Expression of CA IX is normally restricted to the mucosa of alimentary tract, but on the other hand, it takes place in a high percentage of human
cancers derived from tissues which are normally CA IX-negative. It is a transmembrane
protein with two extracellular domains:
carbonic anhydrase (CA) with a high catalytic activity and a
proteoglycan-like segment (PG), mediating cell-cell adhesion. Both CA and PG domains interact with the microenvironment and they could play a role in
tumorigenesis, but their roles are poorly understood. The present work characterizes some newly recognized properties of the PG. One of them is a prevalently negative charge, caused by a high proportion of
dicarboxylic amino acids. This is reflected by easy dissociation of complexes formed by PG either with
monoclonal antibody M75 or with the
cell surface receptor already at slightly acidic pH. This property might facilitate separation of cells from the primary
tumor. Released cells may subsequently attach elsewhere in the organism and eventually start metastatic growth. Another aim of the present study was to identify human tumor cell lines which are expressing the presumed CA IX receptor molecule. The same cell lines were also tested for the presence of CA IX
protein; we found that expression of CA IX and of the receptor is independent of each other. In addition, we examined the species specificity of CA IX receptors. The PG domain, which contains the
epitope of mAb M75 -PGEEDLP- overlapping with the binding site for putative receptor is relatively conserved in evolution: human and rat CA IX cross-react with M75 antibody on western blots. Consistently with this, human and rat cells can attach to purified human CA IX
protein. On the other hand, murine CA IX contains an entirely different equivalent of PG sequence and it does not react with M75 antibody or attach to human CA IX
protein. This is suggestive of the co-evolution of CA IX
protein together with its receptor.