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Parathyroid Hormone-Related Protein Production by Breast Cancers, Improved Survival, and Reduced Bone Metastases

Michael A. Henderson, Janine A. Danks, Jane M. Moseley, John L. Slavin, Toni L. Harris, Martina R. McKinlay, John L. Hopper, T. John Martin

Affiliations of authors: M. A. Henderson, T. L. Harris, M. R. McKinlay, University of Melbourne, Department of Surgery, St. Vincent's Hospital, Fitzroy, Australia; J. A. Danks, J. M. Moseley, T. J. Martin (St. Vincent's Institute of Medical Research), J. L. Slavin (Department of Pathology), St. Vincent's Hospital; J. L. Hopper, Department of General Practice and Public Health, University of Melbourne.

Correspondence to: Michael A. Henderson, M.D., FRACS, Department of Surgery, St. Vincent's Hospital, 41 Victoria Parade, Fitzroy 3065, Victoria, Australia (e-mail: henderson{at}surgerysvh.unimelb.edu.au).

Metastasis of breast cancers is a common and serious clinical problem, with up to one third of women with early-stage breast cancer eventually dying of the disease. Of all patients with distant spread of the cancer, 70% have bone metastases (1,2), reflective of the observations of Paget (3) that breast cancers had a particular predilection to grow in bone. A special property required of cancer cells for growth as metastases in bone is the ability to promote bone resorption by inducing the formation and activity of osteoclasts (4). Parathyroid hormone-related protein (PTHrP) is produced by two thirds of primary breast cancers (5), promotes osteoclast formation and bone resorption, and has been implicated in site-specific metastasis of breast cancer to bone (6). The current study investigated the relationship among detection of PTHrP in primary breast cancers, development of skeletal complications, and overall survival.

From December 1, 1989, through December 31, 1994, a total of 402 consecutively accrued women with operable (stages 1–3) invasive primary breast cancer were followed prospectively for a median period of 67 months (range, 3–120 months). Impalpable tumors detected by screening mammography accounted for 5% of the cancers. Patients with previous malignancy or preoperative therapy before open biopsy were excluded (35 patients). The timing and site of metastases and overall survival were recorded, as were routine prognostic factors. Patients were examined at least twice a year for 3 years and then annually. The study was approved by the Human Research Ethics Committee of St. Vincent's Hospital (Fitzroy, Australia), and patients provided informed consent according to institutional guidelines.

There were 367 eligible patients, and the median age at diagnosis was 60 years (range, 27–92 years). The majority of these patients were postmenopausal (70%), the median tumor size was 26 mm (range, 4–180 mm), and most patients were axillary lymph node negative (60%) (Table 1Go). PTHrP was detected by immunohistochemistry in 265 (72%) of the primary breast tumors and was associated with estrogen receptor (ER) status (odds ratio [OR] = 2.4; 95% confidence interval [CI] = 1.4 to 4.0; P<.001), progesterone receptor (PR) status (OR = 1.9; 95% CI = 1.1 to 3.2; P = .01), and presence of tumor calcification (OR = 1.6; 95% CI = 1.0 to 2.6; P = .05) but not with menopausal status, tumor size, lymph node status, tumor grade, or the presence of lymphatic/vascular invasion by tumor (7). The crude 5-year cancer-specific survival for all patients was 82% (95% CI = 78% to 86%). Prognostic factors associated with survival included American Joint Committee on Cancer stage, tumor size, lymph node status, ER status, PR status, presence of tumor calcification, absence of tumor lymphatic/vascular invasion, and tumor grade but not menopausal status. Patients with tumors containing detectable PTHrP had improved survival compared with patients with PTHrP-negative tumors (Fig. 1Go, A) (8,9). The crude 5-year cancer-specific survivals were 87% (95% CI = 82% to 91%) for patients with PTHrP-positive tumors and 73% (95% CI = 63% to 81%) for those with PTHrP-negative tumors (P = .002). Cox proportional hazards modeling (10) demonstrated that positive PTHrP status was independently associated with improved survival (P = .003), as were the number of axillary lymph nodes involved (P<.001), tumor size (P<.001), and tumor grade (P = .007). After adjustment for these independently predictive prognostic factors, the hazard rate for death from breast cancer in women with a PTHrP-positive tumor was 45% (95% CI = 26% to 80%) of that in women with a PTHrP-negative tumor (P = .003) (10). All statistical tests were two-sided.


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Table 1. Clinical and pathologic characteristics in 367 patients with breast cancer
 


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Fig. 1. A) Kaplan–Meier curves for overall survival as a function of time from diagnosis. Survival was compared for patients with parathyroid-related hormone protein (PTHrP)-positive tumors (N = 265) and patients with PTHrP-negative tumors (N = 102). O = number of observed events. Differences in survival were assessed by the log-rank test (P = .002) (8,9). The numbers of patients at risk in each group at successive survival times are shown. Subjects were considered to have died of breast cancer if there was objective evidence of metastases before death. Bone metastasis was confirmed by x-ray and scintigraphy in patients with bone pain or fracture. Local and regional recurrences were confirmed histologically. Recurrences at other accessible sites (e.g., supraclavicular lymph nodes, distant lymph node recurrence, and pleural effusion or ascites fluid) were confirmed by cytologic examination. Other sites of metastasis were confirmed by appropriate imaging. The mean length of follow-up was 70 months, and the median follow-up was 67 months (range, 3–120 months). At follow-up, 15 patients were alive with metastatic disease, 66 had died of breast cancer, and 22 had died of other causes without any evidence of recurrent breast cancer. The crude 5-year cancer-specific survivals were 87% (95% confidence interval [CI] = 82% to 91%) for patients with PTHrP-positive tumors and 73% (95% CI = 63% to 81%) for those with PTHrP-negative tumors. B) Curves for cumulative risk for development of bone metastases as a function of time from diagnosis. Survival was compared for patients with PTHrP-positive tumors (N = 265) and patients with PTHrP-negative tumors (N = 102). Differences in survival were assessed by the log-rank test (P = .001) (8,9). The crude cumulative 5-year risk for the development of bone metastases was 26% (95% CI = 19% to 37%) for patients with PTHrP-negative primary tumors and 13% (95% CI = 9% to 18%) for those with PTHrP-positive tumors. All statistical tests were two-sided.

 
Patients with PTHrP-positive primary cancers were less likely to develop bone metastases at any time during observation after surgery (Fig. 1Go, B) (8,9). Of the patients with PTHrP-positive tumors, 12% developed bone metastases in the course of observation compared with 27% of patients with PTHrP-negative cancers (P = .001). Patients with PTHrP-negative cancers were also more likely to develop metastases in the lung (22% versus 8%; P<.001), liver (14% versus 5%; P = .01), and other soft-tissue sites (15% versus 6%; P = .02). In patients with PTHrP-positive tumors and in patients with PTHrP-negative cancers, there were no statistically significant differences in development of central nervous system metastases (4% and 4%, respectively), local and/or regional recurrence (11% and 16%, respectively), or ascites (1% and 1%, respectively). Factors predictive of bone metastasis were negative PTHrP status, negative ER and PR status, lymph node status, tumor size, tumor grade, lymphatic/vascular invasion, and absence of calcification in the primary tumor but not menopausal status. The crude cumulative 5-year risk for the development of bone metastases was 26% (95% CI = 19% to 37%) for patients with PTHrP-negative primary tumors and 13% (95% CI = 9% to 18%) for those with PTHrP-positive tumors (P = .001). Cox proportional hazards model analysis (10) demonstrated that absence of PTHrP staining in the primary tumor was independently predictive of bone metastases (P = .002), as was the number of axillary lymph nodes involved with tumor (P<.001), the presence of lymphatic/vascular invasion (P = .002), and the absence of PR staining (P = .04). After adjustment for these independently predictive prognostic factors, the hazard rate for development of bone metastases in women with PTHrP-positive tumors was 39% (95% CI = 21% to 71%; P = .002) (10) of that in women with a PTHrP-negative tumor.

This study was carried out over a 10-year period as a prospective investigation of consecutively accrued patients. Consistent with previous reports (5,1115), approximately two thirds of primary breast tumors contained detectable PTHrP. The new finding is that PTHrP localization in female breast tumors is an independent predictor of improved survival and reduced risk of metastases, whether or not adjustment is made for the recognized prognostic factors—axillary lymph node status, tumor size, tumor grade, and ER and PR status. These observations are contrary to our original hypothesis and indicate that PTHrP-negative cancers are associated with more metastases and less favorable survival. Thus, we conclude that PTHrP production is associated with a less invasive phenotype. A direct causal link remains to be established. Because these results were obtained before the introduction of screening mammography, the importance of PTHrP in very early breast cancer needs to be and is currently being addressed.

When the study began, we hypothesized that patients with PTHrP-positive breast cancers would be more likely to develop bone metastases. In the intervening time, experimental support has been obtained for a local role of PTHrP in bone metastasis formation (16). More frequent and larger bone metastases were found in a nude mouse model of bone metastasis when human breast cancer cells expressing PTHrP were used (1618). Concurrent treatment with bisphosphonates, which are powerful inhibitors of bone resorption, or with neutralizing antibody against PTHrP reduced both the size and the number of metastases in this model (16,17).

The present findings, however, do not exclude a central role for PTHrP in the bone metastasis process. Specific properties of the "soil," or bone microenvironment, for successful establishment of metastases and regulation of PTHrP production have been described. The role of the bone microenvironment is emphasized by studies in the nude mouse model, in which transforming growth factor-{beta} a powerful stimulator of PTHrP production (18,19) released during tumor invasion, increases local PTHrP production by breast cancer cells, thereby promoting bone resorption and facilitating tumor growth.

We conclude that women with PTHrP-positive primary breast cancers have a more favorable outcome and have fewer metastases to bone and other sites. The nature of autocrine actions of PTHrP in primary breast tumors that contribute to this favorable outcome would be of considerable interest, and attention should now be focused on how PTHrP contributes to this protective effect against invasion and metastasis. The greater frequency of bone metastases in patients with PTHrP-negative primary breast cancers remains consistent with a local role for PTHrP in the development of bone metastases because the bone microenvironment has the potential to promote tumor cell production of PTHrP.

NOTES

Supported by the National Health and Medical Research Council of Australia, by the Anti-Cancer Council of Victoria, and by the Thomaïy Breast Cancer Research Foundation.

Presented in part at the 21st Annual Meeting of the American Society for Bone and Mineral Research, St. Louis (MO), in September 1999 and at the 90th Annual Meeting of the American Association for Cancer Research, Philadelphia (PA), in March 1999.

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Manuscript received July 6, 2000; revised November 24, 2000; accepted November 30, 2000.


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