Characterization of three distinct size classes of human 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA: expression of the transcripts in hepatic and nonhepatic cells.

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNA is expressed in two highly differentiated human hepatoma cell lines, HepG2 and Hep3B, at exceptionally high levels relative to human fetal liver and fibroblasts. Blotting experiments revealed that the mRNA consists of three major size classes of approximately 4.7, 4.5, and 4.2 kb that responded coordinately to agents that alter HMG-CoA reductase activity. In view of the markedly elevated levels of reductase mRNA in the hepatoma cell lines, we compared the pattern of transcriptional initiation in these cells with those in normal liver and fibroblasts. These analyses revealed a complex pattern of initiation sites, all of which were suppressed by oxysterols, extending over approximately 300 nucleotides. However, all of the major sites detected in the hepatomas could also be found in human liver and fibroblasts. Heterogeneity of transcriptional initiation does not account for the three major size classes of mRNA detected by RNA blotting. RNase H mapping demonstrates that these are produced by use of three polyadenylation sites. To determine the extent to which these sites have been conserved between the human gene and the previously characterized Chinese hamster gene, we cloned and sequenced the 3' untranslated region of the longest form of the human mRNA. These studies revealed that, despite a high overall degree of sequence conservation, the spectrum of polyadenylation sites used differs qualitatively between the two species. Features of the mRNA sequence that may contribute to these differences are described.