Identification of LRF-1, a leucine-zipper protein that is rapidly and highly induced in regenerating liver.

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Abstract
Liver regeneration provides one of the few systems for analysis of mitogenesis in the fully developed, intact animal. Several proteins have been identified as part of the primary growth response in regenerating liver and in mitogen-stimulated cells. Some of these proteins, such as the Jun and Fos families of transcription factors, are thought to have a role in activating transcription of genes expressed subsequently in the growth response. Through differential screening of a regenerating-liver cDNA library, we have identified a rapidly and highly induced gene encoding a 21-kDa leucine-zipper-containing protein that we have designated liver regeneration factor 1 (LRF-1). LRF-1 has no homology with other leucine-zipper proteins outside the basic and leucine-zipper domains. LRF-1 alone can bind DNA, but it preferentially forms heteromeric complexes with c-Jun and Jun-B and does not interact with c-Fos. In solution, it binds with highest affinity to cAMP response elements but also has affinity for related sites. In cotransfection studies, LRF-1 in combination with c-Jun strongly activates a c-Jun-responsive promoter. The induction of the LRF-1 gene in regenerating liver greatly increases the potential variety of heterodimeric combinations of leucine-zipper transcription factors. While LRF-1 mRNA is rapidly induced in the absence of protein synthesis, its peak induction is later than c-fos mRNA, suggesting that LRF-1 may regulate responsive genes at a later point in the cell cycle. As such, LRF-1 may have a unique and critical role in growth regulation of regenerating liver and mitogen-stimulated cells.