Soluble form of carbonic anhydrase IX (CA IX) in the serum and urine of renal carcinoma patients.

Tumour-associated protein carbonic anhydrase IX (CA IX) has two major forms. One is a cell-associated, transmembrane protein seen on Western blots as a twin band of 54/58 kDa, expressed in gastric mucosa and in several types of cancer. The other is a soluble protein s-CA IX of 50/54 kDa, which is released into the culture medium or into the body fluids, most likely by proteolytic cleavage of the extracellular part from transmembrane and intracellular sequences. While TC media of CA IX-positive tumour cell lines or short-term cultures of tumour explants contain a relatively high concentration of s-CA IX (20-50 ng ml(-1)), the level of this antigen in blood serum and urine of renal clear cell carcinoma patients is about 1000 x lower. The concentration of CA IX in the blood and in urine varies within wide limits and there is no obvious correlation with tumour size. After nephrectomy, s-CA IX is cleared from the blood within a few days. Only an extremely low concentration of CA IX was detectable in the sera and in urine of control individuals.

Cloning and characterization of MN, a human tumor-associated protein with a domain homologous to carbonic anhydrase and a putative helix-loop-helix DNA binding segment.

MN is a transmembrane glycoprotein that has been detected in HeLa cells and in some human carcinomas. The expression of MN protein in HeLa cells is regulated by cell density. In HeLa x fibroblast cell hybrids its expression correlates with tumorigenicity. Using a specific monoclonal antibody we have identified a cDNA clone coding for MN. Analysis of the deduced amino acid sequence revealed strong structural homology between the central region of the MN protein and carbonic anhydrases (CA). MN sequence retains the conserved zinc-binding site as well as the enzyme's active center. In accord with these findings, MN protein from HeLa cells was found to bind zinc and to have carbonic anhydrase activity. The N-terminal region of MN shares some similarity with DNA binding proteins of the helix-loop-helix (HLH) family, and the protein was found to have affinity for DNA by DNA-cellulose chromatography. The region between the CA-like domain and the putative HLH domain is rich in imperfect repeats of serine, proline, glycine and acidic residues with few hydrophobic amino acids, resembling thus an activation region of transcription factors. The fact that MN protein is detectable in several types of human carcinomas, but not in corresponding non-cancerous tissues, suggests its possible role in neoplasia. In addition, the analysis of biological consequences of MN expression of NIH3T3 cells provides the evidence in favour of MN protein involvement in control of cell proliferation and transformation.