Overexpression of parathyroid hormone-related protein causes hypercalcemia but not bone metastases in a murine model of mammary tumorigenesis.

Several lines of evidence suggest that production of parathyroid hormone-related protein (PTHrP) by breast cancer cells contributes to the formation of bone metastases. However, it is not clear if PTHrP promotes access of cancer cells to the skeleton or if it simply promotes bone resorption around cells already within bone. To study the effects of PTHrP on the development of bone metastases, we treated mice overexpressing PTHrP in their mammary glands (K14-PTHrP transgenic mice) with 9,10-dimethyl-1,2-benz-anthracene (DMBA), a known mammary carcinogen. After DMBA treatment, K14-PTHrP mice showed a higher incidence of tumor formation and a shorter latency to tumor formation than wild-type littermates. Transgenic tumors expressed the K14-PTHrP transgene and secreted excess amounts of PTHrP. In response, tumor-bearing transgenic mice became hypercalcemic and had elevated circulating levels of PTHrP. Despite the development of visceral metastases, neither transgenic mice nor wild-type controls developed bone metastases. This was true even if tumor cells were introduced into the arterial circulation of immunodeficient mice. Our results are consistent with the emerging notion that the ability of breast cancer cells to produce PTHrP in response to cues from the bone microenvironment may be more important to the development of skeletal metastases than the production of PTHrP by cells within the primary breast cancer.

Acceleration of mouse mammary tumor virus-induced murine mammary tumorigenesis by a p53 172H transgene: influence of FVB background on tumor latency and identification of novel sites of proviral insertion.

We previously showed that a mammary-specific dominant-negative p53 transgene (WAP-p53(172H)) could accelerate ErbB2-induced mammary tumorigenesis in mice, but was not tumorigenic on its own. To identify other genes that cooperate with WAP-p53(172H) in tumorigenesis, we performed mouse mammary tumor virus (MMTV) proviral mutagenesis. We derived F1, N2, and N4/N5 mice from p53(172H) transgenic FVB mice backcrossed onto MMTV+ C3H/He mice. Results show the latency of MMTV tumorigenesis is correlated with FVB contribution. F1 tumors had the shortest latency (217 days), had a higher rate of metastasis, and were less differentiated than the N2 and N4/N5 tumors. The latency was 269 days in N2 mice, and lengthened to 346 days in N4/N5 mice. p53(172H) significantly accelerated MMTV tumorigenesis only in N2 mice, indicating cooperativity between p53(172H) and MMTV in this cohort. To identify genes that may be causally involved in MMTV-induced mammary tumorigenesis, we identified 60 sites of proviral insertion in the N2 tumors. Among the insertions in p53(172H) transgenic tumors were 10 genes not previously found as sites of MMTV insertion including genes involved in signaling (Pdgfra, Pde1b, Cnk1), cell adhesion (Cd44), angiogenesis (Galgt1), and transcriptional regulation (Olig1, Olig2, and Uncx4.1). These may represent cellular functions that are likely not deregulated by mutation in p53.