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The pattern of expression of homeodomain proteins often exceeds their apparent domain of activity. Tissue-specific proteins that modulate the in vivo activity of homeodomain proteins have been proposed to account for this functional restriction. The first identified example of such an accessory protein is DCoH, which confers transcriptional activity to the hepatocyte nuclear factor 1 and provides a model of how other accessory factors might modulate the function of homeodomain proteins.

Dimerization among transcription factors has become a recurrent theme in the regulation of eukaryotic gene expression. Hepatocyte nuclear factor-1 alpha (HNF-1 alpha) is a homeodomain-containing protein that functions as a dimer. A dimerization cofactor of HNF-1 alpha (DCoH) was identified that displayed a restricted tissue distribution and did not bind to DNA, but, rather, selectively stabilized HNF-1 alpha dimers. The formation of a stable tetrameric DCoH-HNF-1 alpha complex, which required the dimerization domain of HNF-1 alpha, did not change the DNA binding characteristics of HNF-1 alpha, but enhanced its transcriptional activity. However, DCoH did not confer transcriptional activation to the GAL4 DNA binding domain. These results indicate that DCoH regulates formation of transcriptionally active tetrameric complexes and may contribute to the developmental specificity of the complex.

HNF-1 alpha (previously referred to as HNF-1, LPB1, and APF) is a vertebrate transcription factor that contains a divergent homeo domain and plays a prominent role in regulating genes that have the common characteristic of being expressed in hepatocytes and a complex group of endodermally and mesodermally derived tissues. HNF-1 alpha is unique among the vertebrate homeo domain-containing proteins in that it dimerizes in the absence of its DNA recognition sequence, suggesting the possibility that the function of HNF-1 alpha may be diversified by forming heterodimers with other related proteins. We report the initial characterization of HNF-1 beta, which is closely related to HNF-1 alpha and is able to form heterodimers with HNF-1 alpha in vitro. Although HNF-1 alpha, but not HNF-1 beta, is expressed in the liver, HNF-1 alpha and HNF-1 beta are coexpressed in the murine Hepa1A cell line and in the mammalian kidney where a subset of hepatocyte genes are expressed. In contrast, exclusive expression of HNF-1 beta is associated with repression of a subset of hepatocyte-specific genes in the dedifferentiated hepatocyte cell line C2, differentiated F9 cells, in somatic hybrids between hepatocytes and fibroblasts, and in the lung. The extent of heterodimerization may be regulated in a tissue-specific way because freely exchangeable heterodimers are formed in Jurkat T cells transfected with HNF-1 alpha and HNF-1 beta, whereas in liver cells stable homodimers are present. These studies define a pair of homeo domain proteins that have the potential to interact to produce an embryologically complex pattern of gene expression.

Hepatocyte Nuclear Factor-1 alpha (HNF-1 alpha) and HNF-1 beta are homeodomain-containing transcription factors which interact with the GTTAATNATTAAC motif essential to the function of more than 15 promoters selectively expressed in the liver. These homeoproteins can form homo- and heterodimers in solution and share identical DNA-binding domains but have different transcriptional activation properties. During retinoic acid (RA) induced differentiation of F9 embryonal carcinoma (EC) cells, which stimulates aspects of pre-implantation embryogenesis, both HNF-1 beta mRNA and immunoreactive DNA-binding activity are strongly induced approximately 24 h post RA-treatment. In contrast, HNF-1 alpha mRNA increases approximately 4-fold after 5 days, concomitant with elevation of HNF-1 alpha DNA-binding activity and expression of the HNF-1 target gene alpha-fetoprotein. These results indicate that HNF-1 alpha and -1 beta expression can be controlled by regulatory hierarchies downstream of primary RA-response genes, and suggest that independent regulatory mechanisms for these factors can confer distinct and interactive developmental functions.