Research Aims
Multidrug Resistance
Our overall objective is to study the mechanism of multidrug resistance encoded by the ABCB1 gene and its product, P-glycoprotein (P-gp), a transmembrane pump that is responsible for drug efflux and resistance to many natural product chemotherapeutics. Cells enriched with P-gp have altered sensitivities to known anticancer drugs relative to cells that are ABCB1-negative, and ABCB1 is believed to be significant in the clinical response to anticancer therapies involving doxorubicin, paclitaxel and many other compounds.
In an attempt to overcome this form of resistance, we are currently developing modulators of MDR which inhibit the natural function of P-gp and restore sensitivity to drugs in the laboratory.
Our laboratory has identified a mutation in the ABCB1 gene which renders the cell line insensitive to MDR modulation. Studies are on-going to generate other ABCB1 gene mutations in order to determine the mechanism(s) of resistance to MDR modulation associated with alterations in P-gp structure and function. Since these MDR modulators all bind to P-gp, and most are transport substrates, such mutations will be important in defining clinically relevant structure-activity relationships of P-gp.
We are also investigating other mechanisms in cellular models which contribute to drug resistance in the clinic such as the anti-apoptotic proto-oncogene BCL2, and the BIRC family of genes which encode for the inhibitors of apoptosis proteins (e.g. c-IAP1, c-IAP2, XIAP, and Livin).
Taxane Mechanisms of Resistance
The taxanes are widely used as chemotherapy agents and have substantial clinical activity in breast, ovarian, lung, and other cancers. Despite activity in human tumors, development of drug resistance presents a serious clinical problem.
Our objective is to study mechanisms of cellular resistance to taxanes, paclitaxel (Taxol) and docetaxel (Taxotere), and the second generation taxane, cabazitaxel (Jevtana) which was approved for the treatment of hormone-refractory metastatic prostate cancer. Although the major known mechanism of resistance to these agents is multidrug resistance mediated by ABCB1, we hypothesize that the relative expression and composition of tubulin isoforms, the binding targets for these drugs, will differ in resistant cell lines which may alter drug sensitivity.
We have established cellular models of taxane resistance in ovarian and breast cancer cell lines to further study the mechanisms of resistance to this important class of drugs, to explore approaches to overcome drug resistance, and to identify biomarkers for predicting sensitivity or resistance in the clinical setting.
Topographical plots from laser cytometry reveal altered drug accumulation patterns in an ABCB1/P-glycoprotein cell model (top panel) compared to parental cells (bottom panel)
Reference
Lacayo NJ, Duran GE, Sikic BI. Modulation of resistance to idarubicin by the cyclosporin PSC 833 (valspodar) in multidrug-resistant cells. J Exp Ther Oncol 3(3):127-35, 2003.
A mutant ABCB1 gene with deletion of a phenylalanine at amino acid residue 335 in the MES-SA/DxP cell line
Reference
Chen G, Duran GE, Steger KA, Lacayo NJ, Jaffrézou JP, Dumontet C, Sikic BI. Multidrug-resistant human sarcoma cells with a mutant P-glycoprotein, altered phenotype, and resistance to cyclosporins. J Biol Chem 272(9): 5974-82, 1997.
Genomic profiling of human cancers using Stanford's Human Exonic Evidence Based Oligonucleotide (HEEBO) arrays
Reference
Schaner ME, Ross DT, Ciaravino G, Sorlie T, Troyanskaya O, Diehn M, Wang YC, Duran GE, Sikic TL, Caldeira S, Skomedal H, Tu IP, Hernandez-Boussard T, Johnson SW, O'Dwyer PJ, Fero MJ, Kristensen GB, Borresen-Dale AL, Hastie T, Tibshirani R, van de Rijn M, Teng NN, Longacre TA, Botstein D, Brown PO, Sikic BI. Gene Expression patterns in ovarian carcinomas. Mol Biol Cell 14(11): 4376-86, 2003.
Pharmacogenetics
We are further pursuing novel mechanisms of resistance to anticancer drugs by the genomic profiling of drug resistant human ovarian and breast cancer cells using microarray technology. Cellular models of resistance to various agents are being developed by step-wise and single step exposure.
We are also screening cancer specimens from patients at diagnosis and after chemotherapy, to identify gene expression patterns that may be important in the disease process as well as in determining response to treatments in the clinic. This project has the potential of identifying new genetic markers that may be useful in the detection and diagnosis of cancers, as well as genes which may be of prognostic significance, particularly with respect to prediction of sensitivity or resistance to therapies.
Projects are underway to profile the gene expression of ovarian cancers, acute leukemias, germ cell cancers, unknown primary cancers, and brain tumors.