Hepatocyte nuclear factor 1α is expressed in a hamster insulinoma line and transactivates the rat insulin I gene

Systematic mutational analysis previously identified two primary regulatory elements within a minienhancer (-247 to -198) of the rat insulin I promoter that are critical for transcriptional activity. The Far box (-241 to -232) and the FLAT element (-222 to -208) synergistically upregulate transcription and, together, are sufficient to confer tissue-specific and glucose-responsive transcriptional activity on a heterologous promoter. Detailed analysis of the FLAT element further revealed that, in addition to the positive regulatory activity it mediates in tandem with the Far box, it is a site for negative regulatory control. A portion of the FLAT element bears considerable sequence similarity to the consensus binding site for hepatocyte nuclear factor 1α (HNF1α; LF-B1), a liver-enriched homeodomain-containing transcription factor. Here we show that the HNF1-like site within the FLAT element exhibited positive transcriptional activity in both HepG2 and HIT cells and bound similar, but distinguishable, nuclear protein complexes in the respective nuclear extracts. Screening of a hamster insulinoma cDNA library with a PCR-derived probe encompassing the DNA-binding domain of rat HNF1α resulted in isolation of a hamster HNF1α (hHNF1α) cDNA homolog. Specific antiserum identified the HNF1α protein as one component of a specific FLAT-binding complex in HIT nuclear extracts. Expression of the hHNF1α cDNA in COS cells resulted in transactivation of reporter constructs containing multimerized segments of the rat insulin I minienhancer. Thus, HNF1α, one component of a DNA-binding complex involved in transcriptional regulation of the rat insulin I gene, may play a significant role in nonhepatic as well as hepatic gene transcription.