Expression and secretion of parathyroid hormone-related protein by a human cancer cell line

Two human osteoblast-like osteosarcoma cell lines show distinct expression and differential regulation of parathyroid hormone-related protein

Parathyroid hormone (PTH)-related protein (PTHrP) acts as a local regulator of osteoblast function via mechanisms that involve PTH/PTHrP receptors linked to protein kinase A (PKA) and C (PKC). However, the regulation of PTHrP production and mRNA expression in human osteoblasts is poorly understood. Here we have characterized alternative PTHrP mRNA 3' splicing variants, encoding PTHrP isoforms of 139, 141, and 173 amino acids, and studied the regulation of PTHrP and its mRNAs by activated PKA and PKC in two human osteoblast-like cell lines (KPDXM and TPXM). Using exon-specific Northern analysis and reverse transcriptase-coupled polymerase chain reaction, we identified mRNAs encoding PTHrP(1-139) and PTHrP(1-141) in both cell lines. PTHrP(1-139) mRNAs predominated in TPXM cells and PTHrP(1-173) mRNAs were only detected in TPXM cells. Activation of PKA or PKC resulted in different effects on PTHrP and its mRNAs in the two cell lines. In TPXM cells, peptide-specific immunoassays detected high basal levels of PTHrP, increasing by 2-fold in cell extracts and 4-fold in culture media at 7 h and 24 h after exposure to forskolin, respectively, paralleling changes in PTHrP mRNA expression. Phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA), a PKC activator, had no effect. In KPDXM cells, PTHrP was not detected in culture media under basal experimental conditions, and barely detectable amounts were present in cell extracts of TPA-treated cells, although the mRNA levels increased substantially in response to TPA. In the responsive cell lines, the effects on mRNA levels were dose dependent, and increased by 6.9- to 10.5-fold and 2.0- to 4.1-fold at 4 h in TPXM and KPDXM cells after exposure to 10 microM forskolin and 150 nM TPA, respectively. PTHrP mRNA levels then declined but were sustained above controls also at 12 h in both cell lines, albeit at considerably higher levels in TPXM cells. The different responsiveness to agents activating PKA- and PKC-dependent pathways may depend on the cellular state of differentiation, or alternatively, cancer cell line-specific defects. Our data demonstrating distinct differences in mRNA species and the amounts of PTHrP produced by the two cell lines as compared with roughly equivalent overall mRNA levels may suggest that post-transcriptional mechanisms play an important role in limiting the production of intracellular and secreted PTHrPs in human osteoblastic cells.

Regulation of parathyroid hormone-related protein secretion and mRNA expression in normal human keratinocytes and a squamous carcinoma cell line

Parathyroid hormone-related protein (PTHrP) has been identified as a causative factor in the pathogenesis of humoral hypercalcemia of malignancy (HHM). The regulation and mechanisms of PTHrP secretion in most normal and malignant cells are unknown. PTHrP secretion, mRNA expression, and transcription were measured in neoplastic human squamous carcinoma cells (A253) and normal human foreskin keratinocytes (NHFK) by radioimmunoassay, RNase protection assay, and transient transfections of the 5'-flanking region of human PTHrP in a luciferase expression vector. Mechanisms of PTHrP secretion were investigated using chemicals (monensin, colchicine, cytochalasin B, guanosine 5'-[gamma-thio]triphosphate (GTPgammaS)) that interfere with or facilitate intracellular transport. Monensin inhibited PTHrP secretion in both NHFK and A253 cells. Ultrastructurally, monensin caused dilatation of rough endoplasmic reticulum and the formation of numerous cytoplasmic secretory vacuoles in both cell lines. Colchicine decreased PTHrP production in NHFK cells and stimulated PTHrP production and mRNA levels in A253 cells. Colchicine also stimulated transcription of the PTHrP-luciferase reporter gene. Cytochalasin B stimulated PTHrP secretion and mRNA expression in A253 cells, but had no effect in NHFK cells. GTPgammaS had no effect on PTHrP secretion in either cell line. It was concluded that PTHrP secretion is dependent on the constitutive movement of secretory vesicles to the cytoplasmic membrane and regulation of PTHrP secretion and mRNA expression are altered in squamous carcinoma cells compared to normal human keratinocytes in vitro.

Parathyroid hormone-related protein secretion is inhibited by oestradiol and stimulated by antioestrogens in KPL-3C human breast cancer cells

We recently established a human breast cancer cell line, KPL-3C, from a breast cancer patient with humoral hypercalcaemia. This cell line possesses oestrogen receptor (ER) and secretes parathyroid hormone-related protein (PTHrP) into medium. To investigate the effects of oestrogen and antioestrogens on PTHrP secretion, KPL-3C cells were cultured for 48 h in an oestrogen-eliminated medium with 17beta-oestradiol (E2), tamoxifen (TAM) and/or a pure antioestrogen, ICI182,780 (ICI), and PTHrP secretion was measured using an immunoradiometric assay. The effects of these agents on cell cycle progression were also studied using flow cytometry. E2 (1-100 nM) significantly inhibited PTHrP secretion, whereas both TAM (0.1-10 microM) and ICI (1-100 nM) significantly stimulated it. These effects were completely blocked by the simultaneous addition of 1 nM E2 to the medium. At the same time, E2 significantly increased the percentage of cells during the S and G2/M phases, whereas both antioestrogens significantly increased the percentage of cells during the G0/G1 phase. Again, these cytostatic effects were completely reversed by the addition of E2. These findings indicate that antioestrogens inhibit the growth of ER-positive breast cancer cells but may stimulate PTHrP secretion and that these effects may be mediated by ER.

Distribution and functions of parathyroid hormone-related protein in vertebrate cells

Parathyroid hormone-related protein (PTHrP) was isolated from tumors and identified as the agent of humoral hypercalcemia of malignancy (HHM) in 1987. Since then its gene structure in several mammalian and an avian species has been analyzed and its gene expression demonstrated in many adult and embryonic tissues derived from all three germ layers. The composition and structure of PTHrP peptide depends on both differential gene splicing and posttranslational processing, which result in a range of peptides of potentially diverse functions. This chapter describes the distribution of PTHrP in both normal and neoplastic adult and embryonic tissues. PTHrP is of fundamental importance to cell survival because the absence of the gene is fatal; this aspect of PTHrP function in cell physiology becomes overwhelmingly important in neoplasia. Intracrine or paracrine actions for PTHrP seem to be most likely in mammalian and avian physiology, but in fishes high circulating levels suggest classic endocrine functions as well. Much remains to be learned of the biology of this fascinating protein.