Humoral hypercalcemia of malignancy is a common complication of lung and certain other cancers. The hypercalcemia results from the actions of tumor factors on bone and kidney. We report here the isolation of full-length complementary DNA clones of a putative hypercalcemia factor, and the expression from the cloned DNA of the active protein in mammalian cells. The clones encode a prepro peptide of 36 amino acids and a mature protein of 141 amino acids that has significant homology with parathyroid hormone in the amino-terminal region. This previously unrecognized hormone may be important in normal as well as abnormal calcium metabolism.

A protein with biological activities similar to parathyroid hormone (PTH) has been purified from serum-free culture medium obtained from a human lung cancer cell line (BEN). A major protein band of 18 kDa was obtained on NaDodSO4/polyacrylamide gels, with faint bands at 35 kDa and 67 kDa. Biological activity was associated only with the 18-kDa band. Amino acid sequence analysis of the material purified by HPLC revealed that 8 of the 16 residues were identical with those of human PTH. Antibody raised to a corresponding synthetic peptide recognized the PTH-related material but showed <1% cross-reactivity with human PTH amino-terminal peptides. BEN cells contained PTH DNA, but not PTH messenger RNA, indicating involvement of another gene. The purified PTH-related protein had a specific biological activity %-6 times greater than that of bovine PTH(1-34). The PTH-related protein may have a role in the syndrome of humoral hypercalcemia of malignancy.Humoral hypercalcemia of malignancy (HHM) is a very common complication of certain cancers, especially squamous cell carcinoma of the lung, in which it contributes substantially to morbidity and mortality (1). Cancer-derived humoral factors can elevate blood calcium levels by promoting bone resorption and restricting calcium excretion by the kidney (1, 2). Although it was initially thought that "ectopic" production of parathyroid hormone (PTH) by these cancers was the cause of the HHM syndrome, it has become apparent that factors other than PTH are responsible (1-5), including transforming growth factors, which are potent promoters of bone resorption (2, 6). In addition, certain cancers produce a factor that is immunologically distinct from PTH but that resembles PTH in its biological activity. These include tumors from patients with HHM (7,8) and from animal models of HHM (8) and renal cortical carcinoma cells in culture (9).We have found that the BEN cell line, established from a hypercalcemic patient with a squamous cell carcinoma of the bronchus (10), produces appreciable amounts of this PTHlike activity, which stimulates adenylate cyclase in osteoblast-like cells. We report here its purification from BEN cell culture medium and partial structural analysis, revealing the existence of a gene related to the PTH gene.

Peptides corresponding to the amino-terminal region of the parathyroid hormone-related protein (PTHrP) of humoral hypercalcemia of malignancy were synthesized. A 34-amino acid peptide, PTHrP(1-34), was two to four times more potent than bovine or human PTH(1-34) in bioassays promoting the formation of adenosine 3',5'-monophosphate (cAMP) and plasminogen activator activity in osteogenic sarcoma cells and adenylate cyclase activity in chick kidney membranes. Like parathyroid hormone itself, in which the activity resides in the first 34 residues, PTHrP peptides of less than 30 residues from the amino terminus showed substantially reduced activity. PTHrP(1-34) had only 6% of the potency of bovine PTH(1-34) in promoting bone resorption in vitro. PTHrP(1-34) strongly promoted the excretion of cAMP and phosphorus and reduced the excretion of calcium in the isolated, perfused rat kidney consistent with the symptoms seen in malignant hypercalcemia.