3. Specific biological functions of HNF4α P1- and P2-derived isoforms
Given the complex nature of HNF4α isoforms regulation of expression and tissue distribution, a few studies have investigated the specific roles of these isoforms, and with often the limitation of targeting multiple members contained in either P1 or P2 isoform subgroups. Nevertheless, important observations were obtained from these studies. Using an exon swapping strategy, Briancon et al . were able to generate knock-in mouse models switching the AF-1 activation domain between HNF4α1 (1a-α1) and HNF4α7 (2a-α7) isoforms driven by P1 or P2 promoters, respectively [45]. From this strategy, it was found that mice expressing only the P1 isoform resulted in impaired glucose tolerance. In contrast, those expressing only the P2 isoform displayed a significant decrease in serum lipids and steatosis and showed alterations in the transcription of genes involved in lipid metabolism and transport [45]. When the same mouse models were used in the context of gut biology, no basal morphological changes were reported during normal homeostasis of adult mice [46]. This was not unexpected, given that the conditional knockout of Hnf4a in intestinal epithelium did not severely impact this tissue [47]. However, when experimental colitis was inflicted in these mice, an increase in colitis severity and mortality was observed in mice expressing P2-only isoform [46]. In the same report, when cancer-associated colitis was induced among these mouse models, P1-only isoform individuals exhibited a reduction in intestinal tumor size and number compared to controls. On the contrary, P2-only isoform individuals showed a larger and increased number of intestinal tumors when compared to controls [46]. Although these two previous studies highlighted some specific biological roles for P1-derived 1a-α1 and P2-derived 2a-α7 isoforms, much needs to be done to picture the global complexity of whole HNF4α isoforms. In a complementary set of experiments, Babeu et al . investigated the specific roles of P1 versus P2 groups of isoforms in the human context [48]. P1 subgroup of isoforms was found to be expressed in association with differentiated colonocytes, while P2-associated isoforms were preferentially detected in proliferating colonocytes. This pattern of expression in association with proliferation versus differentiation cell status was maintained during colon cancer, with P1 isoforms being repressed and P2 isoforms induced in the neoplastic tissues compared to their normal margins [48]. Under these circumstances, the mechanism involved in the repression of P1 isoforms was found to be dependent on β-catenin expression [48]. Using a small interfering RNA strategy, the same report identified specific classes of genes dependent on whole P1 versus P2 isoforms transcriptional activity [48] . P1-derived isoforms were involved in regulating genes associated with cell metabolism whereas P2-derived isoforms were specifically involved in regulating genes related to cell-cycle progression and survival [48]. Again, although this study highlighted specific global roles for P1 versus P2 whole classes of isoforms, further studies are needed to understand the specific role of every single isoform in these biological contexts.