SUPPRESSION OF LCK PROTOONCOGENE EXPRESSION IN MURINE SOMATIC-CELL HYBRIDS BETWEEN T-LYMPHOMA-CELLS AND FIBROBLASTS

Somatic cell hybrids were obtained by cell fusions between Lck-positiv e EL4 mouse T lymphoma cells and Lck-negative B82 mouse fibroblasts or S194 mouse plasmacytoma cells to examine negative control of lck gene expression in the resulting hybrids. Western blot analysis using a mo noclonal antibody against the Lck protein showed a marked decrease in p56(lck) expression in B82 x EL4 (EEL) hybrids. In contrast to EEL hyb rids, the level of p56(lck) was not changed significantly in S194 x EL 4 (SEL) hybrids and was approximately one-half of that seen in EL4 cel ls. Diminished expression of the Lck protein in EEL hybrids paralleled downregulation of Ick mRNA, which was exclusively transcribed from th e distal promoter in EL4 cells. It is unlikely that the suppression wa s simply a consequence of chromosome segregation critical for lck gene expression, since EEL hybrids retained the EL4-derived lck gene and m ost of the chromosomes from both parental cells. The results from trea tment of EEL hybrids with actinomycin D or cycloheximide suggested tha t suppression of Ick gene expression in the hybrids might not be due t o posttranscriptional control. DNA methylation status in the lck dista l promoter and the coding regions did not appear to correlate with the expression of the gene. Our results suggest that negative control of Ick gene expression differs between fibroblasts and B cells, in that I ck gene expression in T cells can be shut down by transfer of a putati ve repressor factor or factors in fibroblasts but not in B cells.