Parathyroid hormone-related protein (PTH-rP) was purified and cloned 10 years ago as
a factor responsible for the
hypercalcemia associated with
malignancy. Clinical evidence supports another important role for PTH-rP in
malignancy as a mediator of the bone destruction associated with osteolytic
metastasis. Patients with PTH-rP positive
breast carcinoma are more likely to develop bone
metastasis. In addition,
breast carcinoma metastatic to bone expresses PTH-rP in >90% of cases, compared with only 17% of
metastasis to nonbone sites. These observations suggest that PTH-rP expression by
breast carcinoma cells may provide a selective growth advantage in bone due to its ability to stimulate osteoclastic
bone resorption. Furthermore,
growth factors such as
transforming growth factor-beta (
TGF-beta), which are abundant in bone matrix, are released and activated by osteoclastic
bone resorption and may enhance PTH-rP expression and
tumor cell growth. To investigate the role of PTH-rP in the pathophysiology of
breast carcinoma metastasis to bone, the human
breast carcinoma cell line MDA-MB-231 was studied in a murine model of human
breast carcinoma metastasis to bone. A series of experiments were performed in which 1) PTH-rP secretion was altered, 2) the effects of PTH-rP were neutralized, or 3) the responsiveness to
TGF-beta was abolished in MDA-MB-231 cells. Cultured MDA-MB-231 cells secreted low amounts of PTH-rP that increased fivefold in response to
TGF-beta.
Tumor cells inoculated into the left cardiac ventricle of nude mice caused osteolytic
metastasis similar to that observed in humans with
breast carcinoma. When PTH-rP was overexpressed in the
tumor cells, bone
metastases were increased. MDA-MB-231 cells transfected with the
cDNA for human preproPTH-rP secreted a tenfold greater amount of PTH-rP and caused significantly greater bone
metastases when inoculated into the left cardiac ventricle of female nude mice compared with parental cells. In contrast, when the
biologic effects of PTH-rP were neutralized or its production was suppressed, such
metastases were decreased. Treatment of mice with a neutralizing
monoclonal antibody to human PTH-rP resulted in a decrease in the development and progression of bone
metastasis due to the parental MDA-MB-231 cells. Similar results were observed when mice were treated with
dexamethasone, a potent
glucocorticoid that suppresses production of PTH-rP by the MDA-MB-231 cells in vitro. The role of bone-derived
TGF-beta in the development and progression of bone
metastasis was studied by transfecting MDA-MB-231 cells with a
cDNA encoding a
TGF-beta type II receptor lacking a cytoplasmic domain, which acts as a dominant negative to block the cellular response to
TGF-beta. Stable clones expressing this mutant receptor (MDA/TbetaRIIdeltacyt) did not increase PTH-rP secretion in response to
TGF-beta stimulation compared with controls of untransfected MDA-MB-231 or those transfected with the empty vector. Mice inoculated into the left cardiac ventricle with MDA/TbetaRIIdeltacyt had fewer and smaller bone
metastases as assessed radiographically and histomorphometrically compared with controls. Taken together, these data suggest that PTH-rP expression by
breast carcinoma cells enhance the development and progression of
breast carcinoma metastasis to bone. Furthermore,
TGF-beta responsiveness of
breast carcinoma cells may be important for the expression of PTH-rP in bone and the development of osteolytic bone
metastasis in vivo. These interactions define a critical feedback loop between
breast carcinoma cells and the bone microenvironment that may be responsible for the alacrity with which
breast carcinoma grows in bone.