Carbonic anhydrase IX (CAIX) is a
hypoxia and HIF-1-inducible
protein that regulates intra- and extracellular pH under hypoxic conditions and promotes
tumor cell survival and invasion in hypoxic microenvironments. Interrogation of 3,630 human breast
cancers provided definitive evidence of CAIX as an independent poor prognostic
biomarker for distant
metastases and survival.
shRNA-mediated depletion of CAIX expression in 4T1 mouse metastatic
breast cancer cells capable of inducing CAIX in
hypoxia resulted in regression of orthotopic mammary
tumors and inhibition of spontaneous lung
metastasis formation. Stable depletion of CAIX in MDA-MB-231 human
breast cancer xenografts also resulted in attenuation of primary
tumor growth. CAIX depletion in the 4T1 cells led to
caspase-independent cell death and reversal of extracellular
acidosis under hypoxic conditions in vitro. Treatment of mice harboring CAIX-positive 4T1 mammary
tumors with novel CAIX-specific small molecule inhibitors that mimicked the effects of CAIX depletion in vitro resulted in significant inhibition of
tumor growth and
metastasis formation in both spontaneous and experimental models of
metastasis, without inhibitory effects on CAIX-negative
tumors. Similar inhibitory effects on primary
tumor growth were observed in mice harboring orthotopic
tumors comprised of lung metatstatic MDA-MB-231 LM2-4(Luc+) cells. Our findings show that CAIX is vital for growth and
metastasis of hypoxic
breast tumors and is a specific, targetable
biomarker for
breast cancer metastasis.