Expression of different Jun and Fos proteins during the G0-to-G1 transition in mouse fibroblasts: in vitro and in vivo associations.

We have characterized the expression of c-Jun, JunB, JunD, c-Fos, and FosB proteins following serum stimulation of quiescent Swiss 3T3 cells by immunoprecipitation analyses. The synthesis of the three Jun proteins rapidly increases following stimulation, remaining at a significant level for at least 8 h. JunB protein presents the highest expression of all. FosB, like c-Fos, is transiently induced. Pulse-chase experiments show that all of the proteins except JunD are short-lived. We have shown that c-Fos and FosB form complexes in vivo with the different Jun proteins and that JunB complexes are predominant. In vitro association and competition experiments show that the affinities between the different Fos and Jun proteins are similar. This finding, together with the in vivo observations described above, suggests that the proportion of the different Jun/Fos heterodimers is governed by the concentration of the different components. The Fos and Jun proteins are phosphoproteins, and some remain relatively highly phosphorylated in their heterodimeric form.     

Existence of different Fos/Jun complexes during the G0-to-G1 transition and during exponential growth in mouse fibroblasts: differential role of Fos proteins.

We have determined the different Fos/Jun complexes present in Swiss 3T3 cells either following serum stimulation of quiescent cells or during exponential growth by immunoprecipitation analyses. We have shown that while c-Fos is the major Fos protein associated with the Jun proteins (c-Jun, JunB, and JunD) soon after serum stimulation, at later times Fra-1 and Fra-2 are the predominant Fos proteins associated with the different Jun proteins. During exponential growth, the synthesis of Fra-1 and Fra-2 is maintained at a significant level, in contrast to c-Fos and FosB, which are expressed at very low or undetectable levels. Consequently, Fra-1 and Fra-2 are the main Fos proteins complexed with the Jun proteins in asynchronously growing cells. To determine whether the Fos proteins are differentially required during the G0-to-G1 transition and exponential growth for the entrance into S phase, we microinjected affinity-purified antibodies directed against c-Fos, FosB, Fra-1, and Fra-2. We have found that while the activities of c-Fos and FosB are required mostly during the G0-to-G1 transition, Fra-1 and Fra-2 are involved both in the G0-to-G1 transition and in asynchronous growth.     

Interaction between the serotoninergic and dopaminergic systems in d-fenfluramine-induced activation of c-fos and jun B genes in rat striatal neurons

To test for the relative contributions of the dopaminergic and serotoninergic systems in the striatum to the effects of d-fenfluramine, an indirect serotonin receptor agonist, we assessed the expression of Fos/Jun proteins induced by d-fenfluramine given alone or in the presence of dopaminergic or serotoninergic agents. To determine the neuronal targets of d-fenfluramine in the striatum, we identified the phenotypes of striatal neurons in which d-fenfluramine induced Fos expression. Our results demonstrated that d-fenfluramine evokes nuclear expression of Fos/Jun B proteins in the striatum, and that the Fos expression was dose-dependent and accompanied by transient induction of c-fos mRNA. Fos expression was blocked by p-chloroamphetamine, a serotoninergic neurotoxin. Pretreatment with SCH 23390, a D1-dopamine receptor antagonist, led to a marked decrease in Fos/Jun B expression in the caudoputamen, but not in the cortex, whereas pretreatment with methiothepin, a nonselective serotonin 5-HT1 receptor antagonist, blocked Fos expression completely in the cortex and only partially in the caudoputamen. The expression of Fos/Jun B in the striatum occurred mainly in dynorphin-containing neurons and in a subpopulation of striatal interneurons that exhibited NADPH-diaphorase activity. Most of the enkephalin-containing neurons of the striatum did not show Fos/Jun B staining. These results suggest that the mechanism by which d-fenfluramine induces c-fos and jun B expression in the rat caudoputamen depends at least in part on activation of the dopaminergic system by serotonin.     

Transformation by Fos proteins requires a C-terminal transactivation domain.

The Fos family of proteins now includes seven members: the retroviral proteins FBR-v-Fos and FBJ-v-Fos and the cellular proteins c-Fos, FosB, FosB2, Fra1, and Fra2. Four proteins (FBR-v-Fos, FBJ-v-Fos, c-Fos, and FosB) transform established rodent fibroblast cell lines, while three (FosB2, Fra1, and Fra2) do not. As all family members display sequence-specific DNA-binding activity as part of a heterodimeric complex with Jun proteins, other features must account for the differences in transforming potential. We demonstrate here that all transforming members have a C-terminal transactivation domain that is lacking in nontransforming members. The nontransforming proteins Fra1 and Fra2 can be converted to transforming proteins by fusion of a transactivation domain from either FosB or VP16. We also demonstrate that differences in the basic region-leucine zipper domain affecting either the affinity or sequence specificity of DNA binding are not determinants of the difference in transforming potential among members of the Fos family. The results further define the functional requirements for transformation by Fos proteins and suggest that the subunit composition of AP1 complexes is an important determinant of mitogenic signalling capability.     

低、高频电针诱导原癌基因c－fos／c－jun和阿片肽基因表达及其关系的比较研究

本工作对低频（２Ｈｚ）和高频（１００Ｈｚ）电针引起大鼠中枢Ｆｏｓ／Ｊｕｎ表达和三类阿片肽基因表达进行了详细的比较研究,并用针对ｃ－ｆｏｓ／ｃ－ｊｕｎ的反义寡核苷酸（ＯＤＮｓ）对电针引起的Ｆｏｓ／Ｊｕｎ表达进行选择性阻断,然后观察阿片肽基因表达是否受到影响,以确定Ｆｏｓ／Ｊｕｎ复合物在电针引起阿片肽基因表达中的作用。主要结果如下：（１）２Ｈｚ和１００Ｈｚ电针引起脑内不同部位的Ｆｏｓ／Ｊｕｎ表达;（２）２Ｈｚ电针使前脑啡肽原（ＰＰＥ）ｍＲＮＡ表达大幅度增高,１００Ｈｚ电针也能增加ＰＰＥｍＲＮＡ的转录,但不如２Ｈｚ电针的作用显著;但１００Ｈｚ电针能使某些脑区的前强啡肽原（ＰＰＤ）基因转录加速,而２Ｈｚ电针没有这一作用;（３）用ｃ－ｆｏｓ／ｃ－ｊｕｎ反义ＯＤＮｓ特异地阻断Ｆｏｓ／Ｊｕｎ表达后,电针引起的ＰＰＤ转录加速被明显阻断,而ＰＰＥ表达不受影响。提示Ｆｏｓ／Ｊｕｎ是电针引起ＰＰＤ（而非ＰＰＥ）基因表达的转录因子。