PTHrP intracrine actions divergently influence breast cancer growth through p27 and LIFR.

The role of parathyroid hormone (PTH)-related protein (PTHrP) in breast cancer remains controversial, with reports of PTHrP inhibiting or promoting primary tumor growth in preclinical studies. Here, we provide insight into these conflicting findings by assessing the role of specific biological domains of PTHrP in tumor progression through stable expression of PTHrP (-36-139aa) or truncated forms with deletion of the nuclear localization sequence (NLS) alone or in combination with the C-terminus. Although the full-length PTHrP molecule (-36-139aa) did not alter tumorigenesis, PTHrP lacking the NLS alone accelerated primary tumor growth by downregulating p27, while PTHrP lacking the NLS and C-terminus repressed tumor growth through p27 induction driven by the tumor suppressor leukemia inhibitory factor receptor (LIFR). Induction of p27 by PTHrP lacking the NLS and C-terminus persisted in bone disseminated cells, but did not prevent metastatic outgrowth, in contrast to the primary tumor site. These data suggest that the PTHrP NLS functions as a tumor suppressor, while the PTHrP C-terminus may act as an oncogenic switch to promote tumor progression through differential regulation of p27 signaling.

Sex-Specific Differences in Gsα-Mediated Signaling Downstream of PTH1R Activation by Abaloparatide in Bone.

Teriparatide, recombinant parathyroid hormone (PTH[1-34]), and abaloparatide, an analogue of PTH related-peptide (PTHrP[1-34]), are both anabolic medications for osteoporosis that target the PTH receptor PTH1R. PTH1R is a G protein-coupled receptor, and the stimulatory Gs protein is an important mediator of the anabolic actions of PTH1R activation in bone. We have published that mice lacking the α subunit of Gs in osteoprogenitors do not increase bone mass in response to PTH(1-34). Unexpectedly, however, PTH(1-34) still increases osteoblast numbers and bone formation rate in male mice, suggesting that PTH1R may have both Gs-dependent and -independent actions in bone. Here we examine the role of Gs signaling in the anabolic actions of abaloparatide. We find that abaloparatide increases bone formation in male mice with postnatal deletion of Gsα in Osx-expressing osteoprogenitors (P-GsαOsxKO mice) but not in female P-GsαOsxKO mice. Therefore, abaloparatide has anabolic effects on bone in male but not female mice that appear to be independent of Gs-mediated signaling. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

HDAC inhibitors stimulate LIFR when it is repressed by hypoxia or PTHrP in breast cancer.

Breast cancer cells frequently disseminate to the bone marrow, where they either induce osteolysis or enter a dormant state. Downregulation of leukemia inhibitory factor receptor (LIFR), a known breast tumor suppressor, enables otherwise dormant MCF7 human breast cancer cells to become aggressively osteolytic. Hypoxia (low oxygen tensions), which may develop in tumors as a pathological response to the metabolic demands of the proliferating cells and as a physiological state in the bone, downregulates LIFR in breast cancer cells independent of hypoxia-inducible factor (HIF) signaling. However, the mechanism by which LIFR is repressed in hypoxia is unknown. Histone deacetylase (HDAC) inhibitors stimulate LIFR by increasing histone acetylation in the proximal promoter and induce a dormancy phenotype in breast cancer cells inoculated into the mammary fat pad. We therefore aimed to determine whether hypoxia alters histone acetylation in the LIFR promoter, and whether HDAC inhibitors effectively stimulate LIFR in breast cancer cells residing in hypoxic microenvironments. Herein, we confirmed that disseminated MCF7 cells became hypoxic in the bone and that hypoxia increased the epigenetic transcriptional repressor H3K9me3 in the distal LIFR promoter while H3K9ac, which promotes transcription, was significantly reduced. Furthermore, HDAC inhibitor treatment rescued hypoxic repression and dramatically increased expression of LIFR, p38ß, and p21, which regulate tumor dormancy. In a second model of LIFR repression, in which parathyroid hormone-related protein (PTHrP) suppresses LIFR expression, we found that PTHrP binds to the distal LIFR promoter, and that PTHrP suppression of LIFR protein is similarly reversed by HDAC inhibitor treatment. Together, these data suggest that HDAC inhibitors stimulate LIFR regardless of the way it is repressed by the microenvironment.

Hypoxia inducible factor signaling in breast tumors controls spontaneous tumor dissemination in a site-specific manner.

Hypoxia is a common feature in tumors and induces signaling that promotes tumor cell survival, invasion, and metastasis, but the impact of hypoxia inducible factor (HIF) signaling in the primary tumor on dissemination to bone in particular remains unclear. To better understand the contributions of hypoxia inducible factor 1 alpha (HIF1α), HIF2α, and general HIF pathway activation in metastasis, we employ a PyMT-driven spontaneous murine mammary carcinoma model with mammary specific deletion of Hif1α, Hif2α, or von Hippel-Lindau factor (Vhl) using the Cre-lox system. Here we show that Hif1α or Hif2α deletion in the primary tumor decreases metastatic tumor burden in the bone marrow, while Vhl deletion increases bone tumor burden, as hypothesized. Unexpectedly, Hif1α deletion increases metastatic tumor burden in the lung, while deletion of Hif2α or Vhl does not affect pulmonary metastasis. Mice with Hif1α deleted tumors also exhibit reduced bone volume as measured by micro computed tomography, suggesting that disruption of the osteogenic niche may be involved in the preference for lung dissemination observed in this group. Thus, we reveal that HIF signaling in breast tumors controls tumor dissemination in a site-specific manner.