William J. Burtis

Immunochemical Characterization of Circulating Parathyroid Hormone–Related Protein in Patients with Humoral Hypercalcemia of Cancer

Tumors from patients with humoral hypercalcemia of cancer produce a parathyroid hormone-related protein (PTHRP). We have developed two region-specific immunoassays capable of measuring PTHRP in plasma: an immunoradiometric assay directed toward PTHRP amino acid sequence 1 to 74 and a radioimmunoassay directed toward PTHRP amino acid sequence 109 to 138. Sixty normal subjects had low or undetectable plasma PTHRP (1 to 74) concentrations (mean, 1.9 pmol per liter) and undetectable PTHRP (109 to 138) concentrations (less than 2.0 pmol per liter). Patients with humoral hypercalcemia of cancer (n = 30) had elevated levels of both PTHRP (1 to 74) (mean, 20.9 pmol per liter) and PTHRP (109 to 138) (mean, 23.9 pmol per liter). The plasma concentrations of immunoreactive PTHRP correlated with the levels of urinary cyclic AMP excreted; in some patients, the concentrations decreased after the tumors were resected. Patients with chronic renal failure (n = 15) had plasma PTHRP (1 to 74) concentrations similar to those in the normal subjects, but their plasma PTHRP (109 to 138) concentrations were elevated (mean, 29.6 pmol per liter). The levels of both peptides were normal in patients with hyperparathyroidism and those with hypercalcemia due to various other causes. Breast milk contained high concentrations of PTHRP. An anti-PTHRP (1 to 36) immunoaffinity column failed to extract PTHRP (109 to 138) immunoactivity from plasma, suggesting that the C-terminal region circulates as a separate peptide. We conclude that plasma PTHRP concentrations are high in the majority of patients with cancer-associated hypercalcemia and that the circulating forms of PTHRP in such patients include both a large N-terminal (1 to 74) peptide and a C-terminal (109 to 138) peptide. Measuring the concentrations of PTHRPs may be useful in the differential diagnosis of hypercalcemia.

Humoral Hypercalcemia of Cancer

THE syndrome of humoral hypercalcemia of cancer was first described by Fuller Albright in 1941.1 When a patients hypercalcemia and hypophosphatemia resolved after the radiation of a single bone me...

Overexpression of Parathyroid Hormone-related Protein in the Pancreatic Islets of Transgenic Mice Causes Islet Hyperplasia, Hyperinsulinemia, and Hypoglycemia

Parathyroid hormone-related protein (PTHrP) is produced by the pancreatic islet. It also has receptors on islet cells, suggesting that it may serve a paracrine or autocrine role within the islet. We have developed transgenic mice, which overexpress PTHrP in the islet through the use of the rat insulin II promoter (RIP). Glucose homeostasis in these mice is markedly abnormal; RIP-PTHrP mice are hypoglycemic in the post-prandial and fasting states and display inappropriate hyperinsulinemia. At the end of a 24-hour fast, blood glucose values are 49 mg/dl in RIP-PTHrP mice, as compared to 77 mg/dl in normal littermates; insulin concentrations at this time are 6.3 and 3.9 ng/ml, respectively. Islet perifusion studies failed to demonstrate abnormalities in insulin secretion. In contrast, quantitative islet histomorphometry demonstrates that the total islet number and total islet mass are 2-fold higher in RIP-PTHrP mice than in their normal littermates. PTHrP very likely plays a normal physiologic role within the pancreatic islet. This role is most likely paracrine or autocrine. PTHrP appears to regulate insulin secretion either directly or indirectly, through developmental or growth effects on islet mass. PTHrP may have a role as an agent that enhances islet mass and/or enhances insulin secretion.

Humoral hypercalcemia of malignancy.

Excerpt At the Annual Meeting of the American Society for Bone and Mineral Research last June, three groups announced the isolation and amino-acid sequencing of a protein that appears to be respons...

Sensitivity of the Parathyroid Hormone–1, 25-Dihydroxyvitamin D Axis to Variations in Calcium Intake in Patients with Primary Hyperparathyroidism

Previous studies have demonstrated a spectrum of parathyroid responsivity to alterations in the extracellular calcium concentration in patients with primary hyperparathyroidism, but studies employing physiologic amounts of calcium have not, to our knowledge, been reported. We studied 18 unselected patients with primary hyperparathyroidism at the lower (400 mg) and upper (1000 mg) limits of a normal dietary intake of calcium. The diet containing high-normal amounts of calcium induced only a slight increase in 24-hour calcium excretion (from 281 to 337 mg per day) yet was associated with significant reductions in fasting serum levels of immunoreactive parathyroid hormone (from 60 to 50 nleq per milliliter; P less than 0.001), nephrogenous cyclic AMP (from 3.52 to 2.63 nmol per deciliter of glomerular filtrate; P less than 0.001), and plasma levels of 1,25-dihydroxyvitamin D (from 74 to 58 pg per milliliter; P less than 0.001). A wide spectrum of responses was observed, with some patients appearing to have essentially autonomous parathyroid function and others having marked suppressibility (up to 50 per cent) of the parathyroid hormone-vitamin D axis. We conclude that parathyroid function may be suppressed by dietary calcium in some patients with primary hyperparathyroidism.

Nephrogenous Cyclic AMP, Adenylate Cyclase-Stimulating Activity, and the Humoral Hypercalcemia of Malignancy

Publisher Summary This chapter discusses nephrogenous cyclic AMP (NcAMP), adenylate cyclase-stimulating activity (ACSA), and the humoral hypercalcemia of malignancy (HHM). Clinical HHM is a pathophysiologic syndrome which can be readily distinguished from 1°HPT group and local osteolytic hypercalcemia. Human HHM and animal models of HHM are associated with increases in NcAMP or UcAMP excretion. In contrast, NcAMP or UcAMP excretion is normal in humans and animals with HHM. These observations, and the findings that humans with MAHC can be divided into clinically and pathophysiologically distinct groups based upon NcAMP excretion, lead inescapably to the conclusion that the observed elevations in NcAMP are related mechanistically to hypercalcemia in patients with HHM. The ACSA that is present in human and animal HHM-associated tumors is not typically present in tumors derived from normocalcemic patients or animals. This ACSA can be prepared in highly purified form from human and animal tumors and can be shown to result from a basic protein with a molecular weight of approximately 30,000. This highly purified material contains potent in vitro bone-resorbing activity. These observations provide strong evidence that this PTH-like ACSA is related mechanistically to HHM. Human and animal HHM-associated tumors contain mRNA which encodes for a secretary protein which is closely related or identical to the ACSA found in tumor extracts.

Clonal rat parathyroid cell line expresses a parathyroid hormone-related peptide but not parathyroid hormone itself

A novel parathyroid hormone-related peptide has been identified in tumors associated with the syndrome of humoral hypercalcemia of malignancy. Subsequently, mRNAs encoding this peptide have been found to be expressed in a number of normal tissues, including the parathyroids. Using Northern blotting, RNase protection, and immunochemical techniques, we examined a clonal rat parathyroid cell line originally developed as a model system for studying parathyroid cell physiology. We found that this line expresses the parathyroid hormone-related peptide but not parathyroid hormone itself. Secretion of the parathyroid hormone-related peptide varied inversely with extracellular calcium concentration, but neither calcium nor 1,25-dihydroxyvitamin D3 appeared to influence steady-state parathyroid hormone-related peptide mRNA levels. This clonal line may prove to be an interesting system for studying the factors responsible for tissue-specific parathyroid hormone and parathyroid hormone-related peptide gene expression.

Trichlormethiazide and Oral Phosphate Therapy in Patients with Absorptive Hypercalciuria

In a short-term prospective study 36 patients with absorptive hypercalciuria were initially treated with diet alone followed by either trichlormethiazide (4 mg. per day) or oral neutral phosphate (1,500 mg. of elemental phosphorus per day) for 6 weeks. Study subjects were then crossed over to the second drug for an additional 6 weeks. In response to dietary treatment urinary calcium decreased from a pre-treatment value of 346 +/- 63 mg. per 24 hours to 308 +/- 90 mg. per 24 hours. Oral phosphate therapy caused a further decrease in urinary calcium to 218 +/- 85 mg. per 24 hours, an over-all decrease of 37 per cent. Parathyroid function did not change significantly with phosphate administration but circulating levels of 1,25-dihydroxyvitamin D decreased by 22 per cent (73 +/- 12 to 57 +/- 16 pg. per ml., p less than 0.001). Pre-treatment renal phosphate threshold did not correlate with the response to oral phosphate administration. Trichlormethiazide treatment led to a 34 per cent decrease in urinary calcium with a mean value on treatment of 228 +/- 80 mg. per 24 hours. 1,25-Dihydroxyvitamin D levels decreased by 10 per cent. Pre-treatment fasting calcium excretion, parathyroid function and 1,25-dihydroxyvitamin D levels did not correlate with the response to trichlormethiazide. We conclude that both drugs by pharmacological means improve the biochemical abnormalities in absorptive hypercalciuria and should be efficacious in its treatment.