Abstract
We have investigated the possibility that some serum factors might negatively regulate the expression of the insulin-like growth factor-II (IGF-II) gene in 18–54,SF cells. Northern blot analyses indicated that there were three major transcripts (3.8 kb, 1.8 kb, and 1.2 kb) of the IGF-II gene in these cells. We found that incubation of 18–54,SF cells in medium containing very high concentrations (50–100%) of chicken serum greatly inhibited the steady-state level of all three IGF-II mRNA species. In addition, we also found that incubation of 18–54,SF cells in medium containing lower concentrations (10–50%) of chicken serum induced a 3.5 kb IGF-II mRNA. The inhibitory effect of high concentrations of chicken serum on IGF-II mRNA expression was not due to a cytotoxic effect of the serum, because these cells were maintained in 100% chicken serum for up to two weeks without loss of cell viability. The inhibitory effect of chicken serum on IGF-II mRNA was reversible after withdrawl of the serum. Nuclear run-on assays suggested that this negative regulation of IGF-II mRNA in 18–54,SF cells by chicken serum was not the result of transcriptional inhibition. Treatment of 18–54,SF cells that had been previously incubated in 100% chicken serum for 24 h with actinomycin D resulted in a partial restoration of the expression of the 3.8 kb and 1.2 kb IGF-II mRNA in these cells. Finally, we also obtained a crude fraction of chicken serum and showed that treatment of 18–54.SF cells with this fraction not only inhibited the steady-state level of the 3.8 kb, 1.8 kb and 1.2 kb IGF-II mRNA, but also induced the appearance of new species of IGF-II mRNAs in these cells. These observations suggest that chicken serum may serve as a potential source for the isolation of putative factors that regulate IGF-II mRNA expression in cultured cells.
Published Version
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