Hypercalcemia Is of Uncertain Significance in Patients With Advanced Adenocarcinoma of the Prostate
Discussion
Hypercalcemia in the setting of prostate cancer is a rare complication with an uncertain pathophysiology.6 Several mechanisms have been proposed for hypercalcemia of malignancy, these comprise humoral hypercalcemia of malignancy mediated by increased PTHrP; local osteolytic hypercalcemia with secretion of other humoral factors; excess extrarenal activation of vitamin D (1,25[OH]2D); PTH secretion, ectopic or primary; and multiple concurrent etiologies.7
PTHrP is the predominant mediator for hypercalcemia of malignancy and is estimated to account for 80% of hypercalcemia in patients with cancer. This protein shares a substantial sequence homology with PTH; in fact, 8 of the first 13 amino acids at the N-terminal portion of PTH were identical.8 PTHrP has multiple isoforms (PTHrP 141, PTHrP 139, and PTHrP 173). Like PTH, it enhances renal tubular reabsorption of calcium while increasing urinary phosphorus excretion.7 The result is both hypercalcemia and hypophosphatemia. However, unlike PTH, PTHrP does not increase 1,25(OH)2D and thus does not increase intestinal absorption of calcium and phosphorus. PTHrP acts on osteoblasts, leading to enhanced synthesis of receptor activator of nuclear factor-κB ligand (RANKL).7
In one study, PTHrP was detected immunohistochemically in prostate cancer cells. Iwamura and colleagues used 33 radical prostatectomy specimens from patients with clinically localized carcinoma of the prostate.9 None of these patients demonstrated hypercalcemia prior to the surgery. Using a mouse monoclonal antibody to an amino acid fragment, all cases demonstrated some degree of immunoreactivity throughout the cytoplasm of the tumor cells, but immunostaining was absent from inflammatory and stromal cells.9Furthermore, the intensity of the staining appeared to directly correlate with increasing tumor grade.9
Another study by Iwamura and colleagues suggested that PTHrP may play a significant role in the growth of prostate cancer by acting locally in an autocrine fashion.10 In this study, all prostate cancer cell lines from different sources expressed PTHrP immunoreactivity as well as evidence of DNA synthesis, the latter being measured by thymidine incorporation assay. Moreover, when these cells were incubated with various concentrations of mouse monoclonal antibody directed to PTHrP fragment, PTHrP-induced DNA synthesis was inhibited in a dose-dependent manner and almost completely neutralized at a specific concentration. Interestingly, the study demonstrated that cancer cell line derived from bone metastatic lesions secreted significantly greater amounts of PTHrP than did the cell line derived from the metastasis in the brain or in the lymph node. These findings suggest that PTHrP production may confer some advantage on the ability of prostate cancer cells to grow in bone.10
Ando and colleagues reported that neuroendocrine dedifferentiated prostate cancer can develop as a result of long-term ADT even after several years of therapy and has the potential to worsen and develop severe hypercalcemia.8 Neuron-specific enolase was used as the specific marker for the neuroendocrine cell, which suggested that the prostate cancer cell derived from the neuroendocrine cell might synthesize PTHrP and be responsible for the observed hypercalcemia.8
Other mechanisms cited for hypercalcemia of malignancy include other humoral factors associated with increased remodeling and comprise interleukin 1, 3, 6 (IL-1, IL-3, IL-6); tumor necrosis factor α; transforming growth factor A and B observed in metastatic bone lesions in breast cancer; lymphotoxin; E series prostaglandins; and macrophage inflammatory protein 1α seen in MM.
Local osteolytic hypercalcemia accounts for about 20% of cases and is usually associated with extensive bone metastases. It is most commonly seen in MM and metastatic breast cancer and less commonly in leukemia. The proposed mechanism is thought to be because of the release of local cytokines from the tumor, resulting in excess osteoclast activation and enhanced bone resorption often through RANK/RANKL interaction.
