Human prostate carcinoma causes hypercalcemia in athymic nude mice and produces a factor with parathyroid hormone-like bioactivity

Animal Models of Cancer-Associated Hypercalcemia

Cancer-associated hypercalcemia (CAH) is a frequently-occurring paraneoplastic syndrome that contributes to substantial patient morbidity and occurs in both humans and animals. Patients with CAH are often characterized by markedly elevated serum calcium concentrations that result in a range of clinical symptoms involving the nervous, gastrointestinal and urinary systems. CAH is caused by two principle mechanisms; humorally-mediated and/or through local osteolytic bone metastasis resulting in excessive calcium release from resorbed bone. Humoral hypercalcemia of malignancy (HHM) is the most common mechanism and is due to the production and release of tumor-associated cytokines and humoral factors, such as parathyroid hormone-related protein (PTHrP), that act at distant sites to increase serum calcium concentrations. Local osteolytic hypercalcemia (LOH) occurs when primary or metastatic bone tumors act locally by releasing factors that stimulate osteoclast activity and bone resorption. LOH is a less frequent cause of CAH and in some cases can induce hypercalcemia in concert with HHM. Rarely, ectopic production of parathyroid hormone has been described. PTHrP-mediated hypercalcemia is the most common mechanism of CAH in human and canine malignancies and is recognized in other domestic species. Spontaneous and experimentally-induced animal models have been developed to study the mechanisms of CAH. These models have been essential for the evaluation of novel approaches and adjuvant therapies to manage CAH. This review will highlight the comparative aspects of CAH in humans and animals with a discussion of the available animal models used to study the pathogenesis of this important clinical syndrome.

Parathyroid hormone-related protein: a potential autocrine growth regulator in human prostate cancer cell lines

OBJECTIVE

We recently demonstrated that parathyroid hormone-related protein (PTHrP) is widely expressed by human prostate cancer tissue, suggesting that PTHrP might be involved in the growth and development of prostate cancer. To study this further, the production of PTHrP and its biologic effect were investigated using human prostate cancer cell lines.

METHODS

The cell lines used were one androgen-dependent cell line, LNCaP, and two androgen-independent cell lines, PC-3 and DU-145. PTHrP secreted by cancer cells was measured by radioimmunoassay. The effect of PTHrP on DNA synthesis in these cells was determined by thymidine incorporation assay.

RESULTS

All cell lines secreted immunodetectable levels of PTHrP in the culture-conditioned media. PC-3 cells secreted significantly higher amounts than the other two cell lines. A synthetic peptide, PTHrP(1-34), stimulated thymidine uptake in PC-3 and DU-145 cells more than threefold the control under serum-free and steroid-free conditions, whereas LNCaP was not affected. However, in the presence of dihydrotestosterone, DNA synthesis of LNCaP cells was stimulated by PTHrP in a dose-dependent manner. Additionally, this PTHrP-induced DNA synthesis was completely neutralized by a validated mouse monoclonal antibody (8B12) raised against PTHrP(1-34).

CONCLUSIONS

Our data suggest that PTHrP may play a significant role in the growth of prostate cancer by acting locally in an autocrine fashion.

Parathyroidectomy abolishes the increase of renal 25-hydroxyvitamin D-1 alpha-hydroxylase in lactating rats

Serum ionized calcium (Ca), but not inorganic phosphorus or immunoreactive parathyroid hormone, negatively correlates with renal 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) and serum 1,25-dihydroxyvitamin D in intact lactating rats. The present study tested the hypothesis that the presumed stimulation of renal 1 alpha-hydroxylase by hypocalcemia requires the presence of intact parathyroid glands. Lactating and nonlactating rats were surgically parathyroidectomized (PTX) or sham-operated (sham) at 9-10 days of lactation. Later (24 h) the rats were bled, nephrectomized, and killed. In lactating PTX rats, serum ionized Ca decreased to 50% of the level of sham rats, and serum 1,25-dihydroxyvitamin D fell to 37 +/- 5.0 pg/ml compared with 82 +/- 13.0 pg/ml for sham lactating rats but was still 2.5 times the value for nonlactating PTX rats (15 +/- 0.8 pg/ml). In contrast to the still elevated serum 1,25-dihydroxyvitamin D concentration in lactating PTX rats, renal 1 alpha-hydroxylase was suppressed to the same low level as in nonlactating PTX rats, suggesting the existence of extrarenal synthesis of 1,25-dihydroxyvitamin D in lactation. A curvilinear relationship was revealed between serum ionized Ca and renal 1 alpha-hydroxylase in sham lactating and nonlactating rats (r2 = 0.71, P < 0.0001). However, in PTX rats, decreasing ionized Ca did not lead to any increase in 1 alpha-hydroxylase above the low baseline values seen at ionized Ca concentrations between 1.3 and 1.5 mM. We therefore conclude that intact parathyroid glands are required for hypocalcemia to activate renal 1 alpha-hydroxylase in female rats.

Renal vasodilatation and microvessel adenylate cyclase stimulation by synthetic parathyroid hormone-like protein fragments

The hypercalcemia caused by malignancy factor, also called parathyroid hormone-related protein (PTHrP), exhibits most of the biological activities of parathyroid hormone (PTH) in kidney and bone. On the basis of the well-documented vascular action of PTH, we characterized the vasodilator action of human (h) PTHrP-(1-34) on a preparation of the isolated rat kidney, and its activity to stimulate adenylate cyclase in microvessels isolated from rabbit kidney cortex. Injection of sequential cumulative doses of hPTHrP-(1-34) into the isolated kidney preparation produced increasing vasodilatation up to 10(-8) M (EC50 of 3 x 10(-9) M) and decreasing responses thereafter. The maximal effect represented 26% of the reference relaxation induced by papaverine. Single injections of hPTHrP-(1-34) resulted in a greater (over 60%) vasodilatation. These results were reminiscent of the tachyphylaxis that occurs after repeated exposure to the peptide. The (3-34) PTH antagonist inhibited the hPTHrP-induced vasodilatation. Human PTHrP-(1-34) was equipotent with hPTH-(1-34) (EC50 values of 3 x 10(-9) M) but 5-fold less potent than rat (r) PTH-(1-34) in stimulating microvessel adenylate cyclase. GTP enhanced the enzyme responses to the peptides but reduced their potency. Both (3-34) and (7-34) PTH antagonists were inhibitors of hPTHrP- or PTH-stimulated microvascular adenylate cyclase. Synthetic hPTHrP-(1-16) had neither vasodilator nor adenylate cyclase-stimulating activity. This hPTHrP fragment exhibited some inhibitory effect on the hPTHrP-(1-34)-induced stimulation of microvessel adenylate cyclase. These results indicate that hPTHrP possesses PTH-like activity to cause vasorelaxation and to stimulate microvascular adenylate cyclase in the kidney.

89Sr therapy: strontium plasma clearance in disseminated prostatic carcinoma

Strontium plasma clearance is an important factor determining the absorbed dose to metastases and bone marrow in patients receiving 89Sr radionuclide therapy for metastatic bone disease. Amongst male patients with disseminated prostatic carcinoma, the renal component of strontium clearance is frequently greatly reduced compared with values reported for healthy middle aged men. We report a study of renal and gut strontium plasma clearance, renal function, calcium urinary excretion, parathyroid function and extent of skeletal osteoblastic metastatic disease in patients referred for radiostrontium therapy for metastasised prostatic malignancy. The wide variation in net strontium clearance was principally due to variation in the renal component. Low values of strontium renal clearance were found to correlate with the elevation of serum PTH and nephrogenous cyclic AMP, which in turn correlated with extent of skeletal metastatic disease. This suggests that the osteosclerotic metastases characteristic of prostatic carcinoma induce secondary hyperparathyroidism due to the high avidity of the skeleton for calcium. The resulting reduction in strontium excretion may be beneficial to the objectives of radiostrontium therapy.

Establishment of a model to evaluate inhibition of bone resorption induced by human prostate cancer cells in nude mice

A model system of human prostate carcinoma in nude mice for searching out a method to protect the bone from cancer cells is described, in which the transplanted human prostate cancer cells were inoculated subcutaneously over the calvaria in nude mice after the periosteum wa disrupted. The tumor induced osteolysis associated with osteoclast proliferation accompanied with reactive new bone formation. This osteolysis was evaluated by measuring the increased area of bone resorption by its reduced opacity to X-ray, and histology. Etidronate disodium, a diphosphonate derivative, at a dose of three mg./kg. and 10 mg./kg. s.c. protected the bone by decreasing the extent of osteolysis as judged by the above criteria. This inhibition was obtained with no apparent effect on the growth of the tumor. These results are discussed in light of recent clinical work, showing that this animal model is a useful tool to test the effect of new drugs against osteolysis of cancer as well as to study the biology of local interaction between bone and cancer cell.