Identification of novel drug resistance–associated proteins by a panel of rat monoclonal antibodies

Since some multidrug‐resistant (MDR) tumor cell lines show drug accumulation defects but do not over‐express Pgp or MDR protein (MRP), a search was made for novel MDR‐related transporter proteins by immunizing rats with non‐small cell lung cancer SW‐1573/2R120 cells to produce monoclonal antibodies (MAbs). Five rat MAbs (LMR‐4, ‐12, ‐42, ‐44 and ‐94) were generated, showing strong membranous staining of non‐Pgp MDR SW‐1573/2R120 tumor cells and minimal reactivity to the corresponding parental and revertant cell lines. In addition, a 6th MAb (LMR‐5) was isolated, recognizing the MDR‐related lung resistance protein (LRP), previously identified as the major vault protein. The first 5 LMR MAbs show predominantly membranous staining of several non‐Pgp MDR tumor cell lines of different histogenetic origins, except for LMR‐4, which recognizes only MDR sublines of the SW‐1573 cell line. Flow‐cytometric analysis revealed that all MAbs, except LMR‐4 and ‐5, detect outside epitopes. Functional studies showed that these MAbs did not restore the daunorubicin accumulation defect. All but one of the MAbs (LMR‐42) showed staining of distinct normal human tissues, notably epithelial cells lining the airways and digestive tract. In addition, staining of vascular endothelial cells was found with all MAbs except LMR‐4. Three MAbs (LMR‐12, ‐44 and ‐94) showed remarkable immunoreactivity with vincristine‐selected SW‐1573 sublines. By immunoblotting and precipitation, the LMR antigens were found to be in the 42–69 kDa range. Int. J. Cancer 73:249–257, 1997. © 1997 Wiley‐Liss, Inc.