Presence of distinct AP-1 dimers in normal and transformed rat hepatocytes under basal conditions and after epidermal growth factor stimulation

✍ Scribed by F Nadori; B Lardeux; M Rahmani; A Bringuier; A Durand-Schneider; D Bernuau

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 417 KB
• Volume: 26
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.510260614

✦ Synopsis

Activation of the transcriptional regulator AP-1, a dimeric

Activator protein 1 (AP-1) transcription factor is a dimer composed of proteins of the Fos and Jun families. Character-complex formed of various combinations of Fos and Jun proteins, is a key step in the cellular response to mitogens. Be-ized members of these families include c-Jun, JunB, and JunD, [1][2][3][4] and c-Fos, FosB, Fra-1, and Fra-2. [5][6][7][8] These proteins cause different dimers are believed to display different regulatory functions, we hypothesized that transformed cells that have a characteristic leucine zipper domain that is involved in dimer formation and a basic domain allowing binding to lack normal growth constraints might display AP-1 dimers that are different from those of normal cells. We therefore the consensus DNA sequence, the phorbol 12-O-tetradecanoate-13-acetate-responsive element (TRE) or the AP-1 site compared in primary and transformed rat hepatocytes (1) the composition of AP-1 dimers under basal conditions and (2) (TGAC/GTCA; for review, see Angel and Karin 9 ) present on the promoter of responsive genes. Whereas Jun family pro-AP-1 induction by epidermal growth factor (EGF). Under basal conditions, AP-1 contained predominantly Jun homodi-teins are able to bind DNA as homodimers, Fos proteins only form heterodimers with Jun proteins, which are more stable mers in both cell types. However, whereas normal hepatocytes contained only JunD, both JunD and JunB were present in the and have greater transactivating capacities than Jun homodimers. [10][11][12] Members of the Jun and Fos families can also form AP-1 complex of 7777 cells. EGF treatment triggered almost identical programs of fos and jun gene activation at the mes-heterodimers with members of the ATF/CREB family, 13,14 which bind to the adenosine 3,5-cyclic monophosphate senger RNA (mRNA) level in both cell types, with an early accumulation of c-fos, c-jun, and junB mRNAs, but no change (cAMP)-responsive element consensus sequence (TGA-CGTCA) and more weakly to the TRE site. 13-15 in junD mRNA levels. In both cell types, c-Fos and Fra-1 proteins increased after EGF treatment, but differences in the It is very likely that the transcription-modulating properties of the various heterodimeric combinations of Jun and Fos induction of Jun proteins were noted, with an increase of c-Jun in hepatocytes and an increase of JunB in 7777 cells. In proteins are different. [16][17][18] JunD appears to be an inefficient transcription activator, 4 at least in the absence of c-Fos, both cell types, activation of AP-1 DNA binding activity by EGF was accompanied by the recruitment of Fra-1 into AP-whereas JunB is considered to be an inactivator of c-Jun. 16 The Fos family proteins also differ in their transcription-1, detected earlier in 7777 cells than in hepatocytes, and with the transient appearance of c-Fos in 7777 cells only. Finally, regulating functions. 19 Studies in vivo have shown that there are distinct Fos/Jun complexes at specific phases of the cell EGF activated AP-1-dependent transcription in 7777 cells but not in hepatocytes. These data indicate important differences cycle during the proliferative response of fibroblastic cells or hepatocytes. 14,20 These data suggest that specific Jun/Fos in the functional activity of AP-1 in transformed hepatocytes.

heterodimers or Jun/Jun homodimers are involved in the reg-(HEPATOLOGY 1997;26:1477-1483.) ulation of distinct genetic programs during cell stimulation.

The AP-1-transcription factor is centrally involved in the responses of the cell to environmental stimuli, leading to proliferation or differentiation. 9 Growth factors induce Abbreviations: AP-1, activator protein 1; TRE, phorbol 12-O-tetradecanoate-13changes in AP-1 activity that are regulated by transcription acetate-responsive element; cAMP, adenosine 3,5-cyclic monophosphate; EGF, epiof the jun and fos genes and by posttranslational modificadermal growth factor; FBS, fetal bovine serum; cRNA, complementary RNA; Rnase, tions of preexisting AP-1. AP-1 induction is believed to conribonuclease; EDTA, ethylenediaminetetraacetic acid; bp, base pair; GAPDH, glyceralvert immediate external signals into longer-lasting changes dehyde-3-phosphate dehydrogenase; PAGE, polyacrylamide gel electrophoresis; ATP, adenosine triphosphate; SDS, sodium dodecyl sulfate; PBS, phosphate-buffered saline; in the transcription of cellular target genes. 21 Because cellular PBST, phosphate-buffered saline and Tween; CAT, chloramphenicol acetyltransferase; transformation is characterized by the loss of normal growth EMSA, electrophoretic mobility shift assay; OA, okadaic acid.

constraints and by abnormal responses to growth factors and From the Laboratoire de Biologie Cellulaire, INSERM Unite ´327, Faculte ´de Me ´deother environmental proliferative stimuli, it has been sug-