Cell phenotype, binding affinity and promoter structure modulate transactivation by HNF1 and LAP

To evaluate the importance of the transcription factors known to bind to the albumin promoter as well as the parameters involved in their activity, we have used cotransfections with an albumin promoter-cat plasmid combined with expression vectors driving the expression of cDNAs coding for liver-enriched factors known to interact with this promoter. We describe the characteristics of a set of clones of hepatic origin: well differentiated, partial variants or pleiotropic dedifferentiated variants. These lines have been characterized for the accumulation of RNAs corresponding to each of the albumin promoter-binding factors. Only HNF1, and to a lesser extent C/EBP, show differences depending upon the differentiation state of the cells. Overexpression of exogenous HNF1 in these cells reveals that this factor is able to transactivate the albumin promoter only in variant cells where the endogenous protein is limiting. By contrast, if the HNF1-binding site is of weak affinity, overexpression of exogenous HNF1 stimulates the albumin promoter even in the HNF1-rich differentiated cells. Overexpression of exogenous LAP strongly transactivates an artificial promoter containing one LAP-binding site, but surprisingly in all the cell lines, it has little effect upon the albumin promoter. These results demonstrate that the transactivation potential of a given transcription factor depends on the degree of differentiation of the recipient cells, on the promoter structure, and on the affinity of the binding site for this factor.