CHANGES IN EXPRESSION OF THE CELL-ADHESION MOLECULE PECAM-1 (CD31) DURING DIFFERENTIATION OF HUMAN LEUKEMIC-CELL LINES

PECAM-1, a member of the immunoglobulin gene superfamily, is widely distributed on cells of the vascular system, and mediates cellular interactions through bath homotypic and heterotypic adhesive mechanisms. Previous studies have demonstrated that PECAM-1 is initially expressed at high levels on CD34(+) multipotential progenitors in the bone marrow, but is subsequently downregulated in more committed precursors of all lineages. Interestingly, although PECAM-1 expression is high on circulating monocytes and neutrophils, little is known about the upregulation of PECAM-1 expression during terminal myelomonocytic differentiation. We have further characterized this process by examining PECAM-1 expression during chemically-induced differentiation of the U937, HL-60 and HEL cell lines. Quantitative Western blot analysis of cellular lysates indicated that PECAM-1 expression could be upregulated in U937 and HL-60 cells by phorbol esters or dimethyl sulfoxide. Northern blot analysis showed that PECAM-1 mRNA levels appeared to increase in parallel with that of PECAM-1 protein. We also observed a marked difference in the apparent molecular mass of PECAM-1 that was lineage-specific, both in differentiated leukemic cell lines and in their corresponding leukocyte population. Immunofluorescence localization indicated that the cellular distribution of PECAM-1 in U937 and HL-60 cells was similar to that of their normal circulating counterparts, and that the pattern of distribution again displayed lineage fidelity. The ability to induce the expression of PECAM-1 molecules having different glycosylation and surface expression patterns may prove useful for further elucidation of the role of PECAM-1 in hematopoiesis, as well as studies of the cell lineage-specific modulation of PECAM-1 function.