Chemotherapy often fails due to the development of drug resistance. One form of drug resistance, termed multidrug resistance (MDR) is usually associated with the increased expression of a membrane-bound drug pump, P-glycoprotein (Pgp), which functions to rapidly efflux MDR drugs from the cell. There are three classes of Pgp genes; MDR is most often caused by over-expression of the Class I genes. In laboratory models, cells exhibiting constitutive over-expression of Class I genes can be selected by continuous exposure to MDR drugs. Alternatively, Pgp expression can be rapidly and transiently induced by short-term exposure to chemotherapeutics. In patients, constitutive over-expression of Class I pgp has been associated with MDR in several tumor types. Moreover, preliminary studies in this laboratory indicate that rapid induction of Pgp can also occur in human tumors. The applicants have been investigating the cbs elements/transcription factors involved in Class I transcription, and have already identified several transcriptional elements involved in the activation of the hamster Class I promoter. PUP-1 is a bipartite element which interacts with both NF-IL6 and glucocorticoid receptor. MED-1 is a cbs element which is required only for the activation of Pgp transcription in MDR cells. TIGE is an element which is required for efficient activation of the Class I Pgp promoter. Although the initial studies have utilized the hamster Class I promoter as a model system, many of the findings also apply to the human system. Therefore, the long-range goal of this laboratory is to define and characterize the transcriptional components involved in both the constitutive and inducible expression of the human Class I homologue, MDR1. The goals of the present application are 1) to continue the analysis of the MED-1 element and its cognate binding proteins in cultured cells and human tumors; 2) to further evaluate the role of the human PUP-1 element in response to various cytokines and steroid hormones, with the long-range goal of using this approach to down-regulate Pgp expression in human MDR tumors; 3) to evaluate the role of chromatin in the differential expression of Pgp genes; and 4) to continue the analysis of transient induction of Pgp expression in human tumors and cell lines, with particular emphasis on the identification of cbs elements and transcriptional factors involved in MDR1 activation.
