Gordon J. Strewler

Parathyroid hormonelike protein from human renal carcinoma cells. Structural and functional homology with parathyroid hormone.

A variety of solid tumors secrete proteins that are immunochemically distinct from parathyroid hormone (PTH) but activate PTH-responsive adenylate cyclase. Such PTH-like proteins have been proposed as mediators of the hypercalcemia and hypophosphatemia frequently associated with malignancies. We purified to apparent homogeneity a PTH-like protein with a molecular weight of 6,000, that is produced by human renal carcinoma cells. The amino-terminal sequence of the PTH-like protein and that of human PTH were found to display at least five identities in the first 13 positions. The purified protein bound to PTH receptors, activated adenylate cyclase in renal plasma membranes, and stimulated cAMP formation in rat osteosarcoma cells. The PTH-like protein reproduced two additional effects of PTH, stimulation of bone resorption in fetal rat limb bone cultures and inhibition of phosphate uptake in cultured opossum kidney cells. These properties are consistent with a role for PTH-like proteins as mediators of the syndrome of malignancy-associated hypercalcemia.

The Physiology of Parathyroid Hormone–Related Protein

Parathyroid hormone–related protein was identified in the 1980s as a tumor product that had the ability to activate parathyroid hormone receptors and cause hypercalcemia.1 Parathyroid hormone–relat...

Bone disease in alcohol abuse

We evaluated bone disease in eight white men between the ages of 49 and 61 years who had been abusing alcohol for at least 10 years. The mean density of vertebral cancellous bone was 58% of normal, whereas the mean density of appendicular cortical bone was 90% of normal. Marked reduction in active bone resorption and bone formation was seen without evidence of osteomalacia. Serum levels of calcium and magnesium were in the lower range of normal; serum levels of phosphorus, calcifediol, and calcitriol were normal; and serum levels of parathyroid hormone and nephrogenous cyclic adenosine monophosphate were in the higher range of normal. These data suggest that bone disease in these subjects is not due to inhibition of parathyroid hormone secretion or function, or abnormal vitamin D metabolism, but to an inhibition of bone remodeling by a mechanism independent of the calciotropic hormones.

Increased Serum Levels of a Parathyroid Hormone-like Protein in Malignancy-Associated Hypercalcemia

STUDY OBJECTIVE To measure the serum levels of a newly described parathyroid hormone-like protein (PLP) which was isolated from malignant tumors associated with hypercalcemia, and determine whether PLP is a humoral factor in malignancy-associated hypercalcemia. DESIGN A cross-sectional study of serum levels of PLP using a newly developed radioimmunoassay. SETTING A university-affiliated Veterans Administration hospital in San Francisco, California, a University hospital in Hong Kong, and a private hospital in Danville, Pennsylvania. PATIENTS Patients with hypercalcemia (calcium greater than 2.65 mmol/L) and a diagnosis of malignancy were studied. Control groups included normocalcemic patients with malignancy, patients with hyperparathyroidism, and normal subjects. MEASUREMENTS AND MAIN RESULTS Serum immunoreactive PLP (iPLP) levels in normal subjects were less than 2.5 pmol eq/L (10 pg/mL), and 68% of subjects had undetectable levels. The serum concentration of iPLP was normal in 15 of 16 hypercalcemic patients with hyperparathyroidism. Serum iPLP was increased (greater than 2.5 pmol eq/L) in 36 of 65 (55%) patients with malignancy-associated hypercalcemia, with a mean value of 6.1 +/- 0.9 pmol eq/L (24 pg/mL). In a subgroup of patients with solid tumors serum iPLP was increased in 30 (71%) of 42 hypercalcemic patients, with a mean value of 6.5 +/- 0.9 pmol eq/L. Serum iPLP was elevated in only 3 of 23 normocalcemic patients with cancer. In patients with solid malignancies (n = 59), levels of iPLP were positively correlated with the total serum calcium (r = 0.43, P less than 0.01). CONCLUSION The data indicate a relation between the serum concentration of iPLP and the presence of hypercalcemia in solid malignancies. The results support a role for PLP as a humoral mediator of hypercalcemia in most patients with solid tumors. Measurement of iPLP should be useful in the differential diagnosis of hypercalcemia.

Human renal carcinoma cells produce hypercalcemia in the nude mouse and a novel protein recognized by parathyroid hormone receptors.

When grown in nude mice, cultured renal carcinoma cells from a hypercalcemic patient produced marked hypercalcemia that was reversed by resection of tumor. Conditioned medium from this cell line contained a protein with activity in a renal adenylate cyclase bioassay for parathyroid hormone (PTH) which was blocked by the competitive PTH antagonist [8norleucyl, 18norleucyl, 34tyrosinyl]bPTH (3-34)amide. However, the biologically active protein was eluted from gel filtration columns as a larger molecular size component that PTH and was not recognized by any of four region-specific PTH antisera. The properties of this factor resemble those of the postulated PTH-like substance(s) in humoral hypercalcemia of malignancy.

Human transforming growth factor-alpha stimulates bone resorption in vitro.

Tumor-derived transforming growth factors (TGF) have been proposed as possible mediators of hypercalcemia in malignancy. We have studied the action of recombinant human TGF-alpha in cultured bone cells and in bone explant cultures. In clonal UMR-106 rat osteosarcoma cells, TGF-alpha and epidermal growth factor (EGF) were equipotent in binding to the EGF receptor. TGF-alpha and EGF both stimulated resorption of neonatal mouse calvaria, and maximal responses were obtained with 10 ng/ml of TGF-alpha after 72 h in culture. The effects of both TGF-alpha and EGF in calvaria, but not those of parathyroid hormone, were inhibited by 5 X 10(-7) M indomethacin. Fetal rat limb bone cultures were less sensitive to TGF-alpha than neonatal mouse calvaria, with a concentration of 30 ng/ml being required to stimulate resorption in this system. The bone-resorbing activity of TGF-alpha in fetal rat bones was inhibited by 10 ng/ml calcitonin but not by 5 X 10(-7) M indomethacin. EGF at concentrations up to 300 ng/ml did not stimulate resorption of the limb bones at time periods up to 66 h. The results indicate that human TGF-alpha is a potent bone-resorbing agent, and support the concept that this growth factor exhibits some effects distinct from those of EGF. TGF-alpha could play an etiologic role in the hypercalcemia of malignancy.

Endotoxin increases parathyroid hormone-related protein mRNA levels in mouse spleen. Mediation by tumor necrosis factor.

Parathyroid hormone-related protein (PTHrP) causes hypercalcemia in malignancy. However, the role and regulation of PTHrP in normal physiology is just beginning to be explored. PTHrP is found in the spleen and has several other features common to cytokines. Since endotoxin (LPS) causes many of its effects indirectly by inducing cytokines, studies were undertaken to determine whether LPS might also induce splenic PTHrP expression. LPS (100 ng/mouse) increased splenic PTHrP mRNA levels 3.6-fold in C3H/OuJ mice. This effect was maximal at 2 h and returned to baseline by 4 h. PTHrP peptide levels also increased 3.3-fold in splenic extracts in response to LPS (1 microgram/mouse). Murine TNF-alpha and human IL-1 beta, cytokines that mediate many of the effects of LPS, also increased splenic PTHrP mRNA levels. LPS-resistant C3H/HeJ mice, which produce minimal amounts of TNF and IL-1 in response to LPS, were resistant to LPS induction of splenic PTHrP mRNA, while TNF-alpha and IL-1 beta readily increased PTHrP mRNA levels in C3H/HeJ mice. Anti-TNF antibody blocked LPS induction of splenic PTHrP mRNA in C3H/OuJ mice by 68%, indicating that TNF is a mediator of the LPS induction of PTHrP levels. In contrast, an IL-1 receptor antagonist (IL-1ra) was ineffective. The increase in PTHrP in the spleen during the immune response suggests that PTHrP may play an important role in immune modulation, perhaps by mediating changes in lymphocyte proliferation and/or function.

FGF23, hypophosphatemia, and rickets: Has phosphatonin been found?

The mineralization of cartilage and bone matrix requires adequate supplies of calcium and phosphate. Among the causes of defective mineralization of bone (osteomalacia) and defective mineralization of cartilage (rickets) are renal phosphate-wasting disorders that produce hypophosphatemia. Phosphate wasting is either inherited as X-linked hypophosphatemic rickets or autosomal dominant hypophosphatemic rickets, or acquired, as can occur in patients with a variety of benign mesenchymal tumors (hemangiopericytomas, fibromas, angiosarcomas, etc.) (1). Osteomalacia induced by tumors is invariably curable if the tumor can be found and resected, indicating that it has a humoral basis. A paper by Shimada et al. (2) in this issue of PNAS identifies a member of the fibroblast growth factor family, FGF23, as the humoral factor that is secreted by tumors to cause tumor-induced osteomalacia. With the discovery that a protease mutation and a cleavage site mutation may cause the same disease, the puzzle pieces could fit together nicely.

Sodium Intake and Vascular Smooth Muscle Responsiveness to Norepinephrine and Angiotensin in the Rabbit

The effect of dietary sodium restriction on the responses to norepinephrine and angiotensin was assessed in two vascular smooth muscle preparations in the rabbit. Sodium restriction reduced the response to angiotensin of both the limb vessels in vivo and the aorta in vitro. The response to norepinephrine was potentiated by sodium restriction in both preparations. The effects on angiotensin responsiveness did not appear to be related to tachyphylaxis, changes in extracellular electrolyte composition, or nonspecific depression of smooth muscle function. Probably, the change in sodium intake induced a change in the smooth muscle cell membranes which modified the affinity of their receptors for angiotensin and norepinephrine.

THE PARATHYROID HORMONE-RELATED PROTEIN

Many physiologic roles of PTHrP are emerging. The protein functions locally in diverse tissues, often regulating the entry of cells into a differentiation pathway or acting as an epithelial signal in epithelial-mesenchymal interactions. To carry out these functions, PTHrP uses the receptor it shares with PTH or one of several PTHrP receptors that have evolved to recognize selectively the PTH-like region of PTHrP or other domains. Thus, PTHrP is a polyhormone. An exquisite selectivity barrier allows PTHrP to carry out its local tissue functions at the same time PTH uses their shared receptor to regulate systemic calcium homeostasis. This barrier is breached under pathologic circumstances, such as when malignant tumors secrete enough PTHrP into blood to cause PTH-like effects, including hypercalcemia. Powerful genetic models that have been developed in the past 7 years promise to give continuing insights into the physiology and pathophysiology of PTHrP.