Purpose: Parathyroid hormone-related protein (PTHRP) is in part responsible for the clinical syndrome of hypercalcaemia of malignancy and has been implicated as an important factor in the development of bone metastases. The aim of this study was to determine the coexpression of PTHRP and its receptor in early breast cancer (EBC) and bone metastases (BM), and correlate these findings to clinical outcome.
Experimental design: Samples of surgically excised EBC (n = 176) and BM (n = 43) were collected and stored in liquid nitrogen. PTHRP protein was determined using immunohistochemistry and receptor mRNA using in situ hybridization (n = 107) or semiquantitative reverse transcription-PCR (n = 69).
Results: PTHRP protein was expressed in 115 of 170 (68%) EBC compared with 100% of BM (P < 0.001), whereas its receptor mRNA was expressed in 88 of 176 (50%) EBC compared with 35 of 43 (81%) BM (P < 0.001). Coexpression of both PTHRP and its receptor was present in 62 EBC samples (37%) and in 35 BM samples (81%; P < 0.001). The PTHRP receptor correlated well with increasing patient age, but not with tumor size, grade, estrogen receptor, progesterone receptor, or lymph node status. Individually PTHRP and PTHRP receptor both correlated well with a reduced disease-free survival (P < 0.004) and receptor alone with reduced overall survival (P < 0.003). Coexpression of both PTHRP and receptor predicted the worst clinical outcome at 5 years, with a mortality rate of 20 of 62 (32%) compared with the ligand and receptor-negative group with 2 of 32 (6%; P < 0.004).
Conclusions: Overall these results show that the PTHRP receptor is expressed more frequently in BM than EBC, and is associated with poor clinical outcome and survival.