Short term exposure of HeLa S3 cells to sodium butyrate induces accumulation of the mRNAs encoding both the α- and β-subunits of hCG. Both mRNAs accumulate with the same kinetics and reach maximal levels with the same concentration of butyrate, suggesting that the levels of α and hCGβ mRNAs are coordinately regulated. In addition, induction of both mRNAs is tightly coupled with a similar increase in secreted levels of hCG subunit protein, further suggesting that regulation of CG biosynthesis in HeLa cells occurs before translation. To determine whether HeLa cells which have overcome the growth inhibitory properties of butyrate show uniformly high levels of α and hCGβ mRNAs, we isolated clonal variants by stepwise selection with increasing concentrations of butyrate. Of 69 isolated variants, at least 2 (AO and B2) display different patterns of CG gene expression. In AO cells, α-subunit mRNA was not detectable, while in B2 cells, the level of α-subunit mRNA was similar to that of wild-type HeLa S3 cells. Conversely, hCGβ mRNA levels in both variant cell lines approximated levels found in butyrate-treated HeLa S3 cultures, suggesting that the mRNAs for α and hCGβ are regulated independently. Analysis of genomic DNA by blot hybridization indicated that the α-subunit gene was present as a single unrearranged copy in HeLa S3 cells and in both variants, indicating that differences in α-subunit gene expression are not associated with major genomic alterations, but probably reflect more subtle changes.
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