Hypoxia and TGF-beta drive breast cancer bone metastases through parallel signaling pathways in tumor cells and the bone microenvironment.
BACKGROUND
Most patients with advanced breast cancer develop bone metastases, which cause pain, hypercalcemia, fractures, nerve compression and paralysis. Chemotherapy causes further bone loss, and bone-specific treatments are only palliative. Multiple tumor-secreted factors act on the bone microenvironment to drive a feed-forward cycle of tumor growth. Effective treatment requires inhibiting upstream regulators of groups of prometastatic factors. Two central regulators are hypoxia and transforming growth factor (TGF)- beta. We asked whether hypoxia (via HIF-1alpha) and TGF-beta signaling promote bone metastases independently or synergistically, and we tested molecular versus pharmacological inhibition strategies in an animal model.


RESULTS
We analyzed interactions between HIF-1alpha and TGF-beta pathways in MDA-MB-231 breast cancer cells. Only vascular endothelial growth factor (VEGF) and the CXC chemokine receptor 4 (CXCR4), of 16 genes tested, were additively increased by both TGF-beta and hypoxia, with effects on the proximal promoters. We inhibited HIF-1alpha and TGF-beta pathways in tumor cells by shRNA and dominant negative receptor approaches. Inhibition of either pathway decreased bone metastasis, with no further effect of double blockade. We tested pharmacologic inhibitors of the pathways, which target both the tumor and the bone microenvironment. Unlike molecular blockade, combined drug treatment decreased bone metastases more than either alone, with effects on bone to decrease osteoclastic bone resorption and increase osteoblast activity, in addition to actions on tumor cells.


CONCLUSIONS
Hypoxia and TGF-beta signaling in parallel drive tumor bone metastases and regulate a common set of tumor genes. In contrast, small molecule inhibitors, by acting on both tumor cells and the bone microenvironment, additively decrease tumor burden, while improving skeletal quality. Our studies suggest that inhibitors of HIF-1alpha and TGF-beta may improve treatment of bone metastases and increase survival.
Dunn
Lauren K
LK
Division of Endocrinology, Department of Medicine, University of Virginia, Charlottesville, Virginia, United States of America.
Mohammad
Khalid S
KS
Fournier
Pierrick G J
PG
McKenna
C Ryan
CR
Davis
Holly W
HW
Niewolna
Maria
M
Peng
Xiang Hong
XH
Chirgwin
John M
JM
Guise
Theresa A
TA
D000818:Animals; D001859:Bone Neoplasms / Q000378:metabolism / Q000556:secondary*; D001842:Bone and Bones / Q000473:pathology; D001943:Breast Neoplasms / Q000378:metabolism* / Q000473:pathology; D045744:Cell Line, Tumor; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D000860:Hypoxia; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D051379:Mice; D009362:Neoplasm Metastasis; D019718:Receptors, CXCR4 / Q000378:metabolism; D015398:Signal Transduction; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D042461:Vascular Endothelial Growth Factor A / Q000378:metabolism
C527276:CXCR4 protein, human; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D019718:Receptors, CXCR4; D016212:Transforming Growth Factor beta; D042461:Vascular Endothelial Growth Factor A