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Breast cancer bone metastases (BMET) are incurable, primarily osteolytic, and occur most commonly in estrogen receptor-α positive (ER+) breast cancer. ER+ human breast cancer BMET modeling in mice has demonstrated an estrogen (E2)-dependent increase in tumor-associated osteolysis and bone-resorbing osteoclasts, independent of estrogenic effects on tumor proliferation or bone turnover, suggesting a possible mechanistic link between tumoral ERα-driven osteolysis and ER+ bone progression. To explore this question, inducible secretion of the osteolytic factor, parathyroid hormone-related protein (PTHrP), was utilized as an in vitro screening bioassay to query the osteolytic potential of estrogen receptor- and signaling pathway-specific ligands in BMET-forming ER+ human breast cancer cells expressing ERα, ERß, and G protein-coupled ER. After identifying genomic ERα signaling, also responsibility for estrogen's proliferative effects, as necessary and sufficient for osteolytic PTHrP secretion, in vivo effects of a genomic-only ER agonist, estetrol (E4), on osteolytic ER+ BMET progression were examined. Surprisingly, while pharmacologic effects of E4 on estrogen-dependent tissues, including bone, were evident, E4 did not support osteolytic BMET progression (vs robust E2 effects), suggesting an important role for nongenomic ER signaling in ER+ metastatic progression at this site. Because bone effects of E4 did not completely recapitulate those of E2, the relative importance of nongenomic ER signaling in tumor vs bone cannot be ascertained here. Nonetheless, these intriguing findings suggest that targeted manipulation of estrogen signaling to mitigate ER+ metastatic progression in bone may require a nuanced approach, considering genomic and nongenomic effects of ER signaling on both sides of the tumor/bone interface., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Endocrine Society. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

TGFβ signaling promotes progression of bone-metastatic (BMET) breast cancer (BCa) cells by driving tumor-associated osteolysis, a hallmark of BCa BMETs, thus allowing for tumor expansion within bone. Turmeric-derived bioactive curcumin, enriched in bone via local enzymatic deconjugation of inactive circulating curcumin-glucuronides, inhibits osteolysis and BMET progression in human xenograft BCa BMET models by blocking tumoral TGFβ signaling pathways mediating osteolysis. This is a unique antiosteolytic mechanism in contrast to current osteoclast-targeting therapeutics. Therefore, experiments were undertaken to elucidate the mechanism for curcumin inhibition of BCa TGFβ signaling and the application of this finding across multiple BCa cell lines forming TGFβ-dependent BMETs, including a possible role for bioactive curcumin metabolites in mediating these effects. Immunoblot analysis of TGFβ signaling proteins in bone tropic human (MDA-SA, MDA-1833, MDA-2287) and murine (4T1) BCa cells revealed uniform curcumin blockade of TGFβ-induced Smad activation due to down-regulation of plasma membrane associated TGFβR2 and cellular receptor Smad proteins that propagate Smad-mediated gene expression, resulting in downregulation of PTHrP expression, the osteolytic factor driving in vivo BMET progression. With the exception of early decreases in TGFβR2, inhibitory effects appeared to be mediated by oxidative metabolites of curcumin and involved inhibition of gene expression. Interestingly, while not contributing to changes in Smad-mediated TGFβ signaling, curcumin caused early activation of MAPK signaling in all cell lines, including JNK, an effect possibly involving interactions with TGFβR2 within lipid rafts. Treatment with curcumin or oxidizable analogs of curcumin may have clinical relevancy in the management of TGFβ-dependent BCa BMETs., (Copyright © 2021. Published by Elsevier Inc.)

While tumoral Smad-mediated transforming growth factor β (TGFβ) signaling drives osteolytic estrogen receptor α-negative (ER-) breast cancer bone metastases (BMETs) in preclinical models, its role in ER+ BMETs, representing the majority of clinical BMETs, has not been documented. Experiments were undertaken to examine Smad-mediated TGFβ signaling in human ER+ cells and bone-tropic behavior following intracardiac inoculation of estrogen (E 2 )-supplemented female nude mice. While all ER+ tumor cells tested (ZR-75-1, T47D, and MCF-7-derived) expressed TGFβ receptors II and I, only cells with TGFβ-inducible Smad signaling (MCF-7) formed osteolytic BMETs in vivo. Regulated secretion of PTHrP, an osteolytic factor expressed in >90% of clinical BMETs, also tracked with osteolytic potential; TGFβ and E 2 each induced PTHrP in bone-tropic or BMET-derived MCF-7 cells, with the combination yielding additive effects, while in cells not forming BMETs, PTHrP was not induced. In vivo treatment with 1D11, a pan-TGFβ neutralizing antibody, significantly decreased osteolytic ER+ BMETs in association with a decrease in bone-resorbing osteoclasts at the tumor-bone interface. Thus, TGFβ may also be a driver of ER+ BMET osteolysis. Moreover, additive pro-osteolytic effects of tumoral E 2 and TGFβ signaling could at least partially explain the greater propensity for ER+ tumors to form BMETs, which are primarily osteolytic.

Breast cancer (BCa) bone metastases (BMETs) drive osteolysis via a feed-forward loop involving tumoral secretion of osteolytic factors (e.g., PTHrP) induced by bone-matrix-derived growth factors (e.g., TGFβ). In prior experiments, turmeric-derived curcumin inhibited in vivo BMET progression and in vitro TGFβ/Smad-signaling in a TGFβ-stimulated PTHrP-dependent human xenograft BCa BMET model (MDA-SA cells). However, it is unclear whether curcumin or curcumin-glucuronide mediates in vivo protection since curcumin-glucuronide is the primary circulating metabolite in rodents and in humans. Thus, effects of curcumin vs. curcumin-glucuronide on Smad-dependent TGFβ signaling were compared in a series of BCa cell lines forming TGFβ-dependent BMET in murine models, and tissue-specific metabolism of curcumin in mice was examined by LC-MS. While curcumin inhibited TGFβ-receptor-mediated Smad2/3 phosphorylation in all BCa cells studied (human MDA-SA, MDA-1833, MDA-2287 and murine 4T1 cells), curcumin-glucuronide did not. Similarly, curcumin, but not curcumin-glucuronide, blocked TGFβ-stimulated secretion of PTHrP from MDA-SA and 4T1 cells. Because the predominant serum metabolite, curcumin-glucuronide, lacked bioactivity, we examined tissue-specific metabolism of curcumin in mice. Compared to serum and other organs, free curcumin (both absolute and percentage of total) was significantly increased in bone, which was also a rich source of enzymatic deglucuronidation activity. Thus, curcumin, and not curcumin-glucuronide, appears to inhibit bone-tropic BCa cell TGFβ-signaling and to undergo site-specific activation (deconjugation) within the bone microenvironment. These findings suggest that circulating curcumin-glucuronide may act as a prodrug that preferentially targets bone, a process that may contribute to the bone-protective effects of curcumin and other highly glucuronidated dietary polyphenols., (Copyright © 2018 Elsevier Inc. All rights reserved.)