Parathyroid hormone-related
protein (PTHrP)

Parathyroid hormone-like hormone (PTHLH)
Humoral hypercalcemia of malignancy (HHM)

Gene: maps to 12p12.2-p11.2 (Mangin M. et al., 1988). It is believed that chromosome 11, containing the PTH gene, and chromosome 12 are evolutionarily related. In addition, the human PTHrP gene has a similar intron-exon organization as the PTH gene. The PTHrP gene comprises eight exons spanning more than 15 kilobases of genomic DNA. The gene is composed of nine exons of which only two are invariant in PTHrP transcripts. The other seven exons may be represented in the PTHrP mRNA complement as a result of alternative splicing, which allows for the production of up to 15 transcripts. Three spatially-distinct promoters, two TATA and one GC-rich region, are responsible for transcription of the gene and these appear to be differentially regulated. The PTHrP gene contains nucleotide sequence motifs in common with members of the immediate-early response gene family, as well as other hallmark features which include induction by growth factors, serum or cycloheximide and relatively short-lived mRNA (Gillespie M.T. and Martin T.J., 1994).
mRNA: size: ? kb.
Protein: 3 peptide isoforms of 139, 141, and 173 amino acids (aa) have been described. Eight of the first 13 aa are identical with human PTH, although antisera directed to the NH₂ terminus of PTHrP do not recognize PTH; this homology is not maintained in the remainder of the molecule. PTHrP is possibly related to the PTH gene by a gene duplication mechanism (Martin T.J. et al., 1989). The importance of the role of PTHrP in normal physiology is underscored by its broad tissue expression, by its intense evolutionary conservation, by its extremely early expression after fertilization of the ovum, and by the lethal consequences of PTHrP gene disruption. The complexity of the role of PTHrP in normal physiology is reflected in the broad tissue distribution of the peptide, its complex transcriptional regulation and mRNA instability motifs, and its multiple transcripts and isoforms. It is now clear that additional complexity exists at the level of posttranslational processing. Expression of the PTHrP gene leads to the tissue-specific processing and secretion of an increasingly complex family of derivative peptides, each with its own repertoire of cognate receptors, signal transduction pathways, and physiological consequences (Orloff J.J. et al., 1994).

Cell lines:
- By RT-PCR, PTHrP mRNA was found in MCF-7, ZR-75-1, T-47-D and Hs578T breast cancer cell (BCC) lines, but not in SK-BR-3 and MDA-MB231 BCC lines (Birch M.A. et al., 1995).

- 8701-BC BCC, derived from a primary carcinoma of the breast, constitutively express PTHrP gene and N-terminal PTHrP immunoreactivity can be found in cell medium. PTHrP [1-34], and, to a minor extent, [67-86] and [107-139], were found to be anti-mitogenic but 'invadogenic' for 8701-BC cells, suggesting that diverse enzymatic activities may contribute to cell invasion in response to different PTHrP fragments (Luparello C. et al., 1995).

- In KPL-3C BCC, PTHrP was found to be inhibited by oestradiol and stimulated by antioestrogens (Kurebayashi J. et al., 1997).

- Expression of a dominant-negative mutant (TbetaRIIDeltacyt) of the TGF-beta type II receptor rendered the BCC line MDA-MB-231 unresponsive to TGF-beta. In a murine model of bone metastases, expression of TbetaRIIDeltacyt by MDA-MB-231 resulted in less bone destruction, less tumor with fewer associated osteoclasts, and prolonged survival compared with controls. Reversal of the dominant-negative signaling blockade by expression of a constitutively active TGF-beta type I receptor in the breast cancer cells increased tumor production of parathyroid hormone-related protein (PTHrP), enhanced osteolytic bone metastasis, and decreased survival. Transfection of MDA-MB-231 cells that expressed the dominant-negative TbetaRIIDeltacyt with the cDNA for PTHrP resulted in constitutive tumor PTHrP production and accelerated bone metastases. These data suggest an important role for TGF-beta in the development of breast cancer metastasis to bone, via the TGF-beta receptor-mediated signaling pathway in tumor cells, and also suggest that the bone destruction is mediated by PTHrP (Yin J.J. et al., 1999).

Tumors:
- In a series of 102 consecutive invasive breast tumors removed surgically from normocalcemic women, positive PTHrP staining was detected in 60% of the tumors but not in the accompanying normal breast tissue. Positive staining was related to the progesterone receptor status of the tumor and to the prognostic index of the patient and not to estrogen receptor status, patient age, tumor size, histological grade, or nodal status (Southby J. et al., 1990).

- PTHrP was localized by immunohistology in 12 of 13 (92%) breast cancer metastases in bone and in 3 of 18 (17%) metastases in non-bone sites. The statistical difference was highly significant (Powell G.J. et al., 1991).

- Elevated PTHrP levels were found in both humoral hypercalcemia of malignancy and hypercalcemia complicating metastatic breast cancer (Grill V. et al., 1991).

- PTHrP mRNA was studied in paraffin sections of 17 primary breast tumors and 26 metastatic lesions, 11 of which were in bone. 10 of the 17 (59%) primary lesions, 8 of 11 (73%) breast cancer metastases to bone, and 3 of 15 (20%) metastases to non-bone sites showed specific localization of PTHrP mRNA (Vargas S.J. et al., 1992).

- The presence of PTHrP mRNA was assessed by using the polymerase chain reaction in 38 normocalcemic breast cancer patients with long-term follow-up (minimum, 5 years) selected for the presence or absence of later bone metastasis development. In all the patients except one, the PTHrP gene was expressed in the breast tumor. The level of amplified PTHrP complementary DNA was inversely related to age and positively related to the proportion of invaded nodes but was not related to the other usual prognostic factors. The level of PTHrP mRNA was not different between the group of patients without recurrence or metastases (n = 11) and the group of patients who later developed metastases in soft tissues (n = 10). By contrast, patients who subsequently developed bone metastases (n = 17) showed higher PTHrP gene expression than patients in the other two groups (Bouizar Z. et al., 1993).

- It has been concluded conclude that in hypercalcemia of malignancy raised serum levels of PTHrP indicate a reduced hypocalcemic response to bisphosphonates, a more advanced tumor state and, therefore, an extremely poor prognosis (Pecherstorfer M. et al., 1994).

- The immunohistochemical localization of PTHrP was studied in sections of formalin-fixed, paraffin-embedded tissues from 28 breast cancers obtained surgically. Of the 28 patients, 12 developed skeletal metastases, 8 developed lung metastases, and the other 8 were alive and disease-free at the time of this study. Sixteen of the 28 (57%) tumors showed positive PTHrP immunoreactivity, the positive ratio being 83% in the patients who developed skeletal metastases, 38% in those who developed lung metastases, and 38% in those without recurrence, respectively (Kohno N. et al., 1994).

- By RT-PCR, all but 1 of 18 breast tumour samples studied expressed PTHrP, whereas receptor expression was detected in 11 of these. Expression of the PTH/PTHrP receptor was found in three out of four metastatic lesions, including one sample in which no receptor was detected in the primary tumour (Carron J.A. et al., 1997).

- The sensitive detection by RT-PCR of PTHrP-positive disseminated breast tumor cells in the bone marrow was found to lack of specificity, possibly because of autochthonous expression of PTHrP in osteoblastic cells (Wulf G.G. et al., 1997).

Birch M.A. et al. (1995) Parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor expression and mitogenic responses in human breast cancer cell lines. Br. J. Cancer 72, 90-95.
Bouizar Z. et al. (1993) Polymerase chain reaction analysis of parathyroid hormone-related protein gene expression in breast cancer patients and occurrence of bone metastases. Cancer Res. 53, 5076-5078.
Carron J.A. et al. (1997) PTHrP and the PTH/PTHrP receptor are co-expressed in human breast and colon tumours. Br. J. Cancer 76, 1095-1098.
Gillespie M.T. and Martin T.J. (1994) The parathyroid hormone-related protein gene and its expression. Mol. Cell. Endocrinol. 100, 143-147.
Grill V. et al. (1991) Parathyroid hormone-related protein: elevated levels in both humoral hypercalcemia of malignancy and hypercalcemia complicating metastatic breast cancer. J. Clin. Endocrinol. Metab. 73, 1309-1315.
Guise T.A. (1997) Parathyroid hormone-related protein and bone metastases. Cancer (suppl.) 80, 1572-1580.
Kohno N. et al. (1994) The expression of parathyroid hormone-related protein in human breast cancer with skeletal metastases. Surg. Today 24, 215-220.
Kurebayashi J. et al. (1997) Parathyroid hormone-related protein secretion is inhibited by oestradiol and stimulated by antioestrogens in KPL-3C human breast cancer cells. Br. J. Cancer 75, 1819-1825.
Luparello C. et al. (1995) Parathyroid hormone-related peptide and 8701-BC breast cancer cell growth and invasion in vitro: evidence for growth-inhibiting and invasion-promoting effects. Mol. Cell. Endocrinol. 111, 225-232.
Mangin M. et al. (1988) Identification of a cDNA encoding a parathyroid hormone-like peptide from a human tumor associated with humoral hypercalcemia of malignancy. Proc. Nat. Acad. Sci. 85, 597-601.
Martin T.J. et al. (1989) Parathyroid hormone-related protein: isolation, molecular cloning, and mechanism of action. Recent Prog. Horm. Res. 45, 467-502.
Orloff J.J. et al. (1994) Parathyroid hormone-related protein as a prohormone: posttranslational processing and receptor interactions. Endocr. Rev. 15, 40-60.
Pecherstorfer M. et al. (1994) Parathyroid hormone-related protein and life expectancy in hypercalcemic cancer patients. J. Clin. Endocrinol. Metab. 78, 1268-1270.
Powell G.J. et al. (1991) Localization of parathyroid hormone-related protein in breast cancer metastases: increased incidence in bone compared with other sites. Cancer Res. 51, 3059-3061.
Southby J. et al. (1990) Immunohistochemical localization of parathyroid hormone-related protein in human breast cancer. Cancer Res. 50, 7710-7716.
Vargas S.J. et al. (1992) Localization of parathyroid hormone-related protein mRNA expression in breast cancer and metastatic lesions by in situ hybridization. J. Bone Miner. Res. 7, 971-979.
Wulf G.G. et al. (1997) Reverse transcriptase/polymerase chain reaction analysis of parathyroid hormone-related protein for the detection of tumor cell dissemination in the peripheral blood and bone marrow of patients with breast cancer. J. Cancer Res. Clin. Oncol. 123, 514-521.
Yin J.J. et al. (1999) TGF-beta signaling blockade inhibits PTHrP secretion by breast cancer cells and bone metastases development. J. Clin. Invest. 103, 197-206.