Tissue-specific expression of the rearranged c-myc in murine plasmacytomas

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In a mouse plasmacytoma, not only the rearranged c-myc but also the non-rearranged c-myc was deregulated to be unresponsive to serum stimulation. Expression of the rearranged c-myc in the plasmacytoma was repressed after cell fusion with normal fibroblasts accompanied with suppression of tumorigenic activity. This repression of the rearranged c-myc could be also found in hybrids between plasmacytoma and liver cells or T-cell lymphoma but not in hybrids between plasmacytoma and B-cells, indicating involvement of tissue specific factors in the repression. Run-on assay showed the repression was at a transcriptional level. The repression was parallel to loss of plasmacytoma-specific proteins and reduced DNase I sensitivity of the rearranged c-myc in the hybrids. These results suggest that trans-acting factors from non-B cells act directly or indirectly to the rearranged c-myc in plasmacytomas to change chromatin configulation around the gene from open state to closed one.