EXPRESSION CHARACTERISTICS OF THE HUMAN CD97 ANTIGEN

Molecules whose expression is increased upon stimulation on leukocyte populations are of considerable interest because insights into their s tructure and function extend knowledge of the intracellular and interc ellular events that accompany cellular activation. One such molecule i s CD97, a cell surface antigen that is rapidly upregulated upon activa tion on lymphocytes. Increasing CD97 levels on peripheral blood lympho cytes (PBL) induced by different stimuli were found to be largely inde pendent on de novo RNA and protein synthesis during the early stage of activation. Thus, inhibition of CD97 surface expression by cyclohexim ide was not noticeable or insignificant for 15 min to 4 h after stimul ation. Furthermore, a fraction of intracellular CD97 decreased within 2 h suggesting redistribution of CD97 protein. Later, de novo protein synthesis apparently contributes to the induction of high CD97 surface density and inhibition by cycloheximide was more pronounced. Upregula tion of CD97 on PBL involves protein kinase C-, tyrosine protein kinas e-and Ca2+-dependent intracellular pathways. The effect on CD97 surfac e expression of the phorbol ester, phorbol 12-myristate 13-acetate (PM A), is different in PBL and Jurkat T cells. Whereas it stimulated afte r 22 h strong CD97 increase on PBL it suppressed CD97 expression on Ju rkat T cells. CD97 expression, which is strong and constitutive on mye lo-monocytic cells, is shown to be not leukocyte-restricted. Thus, CD9 7 transcripts were found in most of the investigated nonhematopoietic cell types and CD97 protein was detected on the cell surface at low am ount. In a previous report, sequence data of a CD97 cDNA suggested a p rotein homologous to the secretin receptor superfamily and consisting of 722 amino acids with 8 potential glycosylation sites. According to this finding, glycoproteins displaying apparent molecular weights in t he region of 70-85 kDa were detected in all investigated cell types. T hese molecules may represent differentially glycosylated and sialylate d molecular forms of the same polypeptide. The findings support the no tion that CD97 is broadly distributed and possesses a differentially r egulated expression behavior on leukocytes and non-hematopoietic cells .