INTRATUMORAL HETEROGENEITY FOR AND EPIGENETIC MODULATION OF MDR-1 EXPRESSION IN MURINE MELANOMA

We determined whether tumour size in vivo and cell density in vitro mo dulate the expression of the mdr-l gene in B16 melanoma cells. Cells w ere injected subcutaneously into syngeneic mice. Small (5 mm in diamet er) and large (15-20 mm in diameter) tumours were harvested, Tumour ce lls from small subcutaneous tumours exhibited higher levels of mdr-l m RNA (measured using Northern blot and in situ hybridization) and P-gly coprotein (P-gp) (measured using immunohistochemistry and fluorescent activated cell sorter analysis), as well as greater in vitro resistanc e to doxorubicin (DXR) than cells from large subcutaneous tumours. Imm unohistochemical studies using an antibody against proliferating cell nuclear antigen revealed that the small subcutaneous tumours contained a larger fraction of proliferating cells than the large tumours. To d etermine whether cell proliferation correlated with expression of mdr- l, we plated B16-F10 cells to yield sparse and confluent monolayer cul tures. The levels of mdr-l mRNA and P-gp and resistance to DXR and pho sphotyrosine activity were higher in the sparse cultures than in the c onfluent cultures, These results demonstrate an intratumoral heterogen eity for the expression of mdr-l that directly correlates with intratu moral heterogeneity for cell division.