Expression of hypoxia-lnducible cell-surface transmembrane carbonic anhydrases in human cancer

An acidic extracellular pH is a fundamental property of the malignant pheno type. In von Hippel-Lindau (VHL)-defective tumors the cell surface transmem brane carbonic anhydrase (CA) CA9 and CA12 genes are overexpressed because of the absence of pVHL. We hypothesized that these enzymes might be involve d in maintaining the extracellular acidic pH in tumors, thereby providing a conducive environment for tumor growth and spread. Using Northern blot ana lysis and immunostaining with specific antibodies we analyzed the expressio n of CA9 and CA12 genes and their products in a large sample of cancer cell lines, fresh and archival tumor specimens, and normal human tissues. Expre ssion was also analyzed in cultured cells under hypoxic conditions. Express ion of CA IX and CA XII in normal adult tissues was detected only in highly specialized cells and for most tissues their expression did not overlap. A nalysis of RNA samples isolated from 87 cancer cell lines and 18 tumors rev ealed high-to-moderate levels of expression of CA9 and CA12 in multiple can cers. Immunohistochemistry revealed high-to-moderate expression of these en zymes in various normal tissues and multiple common epithelial tumor types. The immunostaining was seen predominantly on the cell surface membrane. Th e expression of both genes was markedly induced under hypoxic conditions in tumors and cultured tumor cells. We conclude that the cell surface trans-m embrane carbonic anhydrases CA IX and CA XII are overexpressed in many tumo rs suggesting that this is a common feature of cancer cells that may be req uired for tumor progression. These enzymes may contribute to the tumor micr oenvironment by maintaining extracellular acidic pH and helping cancer cell s grow and metastasize. Our studies show an important causal link between h ypoxia, extracellular acidification, and induction or enhanced expression o f these enzymes in human tumors.