Parathyroid Hormone-Related Protein

In 1941, Fuller Albright proposed that nonparathyroid malignant lesions might produce a humoral factor, presumably parathyroid hormone (PTH), that could account for hypercalcemia in patients with cancer who have little or no evidence of bone metastatic lesions.1Case records of the Massachusetts General Hospital (Case 27461).N Engl J Med. 1941; 225: 789-791Crossref Google Scholar From the mid-1960s to the present, radioimmunoassays for measurement of PTH were applied to this problem, but confirmation of the concept of “ectopic hyperparathyroidism” proved elusive.2Riggs BL Arnaud CD Reynolds JC Smith LH Immunologic differentiation of primary hyperparathyroidism from hyperparathyroidism due to nonparathyroid cancer.J Clin Invest. 1971; 50: 2079-2083Crossref PubMed Scopus (113) Google Scholar, 3Powell D Singer FR Murray TM Minkin C Potts Jr, JT Nonparathyroid humoral hypercalcemia with neoplastic diseases.N Engl J Med. 1973; 289: 176-181Crossref PubMed Scopus (214) Google Scholar, 4Endres DB Villanueva R Sharp Jr, CF Singer FR Measurement of parathyroid hormone.Endocrinol Metab Clin North Am. September 1989; 18: 611-629PubMed Google Scholar Certain PTH assays provided data that supported detectable but seldom increased serum PTH levels in many cancer patients with hypercalcemia, an indication that this factor might account for the hypercalcemic state.2Riggs BL Arnaud CD Reynolds JC Smith LH Immunologic differentiation of primary hyperparathyroidism from hyperparathyroidism due to nonparathyroid cancer.J Clin Invest. 1971; 50: 2079-2083Crossref PubMed Scopus (113) Google Scholar Other assays indicated that the hormone levels were suppressed in most cancer patients with hypercalcemia.3Powell D Singer FR Murray TM Minkin C Potts Jr, JT Nonparathyroid humoral hypercalcemia with neoplastic diseases.N Engl J Med. 1973; 289: 176-181Crossref PubMed Scopus (214) Google Scholar, 4Endres DB Villanueva R Sharp Jr, CF Singer FR Measurement of parathyroid hormone.Endocrinol Metab Clin North Am. September 1989; 18: 611-629PubMed Google Scholar These apparently conflicting data can now be explained by the fact that numerous forms of immunoreactive PTH exist in the circulation and that the antisera used to measure PTH have a variety of antigenic specificities. If assays directed against the mid or carboxy-terminal region of the molecule are used, serum immunoreactive PTH levels are generally measurable but represent fragments of PTH that lack the biologically active amino-terminal component of the molecule. Amino-terminal assays generally detected no immunoreactive PTH in the circulation unless the patients with hypercalcemia had associated primary hyperparathyroidism. The development, more recently, of extremely sensitive two-site assays for measuring intact PTH has unquestionably demonstrated that most cancer patients with hypercalcemia have suppressed or unmeasurable PTH levels.4Endres DB Villanueva R Sharp Jr, CF Singer FR Measurement of parathyroid hormone.Endocrinol Metab Clin North Am. September 1989; 18: 611-629PubMed Google Scholar In 1987, Suva and colleagues5Suva LJ Winslow GA Wettenhall REH Hammonds RG Moseley JM Diefenbach-Jagger H Rodda CP Kemp BE Rodriguez H Chen EY Hudson PJ Martin TJ Wood WI A parathyroid hormone-related protein implicated in malignant hypercalcemia: cloning and expression.Science. 1987; 237: 893-896Crossref PubMed Scopus (1156) Google Scholar reported the structure of a 141-amino acid PTH-related protein (PTHrP) cloned from a cancer cell line. This protein was homologous with PTH in 8 of the first 13 amino acids but exhibited almost no homology with PTH in the rest of the molecule. In vitro and in vivo studies with synthetic PTHrP amino-terminal fragments demonstrated that these protein fragments had most of the biologic effects of PTH6Horiuchi N Caulfield MP Fisher JE Goldman ME McKee RL Reagan JE Levy JJ Nutt RF Rodan SB Schofield TL Clemens TL Rosenblatt M Similarity of synthetic peptide from human tumor to parathyroid hormone in vivo and in vitro.Science. 1987; 238: 1566-1568Crossref PubMed Scopus (306) Google Scholar (including increased urinary excretion of cyclic adenosine monophosphate) and bound to the PTH receptor.7Jüppner H Abou-Samra A-B Uneno S Gu W-X Potts Jr, JT Segre GV The parathyroid hormone-like peptide associated with humoral hypercalcemia of malignancy and parathyroid hormone bind to the same receptor on the plasma membrane of ROS 17/2.8 cells.J Biol Chem. 1988; 263: 8557-8560Abstract Full Text PDF PubMed Google Scholar Two additional species of PTHrP with a unique carboxy-terminal region have been discovered; they arise from alternate splicing of the PTHrP gene and are 139 and 173 amino acids in length, respectively.8Mangin M Ikeda K Dreyer BE Broadus AE Isolation and characterization of the human parathyroid hormone-like peptide gene.Proc Natl Acad Sci USA. 1989; 86: 2408-2412Crossref PubMed Scopus (172) Google Scholar Three laboratories have previously reported clinical studies that have used specific radioimmunoassays for PTHrP,9Budayr AA Nissenson RA Klein RF Pun KK Clark OH Diep D Arnaud CD Strewler GJ Increased serum levels of a parathyroid hormone-like protein in malignancy-associated hypercalcemia.Ann Intern Med. 1989; 111: 807-812Crossref PubMed Scopus (197) Google Scholar, 10Henderson JE Shustik C Kremer R Rabbani SA Hendy GN Goltzman D Circulating concentrations of parathyroid hormone-like peptide in malignancy and in hyperparathyroidism.J Bone Miner Res. 1990; 5: 105-113Crossref PubMed Scopus (132) Google Scholar, 11Burtis WJ Brady TG Orloff JJ Ersbak JB Warrell Jr, RP Olson BR Wu TL Mitnick ME Broadus AE Stewart AF Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer.N Engl J Med. 1990; 322: 1106-1112Crossref PubMed Scopus (496) Google Scholar and in this issue of the Proceedings (pages 1399 to 1407), Kao and colleagues describe their findings with use of an amino-terminal antiserum to PTHrP to measure the protein in plasma extracts from normal subjects and patients with cancer and primary or secondary hyperparathyroidism. The early results obtained with these assays have been interesting but somewhat difficult to interpret. Application of these assays to the measurement of immunoreactive PTHrP in the circulation of normal subjects has been disappointing. Detectable PTHrP levels have been found in 32%,9Budayr AA Nissenson RA Klein RF Pun KK Clark OH Diep D Arnaud CD Strewler GJ Increased serum levels of a parathyroid hormone-like protein in malignancy-associated hypercalcemia.Ann Intern Med. 1989; 111: 807-812Crossref PubMed Scopus (197) Google Scholar 5%,10Henderson JE Shustik C Kremer R Rabbani SA Hendy GN Goltzman D Circulating concentrations of parathyroid hormone-like peptide in malignancy and in hyperparathyroidism.J Bone Miner Res. 1990; 5: 105-113Crossref PubMed Scopus (132) Google Scholar 47%,11Burtis WJ Brady TG Orloff JJ Ersbak JB Warrell Jr, RP Olson BR Wu TL Mitnick ME Broadus AE Stewart AF Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer.N Engl J Med. 1990; 322: 1106-1112Crossref PubMed Scopus (496) Google Scholar and 90% (Kao and associates, this issue) of normal subjects. As in the case of early PTH assays, interpretation of these data is hampered by the lack of knowledge about the circulating form (or forms) of PTHrP. An additional technical problem is that no ideal technique is available for correcting for nonspecific protein effects on antigen-antibody binding in these assays, unlike the use of serum or plasma from patients with hypoparathyroidism in PTH assays. The difficulty in determining a definitive normal range may, in part, explain several unexpected results reported in the early assay studies. Before the development of these assays, increased urinary excretion of cyclic adenosine monophosphate was used as a biochemical marker for the “humoral hypercalcemia of malignancy” syndrome. In most patients with solid tumors (particularly squamous cell carcinomas) and hypercalcemia, urinary excretion of cyclic adenosine monophosphate is increased, and in most patients with hematologic malignant lesions and hypercalcemia, urinary excretion of cyclic adenosine monophosphate is suppressed or normal. Inexplicably increased PTHrP levels were found in 50%,9Budayr AA Nissenson RA Klein RF Pun KK Clark OH Diep D Arnaud CD Strewler GJ Increased serum levels of a parathyroid hormone-like protein in malignancy-associated hypercalcemia.Ann Intern Med. 1989; 111: 807-812Crossref PubMed Scopus (197) Google Scholar 33%,10Henderson JE Shustik C Kremer R Rabbani SA Hendy GN Goltzman D Circulating concentrations of parathyroid hormone-like peptide in malignancy and in hyperparathyroidism.J Bone Miner Res. 1990; 5: 105-113Crossref PubMed Scopus (132) Google Scholar and 45% (Kao and colleagues, this issue) of patients with hematologic malignant lesions, whereas in patients with solid tumors, PTHrP levels were increased in 71%,9Budayr AA Nissenson RA Klein RF Pun KK Clark OH Diep D Arnaud CD Strewler GJ Increased serum levels of a parathyroid hormone-like protein in malignancy-associated hypercalcemia.Ann Intern Med. 1989; 111: 807-812Crossref PubMed Scopus (197) Google Scholar 53%,10Henderson JE Shustik C Kremer R Rabbani SA Hendy GN Goltzman D Circulating concentrations of parathyroid hormone-like peptide in malignancy and in hyperparathyroidism.J Bone Miner Res. 1990; 5: 105-113Crossref PubMed Scopus (132) Google Scholar and 48% (Kao and co-workers, this issue) of the patients studied. Unfortunately, in only one of the reported studies was concomitant urinary cyclic adenosine monophosphate data obtained.11Burtis WJ Brady TG Orloff JJ Ersbak JB Warrell Jr, RP Olson BR Wu TL Mitnick ME Broadus AE Stewart AF Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer.N Engl J Med. 1990; 322: 1106-1112Crossref PubMed Scopus (496) Google Scholar In this study, a positive correlation existed between urinary cyclic adenosine monophosphate and PTHrP levels. The authors, however, did not present the individual data obtained in the patients with specific malignant lesions; thus, at this time, it is not possible to determine whether patients with multiple myeloma might have increased PTHrP levels but low urinary excretion of cyclic adenosine monophosphate. Burtis and colleagues11Burtis WJ Brady TG Orloff JJ Ersbak JB Warrell Jr, RP Olson BR Wu TL Mitnick ME Broadus AE Stewart AF Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer.N Engl J Med. 1990; 322: 1106-1112Crossref PubMed Scopus (496) Google Scholar found evidence of both amino-terminal and carboxy-terminal fragments of PTHrP in samples from patients with cancer, but they detected no evidence of intact PTHrP. Using an amino-terminal immunoradiometric assay, they found normal PTHrP levels in patients with chronic renal failure, in contrast with high levels with a carboxy-terminal assay in the same patients. This result is somewhat analogous to data in patients with chronic renal failure obtained with amino- and carboxy-terminal assays for PTH, in which carboxy-terminal assays yield much higher hormone levels than do amino-terminal assays. This outcome suggests that an important site of metabolic clearance of PTHrP carboxy-terminal fragments is the kidney. In three of the four assays reported, patients with primary hyperparathyroidism had normal levels of PTHrP. Henderson and colleagues,10Henderson JE Shustik C Kremer R Rabbani SA Hendy GN Goltzman D Circulating concentrations of parathyroid hormone-like peptide in malignancy and in hyperparathyroidism.J Bone Miner Res. 1990; 5: 105-113Crossref PubMed Scopus (132) Google Scholar using an amino-terminal PTHrP antiserum, were the only investigators who detected high levels in patients with either primary or secondary hyperparathyroidism. This finding seems concordant with the observation that parathyroid adenomas and glands from patients with secondary hyperparathyroidism often have detectable PTHrP,12Danks JA Ebeling PR Hayman JA Diefenbach-Jagger H Collier FM Grill V Southby J Moseley JM Chou ST Martin TJ Immunohistochemical localization of parathyroid hormone-related protein in parathyroid adenoma and hyperplasia.J Pathol. 1990; 161: 27-33Crossref PubMed Scopus (57) Google Scholar but the explanation for the observation of normal PTHrP levels in such patients by three laboratories remains unknown. Clearly, much more work will be necessary to identify the exact details of the pathogenesis of the humoral hypercalcemia of malignancy syndrome produced by PTHrP. More sensitive antisera or monoclonal antibodies should provide a means for establishing the normal range. Equally important will be the use of appropriate protein standards, which will necessitate defining the exact molecular species of biologically active PTHrP in the circulation. After these second-generation assays have been developed, it will be possible to determine what proportion of cancer patients with hypercalcemia represents, in fact, a manifestation of PTHrP excess. PTHrP was discovered as a consequence of studies in which the cause of the humoral hypercalcemia of malignancy syndrome was pursued. A burgeoning area of research involves the defining of the likely role of this protein in normal physiologic processes. Unlike PTH, whose synthesis in normal persons is restricted to the parathyroid glands, PTHrP messenger RNA is widely distributed in normal tissues, including the skin, thyroid, bone marrow, hypothalamus, pituitary, parathyroid, adrenal cortex, adrenal medulla, and stomach.13Ikeda K Weir EC Mangin M Dannies PS Kinder B Deftos LJ Brown EM Broadus AE Expression of messenger ribonucleic acids encoding a parathyroid hormone-like peptide in normal human and animal tissues with abnormal expression in human parathyroid adenomas.Mol Endocrinol. 1988; 2: 1230-1236Crossref PubMed Scopus (206) Google Scholar Strikingly high levels of PTHrP have been found in human, cow, pig, rat, mouse, opossum, water buffalo, and goat milk.9Budayr AA Nissenson RA Klein RF Pun KK Clark OH Diep D Arnaud CD Strewler GJ Increased serum levels of a parathyroid hormone-like protein in malignancy-associated hypercalcemia.Ann Intern Med. 1989; 111: 807-812Crossref PubMed Scopus (197) Google Scholar, 14Thiede MA Rodan GA Expression of a calcium-mobilizing parathyroid hormone-like peptide in lactating mammary tissue.Science. 1988; 242: 278-280Crossref PubMed Scopus (347) Google Scholar, 15Budayr AA Halloran BP King JC Diep D Nissenson RA Strewler GJ High levels of a parathyroid hormone-like protein in milk.Proc Natl Acad Sci USA. 1989; 86: 7183-7185Crossref PubMed Scopus (187) Google Scholar, 16Thurston AW Cole JA Hillman LS Im JH Thorne PK Krause WJ Jones JR Eber SL Forte LR Purification and properties of parathyroid hormone-related peptide isolated from milk.Endocrinology. 1990; 126: 1183-1190Crossref PubMed Scopus (54) Google Scholar Prolactin, either induced by suckling or exogenously administered, dramatically induces PTHrP messenger RNA in the mammary gland.17Thiede MA The mRNA encoding a parathyroid hormone-like peptide is produced in mammary tissue in response to elevations in serum prolactin.Mol Endocrinol. 1989; 3: 1443-1447Crossref PubMed Scopus (117) Google Scholar In a study of PTHrP physiologic features in lactating women, Khosla and colleagues also present their findings in this issue of the Proceedings (pages 1408 to 1414). Using the amino-terminal assay for PTHrP, as described by Kao and colleagues in the companion article herein, they measured plasma PTHrP in sequential plasma samples obtained from prepartum and postpartum lactating women. In 10 prepartum women, the mean plasma PTHrP levels were in the high-normal range, but the mean levels were identical in the lactating state. Similarly, in seven women with pathologic hyperprolactinemia, plasma PTHrP levels were not frankly increased. Thus, apparently the high level of PTHrP in milk does not produce a significant increase of the protein in the extracellular fluid of the mother. The role of the protein in the production of milk or in newborn physiologic processes remains to be established. A series of elegant experiments in fetal lambs has shown that PTHrP is an important modulator of fetal extracellular calcium homeostasis. It has been proposed and supported by experimental observations that both the fetal lamb parathyroid glands and the sheep placenta may secrete PTHrP and thereby regulate the placental calcium pump that maintains the fetus in a relatively hypercalcemic state in comparison with the mother.18Rodda CP Kubota M Heath JA Ebeling PR Moseley JM Care AD Caple IW Martin TJ Evidence for a novel parathyroid hormone-related protein in fetal lamb parathyroid glands and sheep placenta; comparisons with a similar protein implicated in humoral hypercalcaemia of malignancy.J Endocrinol. 1988; 117: 261-271Crossref PubMed Scopus (248) Google Scholar, 19Loveridge N Caple IW Rodda C Martin TJ Care AD Further evidence for a parathyroid hormone-related protein in fetal parathyroid glands of sheep.Q J Exp Physiol. 1988; 73: 781-784PubMed Google Scholar, 20Abbas SK Pickard DW Rodda CP Heath JA Hammonds RG Wood WI Caple W Martin TJ Care AD Stimulation of ovine placental calcium transport by purified natural and recombinant parathyroid hormone-related protein (PTHrP) preparations.Q J Exp Physiol. 1989; 74: 549-552Crossref PubMed Scopus (92) Google Scholar, 21Abbas SK Pickard DW Illingworth D Storer J Purdie DW Moniz C Dixit M Caple TW Ebeling PR Rodda CP Martin TJ Care AD Measurement of parathyroid hormone-related protein in extracts of fetal parathyroid glands and placental membranes.J Endocrinol. 1990; 124: 319-325Crossref PubMed Scopus (70) Google Scholar Thus, PTHrP may, in fact, be the main fetal “parathyroid hormone” because classic PTH does not seem to alter placental calcium transport. A point of considerable interest in these studies is that PTHrP(1–34) is ineffective in increasing calcium transport across the sheep placenta, whereas PTHrP(1–84), PTHrP(1–108), and PTHrP(1–141) are effective.20Abbas SK Pickard DW Rodda CP Heath JA Hammonds RG Wood WI Caple W Martin TJ Care AD Stimulation of ovine placental calcium transport by purified natural and recombinant parathyroid hormone-related protein (PTHrP) preparations.Q J Exp Physiol. 1989; 74: 549-552Crossref PubMed Scopus (92) Google Scholar This finding suggests that specific receptors for PTHrP bind to the protein at a site distal to amino acid 34. Investigators have also proposed that PTHrP may serve as an important autocrine-paracrine factor in numerous biologic systems. For example, the protein has been found to be produced by lectin-stimulated normal T lymphocytes, and when these lymphocytes were incubated with PTHrP(1–34), DNA synthesis was suppressed.22Adachi N Yamaguchi K Miyake Y Honda S Nagasaki K Akiyama Y Adachi I Abe K Parathyroid hormone-related protein is a possible autocrine growth inhibitor for lymphocytes.Biochem Biophys Res Commun. 1990; 166: 1088-1094Crossref PubMed Scopus (64) Google Scholar These experiments suggest that PTHrP may act as a cytokine that inhibits the replication of T lymphocytes. Thus, PTHrP could conceivably be both a fetal hormone that controls calcium homeostasis in the mammalian fetus and an autocrine-paracrine factor in postnatal life. The function of this protein as a circulating hormone seems to return when neoplastic or hyperplastic changes occur in organs that normally synthesize PTHrP. In the next few years, we can expect many new insights into the physiologic and pathophysiologic aspects of this fascinating protein.