CONSTITUTIVE EXPRESSION OF MULTIDRUG-RESISTANCE IN HUMAN COLORECTAL TUMORS AND CELL-LINES

In this study we report detection of mdr1 gene expression in the liver metastases of 7/11 patients with colon carcinoma and characterise the MDR phenotype associated with a panel of 19 human colon carcinoma cel l lines. Within this panel, mdr1 mRNA biosynthesis and surface localis ation of Pgp were assessed with respect to MDR functionality where the cell lines are representative of different clinical stages of tumour progression, metastatic potential and differentiation. The data indica tes that constitutive levels of mdr1 mRNA/Pgp expression may not neces sarily result in the functional expression of the MDR phenotype. While low levels of mdr1 mRNA/Pgp were detected in 5/8 well differentiated colon cell lines, only 2/8 were functionally MDR. In contrast, 10/11 m oderate and poorly differentiated lines expressed mdr1 mRNA/Pgp and of these, 9/11 were functionally MDR. The phosphorylation status of the mature 170 kD P-glycoprotein and the surface localisation of this glyc oprotein showed the strongest correlation with functionality. Analysis of cell lines for cross-resistance and chemosensitivity profiles agai nst a battery of chemotherapeutic drugs suggests multiple mechanisms, in addition to Pgp, contribute to the overall resistance of colorectal cancer.