DETECTION OF A SECRETED MUC1 SEC PROTEIN BY MUC1 ISOFORM-SPECIFIC MONOCLONAL-ANTIBODIES/

Although MUC1 proteins are known to be secreted by breast cancer cells , the mechanism of their release from the cell is still obscure. Our p reviously reported MUC1 cDNA sequences suggested the existence of a se creted MUC1 isoform, MUC1/SEC, that includes a sequence of intron 2, a nd terminates prematurely at a stop codon within this intron. It is th us devoid of a transmembrane domain. As no formal evidence for MUC1/SE C expression at the protein level had been provided, we generated mono clonal antibodies (mAbs) against a peptide sequence (sec peptide) that is unique for the MUC1/SEC protein. Two anti-sec peptide mAbs were ob tained which reacted strongly with (a) the immunizing peptide, (b) rec ombinant MUC1/SEC protein, and (c) MUC1 proteins secreted from breast cancer cells. The immunoreactivity of the anti-sec peptide mAbs with M UC1 proteins secreted by breast cancer cells was specifically inhibite d by the sec peptide-it was completely unaffected by a peptide sequenc e that represents a MUC1 repeat motif. Significantly, the anti-sec pep tide mAbs also detected MUC1/SEC protein in sera of breast cancer pati ents. We have established here that these mAbs recognize the MUC1/SEC isoform via a peptide sequence which is unique for the MUC1/SEC protei n. Our studies thus demonstrate that the MUC1/SEC protein is a bona-fi de MUC1 isoform and that its expression may contribute to the secretio n of MUC1 proteins by secretory epithelial cells in general and breast cancer cells in particular. (C) 1996 Academic Press, Inc.