IDENTIFICATION OF PECAM-1 IN SOLID TUMOR-CELLS AND ITS POTENTIAL INVOLVEMENT IN TUMOR-CELL ADHESION TO ENDOTHELIUM

PECAM-1 (CD31/EndoCAM) is an adhesion molecule in the immunoglobulin s upergene family that is expressed on endothelial cells, platelets, and some hematopoietic lineage cells. In this paper, using several polycl onal and monoclonal antibodies against PECAM-1, we identified PECAM-1 molecules on human, rat, and murine solid tumor cell lines. Immunocyto chemical labeling and flow cytometric analysis using either polyclonal , monoclonal, or Fab portion of the antibodies against PECAM-1 detecte d a distinct distribution on tumor cell surface. Immunoblotting reveal ed proteins ranging from 120 to 130 kDa in tumor cells derived from di fferent species. Immunoprecipitation and subcellular fractionation stu dies indicated that PECAM-1 is constitutively expressed on the surface of human tumor cells (i.e. colon adenocarcinoma). The specificity of a major polyclonal anti-PECAM-1 used in the current study (i.e. SEW-3) was confirmed by the preabsorption studies. PECAM-1 molecules on tumo r cells appear to bear terminal carbohydrate moieties (i.e. sialic aci d residues) different from those on platelets, since neuraminidase tre atment of tumor cells, unlike platelets, did not result in a mobility shift. Polymerase chain reaction (PCR) analysis of genomic DNA derived from tumor cell lines of different species revealed the presence of P ECAM-1 gene in the genome. The mRNAs of PECAM-1 in tumor cells were de tected by reverse transcription-PCR followed by Southern hybridization . Screening of more than 20 human, rat, and murine solid tumor cell li nes indicated that PECAM-1 is widely expressed, although the level of expression varies considerably among different cell lines. The express ion of PECAM-1 message in tumor cells was confirmed by Northern blotti ng. DNA sequencing of the PCR fragment revealed that human tumor cell PECAM-1 matches 100% to the human endothelial cell counterpart. Finall y, it was demonstrated that tumor cell PECAM-1 is involved in mediatin g tumor cell adhesion to endothelium, as evidenced by the ability of a nti-PECAM-1 antibodies to decrease the adhesion of unstimulated tumor cells to microvascular endothelial cells.