Curcumin Inhibition of TGFβ signaling in bone metastatic breast cancer cells and the possible role of oxidative metabolites

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.