Properties of parathyroid hormone receptors on circulating bovine lymphocytes

New insights into the role of T cells in the vicious cycle of bone metastases

PURPOSE OF REVIEW

Bone metastases interact with the bone microenvironment. Cancer cells modulate the functions of osteoblasts and osteoclasts to induce new bone formation or bone resorption, leading to secondary stimulation of tumor development. Recent findings suggest the involvement of T cells in this process.

RECENT FINDINGS

Bone metastatic cancer cells produce factors such as parathyroid hormone-related protein, interleukin-7, and interleukin-8 that can recruit or activate T cells. T cells are involved in bone remodeling and can induce osteoclastic resorption. Bone resorption releases transforming growth factor-beta, however, which could suppress T-cell antitumor immune responses. Bisphosphonate antiresorptive drugs are the approved treatment for solid tumor bone metastases. They have recently been found to activate the cytolytic activity of gammadelta T cells. Thus, inhibitors of transforming growth factor-beta or antiresorptive therapies may be effective enhancers of antitumor immune responses in bone.

SUMMARY

T cells at the site of bone metastases may be functionally suppressed by factors in the bone microenvironment. Instead of acting against tumor cells, they may increase bone resorption, making bone a privileged site for tumor growth.

A longitudinal study on the effect of nifedipine therapy and its discontinuation on [Ca2+]i and proliferation of B lymphocytes of dialysis patients

The abnormalities in cytosolic calcium ([Ca2+]i) and proliferation of B cells in uremic patients are significantly improved by treatment with nifedipine. The rapidity with which this agent induces its beneficial effect and whether these derangements reemerge after cessation of therapy are not known. We studied six hemodialysis patients before, during, and after treatment with nifedipine. Before treatment, [Ca2+]i of B cells was markedly elevated (125 +/- 4.3 nmol/L) and their proliferation markedly reduced (5.2 +/- 0.36 x 10(3) cpm). After 1 month of therapy, [Ca2+]i fell significantly (P < 0.01) to 95 +/- 1.7 nmol/L and to a normal value of 84 +/- 1.6 after 2 months. The levels of [Ca2+]i rose significantly (P < 0.01) to 95 +/- 2.3 nmol/L after 1 month of cessation of therapy and were 115 +/- 2.8 nmol/L by 2 months. Proliferation of B cells improved significantly (P < 0.01) after 1 month of therapy (9.4 +/- 1.1 x 10(3) cpm) with further improvement during the subsequent month, reaching a normal value (12.2 +/- 1.1 x 10(3) cpm) by the end of the 2 months. Proliferation of B cells decreased after cessation of therapy and was 5.2 +/- 0.17 x 10(3) cpm after 2 months, a value similar to the pretreatment level. The blunted inhibitory effect of PTH-(1-84) on B cell proliferation was reversed by nifedipine treatment and reappeared after discontinuation of therapy. Also, serum globulin levels increased after administration of nifedipine and decreased again after cessation of treatment. The results show that nifedipine rapidly reversed the elevation in [Ca2+]i of B cells, the impairment in their proliferation, and the blunted inhibitory effect of PTH on B cell proliferation, and was associated with increased serum globulin levels. These derangements reemerged after cessation of therapy. These data indicate that nifedipine therapy is effective in the management of the abnormalities in B cell metabolism and function in hemodialysis patients. The treatment with this drug must be maintained to sustain its beneficial effects. Other calcium channel blockers may also be effective, but their effects were not examined in the current study.

Identification and characterization of parathyroid hormone receptors in rat renal cortical plasma membranes using radioligand binding

Parathyroid hormone (PTH) receptors have been described in renal tissue from several species, but not in the rat. In this study, radioligand binding techniques were used to identify and characterize PTH receptors in rat kidney cortical membranes. The sulfur-free PTH analog [Nle8,18Tyr34]bovine PTH-(1-34)amide was iodinated using the iodogen method. This ligand was suitable for use in identifying PTH receptors in canine renal membranes, but not rat renal membranes. Synthetic, unsubstituted rat PTH-(1-34) was iodinated using the milder, lactoperoxidase technique and was purified by HPLC on a C8 column. [125I]rat PTH-(1-34) bound rapidly to both rat and dog renal membranes. At 22 degrees C reaction reached steady state within 20 minutes, and this level was maintained for at least 3 h. Specific binding was routinely greater than 90% for rat kidney and greater than 95% for dog kidney. Similar results were obtained at 4 degrees C with a longer time required to attain steady state (approximately 45 minutes). Binding was reversible as demonstrated by dissociation of bound ligand after either infinite dilution or displacement with excess nonradioactive PTH. Binding was saturable and of high affinity (rat kidney: Bmax = 2.3 pmol/mg protein, Kd = 3.1 nM, dog kidney: Bmax = 2.1 pmol/mg protein, Kd = 3.7 nM). Rat renal cortical adenylate cyclase activity was stimulated by rat PTH in a dose-dependent manner with an EC50 of 4 nM, a value in good agreement with the binding data. This study demonstrates the feasibility of identifying and characterizing parathyroid hormone receptors in rat renal cortical plasma membranes using radioligand binding techniques.