The biology and mechanism of chemoresistance of brain metastases.

摘要

Primary brain neoplasms and metastases to the brain are generally resistant to systemic chemotherapy. The purpose of theses studies was to determine the mechanism(s) for this resistance. We have developed a model to study the biology of brain metastasis by injecting metastatic K1735 melanoma cells into the carotid artery of syngeneic C3H/HeN or nude mice. The resulting brain lesions are produced in the parenchyma of the brain. Mice with subcutaneous or brain melanoma lesions were treated intravenously with doxorubicin (DXR) (7 mg/kg). The s.c. lesions regressed in most of the mice whereas no therapeutic benefits were produced in mice with brain metastases. The intravenous injection of sodium fluorescine revealed that the blood-brain barrier (BBB) is intact in and around brain metastases smaller than 0.2 mm;Western blot and FACS analyses revealed that K1735 melanoma brain metastases expressed high levels of P-glycoprotein (P-gp) as compared to s.c. tumors or in vitro cultures. Similarly, K1735 cells from brain metastases expressed higher levels of mdrl mRNA. This increased expression of mdrl was due to adaptation to the local brain environment. We base this conclusion on the results of two studies. First, K1735 cells from brain metastases cultured for 7 days lost the high mdrl expression. Second, in crossover experiments K1735 cells from s.c. tumors (low mdrl expression) implanted into the brain exhibited high levels of mdrl expression whereas cells from brain metastases implanted s.c. lost the high level mdrl expression.;To investigate the mechanism by which the brain environment upregulates mdrl expression of the K1735 cells we first studied the regulation of P-gp in brain endothelial cells. Since astrocytes are closely linked with the BBB we cocultured brain endothelial cells for 3 days with astrocytes. These endothelial cells expressed high levels of mdrl mRNA and protein whereas endothelial cells cocultured with endothelial cells or fibroblasts did not. We next cocultured K1735 melanoma cells with astrocytes. Here again, astrocytes (but not fibroblasts or tumor cells) uprelated the mdrl expression in K1735 tumor cells. This upregulation inversely correlated with intracellular drug accumulation and sensitivity to DXR.;The data conclude that the resistance of melanoma brain metastases to chemotherapy is not due to an intact BBB but to the upregulation of the mdrl gene by the organ microenvironment, i.e., the astrocytes. This epigenetic mediated resistance to chemotherapy has wide implications for the therapy of brain metastases.