vitamin D receptor (VDR) was isolated, analyzed, and found to be identical in normal, immortalized, and ras-transformed keratinocytes. Subsequently, nuclear extracts from immortalized and ras-transformed keratinocytes were analyzed in gel mobility shift assays utilizing labeled vitamin D response elements or thyroid hormone response elements. A specific protein.DNA complex that was shown to contain VDR using an anti-VDR antibody was identified in both types of extracts; however, the addition of an anti-retinoid X receptor (RXR) antibody identified RXR in the complex of both normal and immortalized keratinocyte cell extracts, but not in ras-transformed keratinocytes. Furthermore, transfection of ras-transformed keratinocytes with wild-type human RXRalpha rescued VDR.RXR and thyroid hormone receptor.RXR complexes as demonstrated by a supershift in the presence of the anti-RXR antibody. Both cell lines were found to express RXRalpha message in equal amounts. Western blot analysis revealed that RXRalpha protein from ras-transformed keratinocytes was indistinguishable from that from immortalized keratinocytes and from control cells. These results suggest a causal relationship between resistance to the growth inhibitory influences of 1,25-dihydroxyvitamin D3 and disruption of the VDR.RXR complex in malignant keratinocytes.

Ruptured Salmonella mycotic aneurysm of the extracranial carotid artery

Mycotic aneurysms of the extracranial carotid artery are rare and difficult to diagnose and can lead to significant medical morbidity. Treatment of these lesions requires expert surgical management and necessitates an assiduous search for an underlying source. We report a case of a ruptured mycotic aneurysm of the cervical carotid artery due to Salmonella infection successfully treated by wide excision and saphenous vein patch angioplasty.

ras Activation of human prostate epithelial cells induces overexpression of parathyroid hormone-related peptide

Immortalized adult and fetal prostate cell lines grown in serum-free conditions produce low levels of parathyroid hormone-related peptide (PTHRP) in the presence of growth factors as assessed by mRNA analysis, PTHRP immunoreactivity, and immunohistochemistry. Subsequent infection of these cells with Kirsten murine sarcoma virus containing an activated Ki-ras oncogene induces at least a 10-20-fold increase in PTHRP expression and production of both adult and fetal immortalized cell lines in the presence of the same growth factors. These results provide the first evidence of direct activation of PTHRP by the ras oncogene in human prostate cells and suggest its potential usefulness as a tumor marker in prostate malignancies.

Expression of parathyroid hormone-related peptide (PTHrP) and PTH/PTHrP receptor in newborn human calvaria osteoblastic cells

We examined the expression of parathyroid hormone-related peptide (PTHrP) and its receptor in normal newborn human calvaria osteoblastic (NHCO) cells. Northern blot analysis showed that NHCO cells express a single 1.6 kb transcript of PTHrP, which was increased within 1 h (2x) and peaked at 6 h (7x) after serum treatment. In the culture media, the release of PTHrP peptide was maximally increased (4x) 24 h after the addition of serum, as determined by immunoradiometric assay. NHCO cells exhibited a cytoplasmic immunostaining for PTHrP in the presence of serum, and most PTHrP-positive cells were alkaline phosphatase-negative, suggesting that PTHrP was expressed in undifferentiated cells. Furthermore, RT-PCR analysis showed that both PTHrP and PTH/PTHrP receptor were expressed in NHCO cells in basal conditions or after stimulation with serum. The maximal PTHrP expression induced by serum suppressed PTH/PTHrP receptor expression, suggesting that PTHrP down-regulated its receptor in NHCO cells. Treatment with 10 nM human PTH(1-34) which binds to PTH/PTHrP receptors, increased intracellular cAMP levels and alkaline phosphatase activity, and decreased cell growth, indicating that ligand binding to PTH/PTHrP receptors regulates NHCO cell proliferation and differentiation. The expression and synthesis of PTHrP and the presence of functional PTH/PTHrP receptors suggest a possible paracrine mechanism of action of PTHrP in normal human calvaria osteoblastic cells.

Anti-proliferative activity and target cell catabolism of the vitamin D analog 1 alpha,24(S)-(OH)2D2 in normal and immortalized human epidermal cells

Vitamin D analogs represent valuable new agents for the suppression of proliferation of a variety of cell types, including those of the skin. One such analog is the vitamin D2 metabolite, 1 alpha,24(S)-dihydroxyvitamin D2, which binds strongly to the vitamin D receptor and induces vitamin D-dependent gene expression in vitro. In the work described here, we studied the anti-proliferative activity and target cell metabolism of 1 alpha,24(S)-dihydroxyvitamin D2 in cells of human epidermal origin. We found this analog to be equally potent in its anti-proliferative effect to the hormone 1 alpha,25-dihydroxyvitamin D3. Furthermore, 1 alpha,24(S)-dihydroxyvitamin D2 was metabolized by the human keratinocyte cell line HPK1A-ras at a slower rate than either 1 alpha,25-dihydroxyvitamin D3 or calcipotriol, a drug used effectively in the treatment of psoriasis. We characterized the metabolic products of 1 alpha,24(S)-dihydroxyvitamin D2 as a mixture of side-chain truncated and hydroxylated products. The main product was identified by GC-MS and NMR techniques as 1 alpha,24(S),26-trihydroxyvitamin D2. The biological activity of this main product was determined in a vitamin D-dependent, growth-hormone reporter gene expression system to be lower than that of the parent molecule. We conclude from these data that 1 alpha,24(S)-dihydroxyvitamin D2 is a valuable new anti-proliferative agent with a slower rate of catabolism by cells of epidermal origin. Preliminary evidence suggests that the parent molecule, and not its products, is responsible for this biological activity in vitro.

Identification and characterization of 1,25-dihydroxyvitamin D3-responsive repressor sequences in the rat parathyroid hormone-related peptide gene

Parathyroid hormone-related peptide (PTHRP) gene transcription is suppressed by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active metabolite of vitamin D3. In the present report, we examined 1, 25(OH)2D3-mediated repression of PTHRP expression by transfection of PTHRP promoter/reporter constructs in normal human keratinocytes and by DNA binding. We localized an element conferring 1, 25(OH)2D3-mediated repression in vivo to a 47-base pair (bp) region located -1121 to -1075 from the transcriptional start site. Mobility shift analysis revealed that this vitamin D response element (VDRE) forms DNA-protein complexes. The addition of a monoclonal antibody that recognizes the DNA binding region of the vitamin D receptor (VDR) attenuated binding of the receptor to the 47-bp sequence, whereas the addition of monoclonal antibody raised against the retinoid X receptor (RXR) further retarded the mobility of the protein-DNA complex. Consequently, the PTHRP promoter element binds a VDR.RXR heterodimer. Examination of this VDRE revealed complete sequence homology with a half-site of the human and rat osteocalcin VDRE (GGGTGA). Furthermore, mutation analysis suggests that a 16-bp domain consisting of an almost perfect repeat separated by a 3-base pair "spacer" GGGTGGAGAGGGGTGA is responsible for the DNA-protein interaction within this 47-bp sequence. Our results therefore indicate the existence of an inhibitory VDRE within the PTHRP promoter that is similar in sequence composition and cellular factor requirement to classical up-regulatory VDREs.

Expression and action of parathyroid hormone-related peptide in human cervical epithelial cells

Parathyroid hormone-related peptide (PTHRP) expression and activity were analyzed in normal human ectocervical keratinocytes (HCX) and keratinocytes immortalized by transfection with human papillomavirus (HPV) types 16 and 18 DNAs. In normal cells, trans-retinoic acid (RA) and 2.0 mM Ca2+ significantly stimulated PTHRP mRNA expression and secretion and led to a significant reduction in the rate of proliferation. In contrast, the basal level of PTHRP production decreased sharply in confluent HCX, and induction by Ca2+ or exogenous growth factors was reduced or lost. After stable transfection with HPV16 and HPV18 DNAs, we observed a sharp decrease of PTHRP production in high-passage poorly differentiated HCX. Finally, addition of exogenous PTHRP-(1-141) inhibited proliferation of both normal cells and low-passage well-differentiated HPV16 immortalized cells. High-passage poorly differentiated cells were refractory to PTHRP. These results demonstrate that PTHRP production varies greatly with the degree of cell proliferation and differentiation and suggest that this peptide acts as an autocrine negative growth regulator for cervical keratinocytes.

In vitro metabolism of the vitamin D analog, 22-oxacalcitriol, using cultured osteosarcoma, hepatoma, and keratinocyte cell lines

Using four cultured cell models representing liver, keratinocyte, and osteoblast, we have demonstrated that the vitamin D analog, 22-oxacalcitriol is degraded into a variety of hydroxylated and side chain truncated metabolites. Four of these metabolic products have been rigorously identified by high pressure liquid chromatography, diode array spectrophotometry, and gas chromatography-mass spectrometry analysis as 24-hydroxylated and 26-hydroxylated derivatives as well as the cleaved molecules, hexanor-1alpha,20-dihydroxyvitamin D3 and hexanor-20-oxo-1alpha-hydroxyvitamin D3. Comparison with chemically synthesized standards has revealed the stereochemistry of the biological products. Although differences exist in the amounts of products formed with the different cell types, it is apparent that 22-oxacalcitriol is subject to metabolism by both vitamin D-inducible and noninducible enzymes. Time course studies suggest that the truncated 20-alcohol is derived from a side chain hydroxylated molecule via a hemiacetal intermediate and the 20-oxo derivative is likely formed from the 20-alcohol. Biological activity measurements of the metabolites identified in our studies are consistent with the view that these are catabolites and that the biological activity of 22-oxacalcitriol is due to the parent compound. These results are also consistent with recent findings of others that the biliary excretory form of 22-oxacalcitriol is a glucuronide ester of the truncated 20-alcohol.

Parathyroid-hormone-related peptide in hematologic malignancies

PURPOSE

To determine whether parathyroid-hormone-related peptide (PTHRP) is an important pathogenetic mediator of hypercalcemia in patients with hematologic malignancies.

PATIENTS AND METHODS

We conducted a cohort analytic study in 76 consecutive patients with non-Hodgkin's lymphoma (NHL), Hodgkin's disease, multiple myeloma, and Waldenstrom's macroglobulinemia, 14 of which were hypercalcemic. Thirty normal subjects served as a control group.

RESULTS

Using the NH2 -terminal radioimmunoassay, PTHRP concentrations in heavy controls were undetectable (<7.5 pmol equivalents of PTHRP [fragment 1-34] per liter). The majority of hypercalcemic patients (8/14) had non-Hodgkin's lymphoma, and 62.5% of these (5/8) had significant elevations of circulating PTHRP concentrations (mean 70.5 +/- 38.5 pmol equivalents of PTHRP per liter) (P <0.01). In non-Hodgkin's lymphoma, 11 of 30 patients with advanced disease (stage IV) had elevated PTHRP concentrations, and of these, 8 of 11 had high-grade pathology. In contrast, only 3 or 21 patients with less advanced disease (stage I to III) had elevated PTHRP concentrations. In 4 NHL patients with less advance PTHRP concentrations sampled prior to cytotoxic chemotherapy, tumor response was associated with a decrease in PTHRP. Concomitant suppression of 1,25(OH)2D3 concentrations was observed in 66% of hypercalcemic patients with non-Hodgkin's lymphoma.

CONCLUSIONS

These results suggest that PTHRP may be an important pathogenetic factor in the development of hypercalcemia in hematologic malignancies, notably in non-Hodgkin's lymphoma.

Surgical treatment for acute native aortic valvular infective endocarditis: long-term follow-up

During the period 1970-1993, 116 patients (63 men, 53 women) with native aortic valvular infective endocarditis were treated surgically. The mean age was 37 years. The main causative organisms were streptococci and staphylococci. Indication for surgery was cardiac failure (70 cases), uncontrolled sepsis (30), peripheral emboli (11) and overwhelming destruction of the aortic valve (five). Hospital and late mortality rates were 8% and 11% respectively. Patients who died in hospital and those who presented a paravalvular leakage had a ring abscess associated with aortic wall destruction. Among 34 patients screened for cerebral septic emboli the condition was confirmed in 15, of whom six were symptom-free. Thus, it is believed that in the presence of root abscess, surgery should be undertaken promptly, regardless of the cardiac status. It is confirmed that cerebral septic emboli should be systematically screened for in the presence of any infective endocarditis.

[Recommendations for prevention of thromboembolism in heart valve diseases. Working Group on Valvular Heart Disease, European Society of Cardiology]

Thromboembolic events are still a major cause of morbidity and mortality in patients with native valvular heart disease and in patients with prosthetic heart valves. Although the introduction of oral anticoagulation reduced this risk, thromboembolism and anticoagulation-related hemorrhages still represent significant problems in the management of these patients. In this article the guidelines developed by the Working Group on Valvular Heart Disease of the European Society of Cardiology for the management of antithrombotic therapy in heart valve disease are thoroughly discussed. The indication for and intensity of anticoagulation in various clinical situations, the concept of risk factor-adjusted intensity of anticoagulation, and the concept of control of oral anticoagulation with the International Normalized Ratio are presented.

Different mechanisms of hydroxylation site selection by liver and kidney cytochrome P450 species (CYP27 and CYP24) involved in vitamin D metabolism

A series of homologated 1 alpha-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 molecules with one to three extra carbons in the side chain were used to examine the substrate preferences and hydroxylation site selection mechanisms of the liver vitamin D3-25-hydroxylase (CYP27) and the target cell 25-hydroxyvitamin D3-24-hydroxylase (CYP24). Cultured and transfected cell models, used as sources of these hydroxylases, gave 23-, 24-, 25-, and 27-hydroxylated metabolites which were identified by their high performance liquid chromatography and GC-MS characteristics. Lengthening the side chain is tolerated by each cytochrome P450 isoform such that 25-hydroxylation or 24-hydroxylation continues to occur at the same rate as in the native side chain, while the site of hydroxylation remains the same for the liver enzyme in that CYP27 continues to hydroxylate at C-25 and C-27 (minor) despite the two-carbon-atom extension. Somewhat surprising is the finding that C-24 and C-23 (minor) hydroxylations also do not change as the side chain is extended by as much as three carbons. We conclude that CYP24 must be directed to its hydroxylation site(s) by the distance of carbon 24 from the vitamin D ring structure and not as in CYP27 by the distance of the hydroxylation site from the end of the side chain.

Extension of native aortic valve endocarditis: surgical considerations

Among 101 consecutive patients operated on for native infective aortic valve endocarditis (53 males, 48 females, mean age 39 years), 69 presented various forms of infectious extension to the surrounding areas. Twenty-six lesions were noted in the aortic roots: 18 annular abscesses, one abscess of the Valsalva sinus and seven aortic wall destructions. Among the subaortic valve pathology, 27 cases of septal lesions were noted and in one case the mitral fibrous trigone was involved. The mitral apparatus was infected in 26 cases, the tricuspid valvule in one case. Both tricuspid and mitral valvular replacements had to be performed in five cases. Among the 16 postoperative atrioventricular blocks, 14 needed a pacemaker. The most frequent causative microorganisms were Staphylococcus aureus and Streptococcus. Surgical management of the lesions consisted of extensive debridement followed by either simple repair of defects or complex reconstructions involving pericardial or synthetic patches or other more complex operations. Early and late mortality rates were 8.5% and 16%; early and late reoperation rates were 6% and 9.5%, respectively. The mean follow-up time was 148 months (12-265 months) with a survival rate of 74% (SE: +/- 0.08) at 10 years. We conclude that, although surgical correction of infective endocarditis may need a complex approach, it provides good results with an acceptable surgical risk.

Vitamin D analogs: new therapeutic agents for the treatment of squamous cancer and its associated hypercalcemia

We have examined the in vitro effects of 1,25 dihydroxy-vitamin D3 [1,25(OH)2D3] and of two side-chain modified analogs of 1,25(OH)2D3 (EB1089 and MC903) on cell growth and parathyroid hormone related peptide (PTHRP) production in immortalized (HPK1A) and neoplastic (HPK1A-ras) keratinocytes. Cell proliferation was strongly inhibited by 1,25(OH)2D3 and its analogs in HPK1A cells, and in this system EB1089 was 10-100 times more potent than 1,25(OH)2D3 or MC903. A similar effect on cell proliferation was observed in HPK1A-ras cells; however, 10-fold higher concentrations of 1,25(OH)2D3 or its analogs were required. We also observed a strong and dose-dependent inhibitory effect of these compounds on PTHRP secretion and gene expression. In both immortalized and neoplastic keratinocytes, EB1089 was 10-100 times more potent than 1,25(OH)2D3 or MC903 on inhibiting PTHRP production. However, although effective in HPK1A-ras cells, 10-fold higher concentrations of 1,25(OH)2D3 or its analogs were required to produce similar actions in this neoplastic model. These studies therefore demonstrate that a 1,25(OH)2D3 analog with low calcemic potency in vivo (EB1089) can inhibit keratinocyte proliferation and PTHRP production by such cells with greater potency than 1,25(OH)2D3. The observed effects of such analogs in neoplastic keratinocytes predicts their potential usefulness in vivo in inhibiting squamous cancer growth and its associated hypercalcemia.

Regulation of parathyroid hormone-related peptide production in normal human mammary epithelial cells in vitro

We have examined the expression and production of parathyroid hormone-related peptide (PTHRP) in primary cultures of normal human mammary epithelial cells (HMEC) derived from nonlactating breast tissue. In response to growth factors such as insulin, insulin-like growth factor I (IGF-I), and epidermal growth factor (EGF), immunoreactive PTHRP was released into conditioned medium, and PTHRP mRNA rapidly increased. In contrast, hydrocortisone and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] inhibited these effects in a dose-dependent manner. Addition of prolactin (PRL) in the presence or absence of insulin, IGF-I, or EGF did not influence PTHRP production during the time course studied. To investigate whether these factors were acting at the transcriptional level, we performed nuclear run-on assays and demonstrated that IGF-I increased PTHRP gene transcription whereas hydrocortisone and 1,25(OH)2D3 inhibited this effect. These studies therefore demonstrate that IGF-I, EGF, 1,25(OH)2D3, and hydrocortisone modulate PTHRP expression in HMEC and that these effects occur in part at the level of gene transcription. Additionally, PRL, a known stimulator of PTHRP expression in vivo, has no effect in this in vitro model.

Effect of 1,25 dihydroxyvitamin D3 and calcium on growth and differentiation and on c-fos and p53 gene expression in normal human keratinocytes

Calcium enhances keratinocyte differentiation, and 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) is both antiproliferative and prodifferentiative in many cell types, including normal human keratinocytes. In the present study, we examined the combined effects of calcium and 1,25(OH)2D3 on parameters of growth and differentiation and on c-fos and p53 gene expression in normal human keratinocytes. Exposure of normal human keratinocytes to 1,25(OH)2D3 markedly reduced [3H] thymidine incorporation and cell number at low and high medium Ca++ concentrations. Simultaneously, cells in the G0/G1 phase of the cell cycle increased significantly and those in the S phase fell precipitously. 1,25(OH)2D3 and calcium also induced keratinocyte differentiation independently, as assessed by immunocytochemistry and by induction of involucrin mRNA. Both Ca++ and 1,25(OH)2D3 were shown, by nuclear run-on assays, to increase involucrin gene transcription. A rapid, transient elevation in c-fos protooncogene expression preceded these effects when epidermal growth factor was present alone. When 1,25(OH)2D3 was added to quiescent keratinocytes, there was a marked augmentation of c-fos mRNA accumulation at low and high medium Ca++ concentrations. Varying medium Ca++ concentrations had no effect on c-fos mRNA levels. Increasing medium Ca++ concentrations from 0.15 to 2.0 mM produced marked elevations of p53 mRNA accumulation and of the rate of p53 gene transcription, whereas 1,25(OH)2D3 had no effect. These results, therefore, suggest that 1,25(OH)2D3 and calcium act in concert to modulate the expression of two important cell-cycle-associated genes, which may be important components in the initial programming of growth and differentiation of normal human keratinocytes.

In vitro metabolism of the anti-psoriatic vitamin D analog, calcipotriol, in two cultured human keratinocyte models

Calcipotriol (MC903) is a side chain analog of the vitamin D hormone calcitriol, containing a 22-23 double bond, a 24(S)-hydroxyl function, and carbons 25, 26, and 27 incorporated into a cyclopropane ring which has been developed for treating psoriasis. The in vitro metabolism of calcipotriol was studied in two keratinocyte cell models, HPK1A and HPK1A-ras. Calcipotriol was initially converted into the 24-ketone (MC1046) and its 22,23-hydrogenated derivative (MC1080), metabolites observed in osteosarcoma, kidney, and hepatoma cell lines. We also observed the formation of further metabolites, identified as the two 23-hydroxylated derivatives of MC1080 (MC1439 and MC1441), the two 23,24-dihydroxylated compounds (MC1575 and 1577), and the side chain-cleaved compounds, tetranor-1,23-(OH)2D3 and calcitroic acid, the end products of catabolism of calcitriol. These findings suggest that calcitriol and calcipotriol may share catabolic enzymes. The biological activity of each of the principal metabolites of calcipotriol, assessed using a growth hormone reporter gene transcriptional activation system and a vitamin D receptor assay, was found to be lower than that of calcipotriol. If the extensive in vitro metabolism of calcipotriol is also found in normal and psoriatic keratinocytes in vivo, then this may explain the lack of systemic calcemic activity of topically applied drug.

Antisense-mediated inhibition of parathyroid hormone-related peptide production in a keratinocyte cell line impedes differentiation

We have used antisense RNA technology to inhibit endogenous PTH-related peptide (PTHRP) production in an established human keratinocyte cell line, HPK1A, to assess the role of PTHRP as a modulator of cell differentiation. Initially to determine the specificity of any alterations in cell function that might be observed, HPK1A cells and Rat-2 fibroblasts (which do not synthesize PTHRP) were both infected with the same retrovirus (pYN) containing antisense PTHRP. In contrast to antisense-infected HPK1A cells (HPK1A-AS), which show accelerated growth indices when endogenous PTHRP production is blocked, antisense-infected Rat-2 cells (Rat-2-AS) displayed no increase in cell proliferation. Consequently, this alteration in HPK1A cell function appeared to be specific to the inhibition of PTHRP production. In HPK1A-AS cells, no PTHRP transcript was observed in cytoplasmic RNA, and none was sequestered in a nuclear RNA preparation. Therefore, hybridization with the antisense strand appears to destabilize PTHRP mRNA, leading to rapid disappearance of the sense-antisense heteroduplex. We then examined the effect of PTHRP inhibition on keratinocyte differentiation using three indices. PTHRP inhibition in HPK1A-AS cells resulted in reduced high mol wt keratin production, as assessed by immunocytochemistry. Expression of mRNA encoding transglutaminase and involucrin was decreased in HPK1A-AS cells compared to that in control cells under conditions of high ambient calcium. Involucrin protein levels were also diminished in HPK1A-AS cells in parallel with the reduced levels of involucrin gene expression. These data, therefore, show that interference with PTHRP production inhibits expression of maturation-specific keratinocyte indices and indicate that endogenous PTHRP acts to enhance differentiation in this keratinocyte model.