title: Wnt signaling induces gene expression of factors associated with bone destruction in lung and breast cancer.
abstract: Parathyroid hormone-related protein (PTHrP) is an important regulator of bone destruction in bone metastatic tumors. Transforming growth factor-beta (TGF-β) stimulates PTHrP production in part through the transcription factor Gli2, which is regulated independent of the Hedgehog signaling pathway in osteolytic cancer cells. However, inhibition of TGF-β in vivo does not fully inhibit tumor growth in bone or tumor-induced bone destruction, suggesting other pathways are involved. While Wnt signaling regulates Gli2 in development, the role of Wnt signaling in bone metastasis is unknown. Therefore, we investigated whether Wnt signaling regulates Gli2 expression in tumor cells that induce bone destruction. We report here that Wnt activation by β-catenin/T cell factor 4 (TCF4) over-expression or lithium chloride (LiCl) treatment increased Gli2 and PTHrP expression in osteolytic cancer cells. This was mediated through the TCF and Smad binding sites within the Gli2 promoter as determined by promoter mutation studies, suggesting cross-talk between TGF-β and Wnt signaling. Culture of tumor cells on substrates with bone-like rigidity increased Gli2 and PTHrP production, enhanced autocrine Wnt activity and led to an increase in the TCF/Wnt signaling reporter (TOPFlash), enriched β-catenin nuclear accumulation, and elevated Wnt-related genes by PCR-array. Stromal cells serve as an additional paracrine source of Wnt ligands and enhanced Gli2 and PTHrP mRNA levels in MDA-MB-231 and RWGT2 cells in vitro and promoted tumor-induced bone destruction in vivo in a β-catenin/Wnt3a-dependent mechanism. These data indicate that a combination of matrix rigidity and stromal-secreted factors stimulate Gli2 and PTHrP through Wnt signaling in osteolytic breast cancer cells, and there is significant cross-talk between the Wnt and TGF-β signaling pathways. This suggests that the Wnt signaling pathway may be a potential therapeutic target for inhibiting tumor cell response to the bone microenvironment and at the very least should be considered in clinical regimens targeting TGF-β signaling.
authors.affiliation: Department of Veterans Affairs, Tennessee Valley Healthcare System (VISN 9), Nashville, TN, USA.
authors.forename: Rachelle W
authors.initials: RW
authors.lastname: Johnson
authors.forename: Alyssa R
authors.initials: AR
authors.lastname: Merkel
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Page
authors.forename: Nazanin S
authors.initials: NS
authors.lastname: Ruppender
authors.forename: Scott A
authors.initials: SA
authors.lastname: Guelcher
authors.forename: Julie A
authors.initials: JA
authors.lastname: Sterling
mesh_terms: D000818:Animals; D015153:Blotting, Western; D001859:Bone Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D005260:Female; D005786:Gene Expression Regulation / Q000502:physiology*; D006801:Humans; D051741:Kruppel-Like Transcription Factors / Q000235:genetics* / Q000378:metabolism; D008175:Lung Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D051379:Mice; D008819:Mice, Nude; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D044162:Parathyroid Hormone-Related Protein / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics; D034741:RNA, Small Interfering / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000502:physiology*; D017154:Stromal Cells / Q000378:metabolism / Q000473:pathology; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism; D014407:Tumor Cells, Cultured; D060509:Wnt3A Protein / Q000502:physiology*; D023041:Xenograft Model Antitumor Assays; D000075746:Zinc Finger Protein Gli2; D051176:beta Catenin / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism
chemical_list: C082593:GLI2 protein, human; D051741:Kruppel-Like Transcription Factors; D009687:Nuclear Proteins; D044162:Parathyroid Hormone-Related Protein; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D016212:Transforming Growth Factor beta; C555510:WNT3A protein, human; D060509:Wnt3A Protein; D000075746:Zinc Finger Protein Gli2; D051176:beta Catenin