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* Department of Biochemistry, Cell adhesion molecules of the Ig superfamily are implicated in a wide variety of biological processes, including cell migration, axon guidance and fasciculation, and growth control and tumorigenesis. Expression of these proteins can be highly dynamic and
cell type specific, but little is known of the signals that
regulate such specificity. Reported here is the molecular cloning and characterization of rat CDO, a novel
cell surface glycoprotein of the Ig superfamily that contains five Ig-like repeats, followed by three fibronectin type III-like repeats in its extracellular region, and a
256-amino acid intracellular region that does not resemble other known proteins. In rat embryo fibroblasts, cdo mRNA expression is maximal in confluent, quiescent cells. It is rapidly and transiently down-regulated by serum stimulation of such cells, and is constitutively down-regulated in oncogene-transformed derivatives of these cells. CDO protein levels are also
dramatically regulated by cell-substratum adhesion, via
a mechanism that is independent of cdo mRNA expression. The amount of CDO produced at the surface of a
cell may therefore be governed by a complex balance of
signals, including mitogenic stimuli that regulate cdo
mRNA levels, and substratum-derived signals that regulate CDO protein production. cdo mRNA is expressed at low levels in most adult rat tissues. A closely
related human gene maps to chromosome 11q23-24, a
region that displays frequent loss of heterozygosity in
human lung, breast, and ovarian tumors. Taken together, these data suggest that loss of CDO function
could play a role in oncogenesis.
Department of Human Genetics, Mount Sinai School of Medicine, New York 10029; and § Zymed Laboratories, South San Francisco, California 94080
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