Regulation of the c-myc proto-oncogene in murine lymphoid cell lines
McCormack, James E.
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The c-myc proto-oncogene, because of its activity as a transforming gene, has been implicated as a factor involved in the normal regulation of cellular proliferative status. Studies of the c-myc gene were initiated using the early B-cell lymphoma line WEHI 231, which can be induced to cease proliferation by crosslinking of its surface immunoglobulin; this is achieved here by treatment with goat anti-(mouse immunoglobulin) [GaMIg]. It is shown here that such treatment leads first to a dramatic and specific rise in the levels of c-myc mRNA, followed by a rapid drop to levels below those found in untreated cells. This drop in c-myc expression precedes a decline in DNA synthesis; these findings demonstrate a correlation between c-myc gene expression and cellular proliferative status. Further studies were aimed at examining the means by which the expression of this gene is regulated. The major findings presented here concern the decay of c-myc mRNA. This molecule is shown to decay very rapidly in exponentially growing WEHI 231 cells (T1/2 est'd 12-15 min.). Furthermore, this turnover rate is subject to modulation: A 2-3 fold increase in T1/2 is apparent in WEHI 231 shortly after exposure to GaMIg. This increase therefore partially accounts for the increase in c-myc mRNA seen following this treatment. Additional studies indicate that the 5' end of the normal c-myc transcript plays a role in conferral of this extreme lability: c-myc transcripts decay more slowly in murine plasmacytomas bearing translocated c-myc genes, which give rise to aberrant c-myc transcripts missing the normal c-myc 5' sequences (T1/2 est'd 45-70 min.). Nevertheless, these truncated transcripts turn over relatively quickly; hence, there must be other factors involved in the targeting of these transcripts for rapid degradation. A second set of studies focussed on transcriptional regulation of the c-myc gene in WEHI 231 cells following exposure to GaMIg. In vitro nuclear run-on transcription analysis demonstrates that regulation of transcription occurs at two levels--initiation of transcription, and attenuation of elongation. Taken as a whole, the work presented here demonstrates that the regulation of the c-myc gene is effected through multiple mechanisms. In the case of WEHI 231 following exposure to GaMIg, rapid and dramatic fluctuations in the levels of c-myc mRNA are the result of coordinate regulation at several sites.
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