Top Ten Misconceptions about Immediate Early Genes (IEG's)
10. Everything induces c-fos.
Considered in aggregate it appears that almost any cellular treatment will induce c-fos. However, for a given cell type and inducer this is not at all the case. In 3T3 cells for instance, IL-1 is a tremendous inducer of the immdiate early genes JE and KC, but fails to induce c-fos. Calcium ionophores on the other hand induce c-fos in PC12 cells, but have no effect on c-fos in Balb/c-3T3 cells. It should also be noted that neuronal firing is not a sufficient condition for c-fos induction in the nervous system.
9. IEG's are coordinately regulated.
Each IEG gene appears to have a unique regulatory pattern. Serum will induce c-fos, c-jun, and junB in fibroblasts, but in the striatum cocaine induces c-fos and junB but fails to induce c-jun. TNF induces jun, but not fos. In addition, some IEG's (IL-2 for instance) have tissue specific expression patterns.
8. All IEG's are transcription factors
Some IEG's are secreted (JE and KC).
Some are ras-like.
Some are enzymes like 2-5A synthetase.
See table for a more complete list.
7. c-fos induction is mediated entirely through the SRE and SRF.
Though the SRE is a well characterized element upstream of c-fos there are several other elements critical for c-fos regulation. Moreover, several proteins have been shown to interact with the SRE. Both mutations within the SRE and the context which it lies in can dramatically alter its function. Some IEG's have no SRE's at all.
6 FOS/JUN and CREB activities are readily distinguishable
Although c-fos, jun and CREB are different proteins and they differ in their DNA binding specificities, it is not easy to distinguish which protein is acting at a particular site. This is because both CREB and AP-1 can bind to an overlapping set of sequences. Some CREB and ATF family proteins can bind AP-1 sites. There are now so many CREB-like activities (ATF's) and c-fos/jun related proteins that it is virtually impossible to tell which protein is interacting at a given gene at a given time. Moreover, it has been shown that CREB and the c-fos/jun family of proteins can directly heterodimerize, further complicating the picture. A corollary of this is that the presence of an AP-1 site in a promoter does not guarantee that either CREB or AP-1 is bound there. Some AP-1 sites are inaccessible due to occlusion by other factors.
5. c-fos/jun functions require AP-1 sites.
Although c-fos and jun can interact and bind to AP-1 sites, they can also bind to related sites like CRE sites. Moreover, jun and c-fos have effects on sites to which other transcription factors bind but which do not contain an AP-1 site, such as glucocorticoid receptor elements, myoD sites, and the SRE. Thus, jun and c-fos can be important regulators of genes that have no AP-1 sites whatsoever.
4. Transformation by FOS/JUN is do to AP-1 site mediated transactivation.
Even though both fos and jun are oncogenes and can transactivate and bind to AP-1 sites, this activity is not sufficient for transactivation. GCN4 does not transform and there are mutants of fos which do transactivate, but do not transform.
3. c-fos/jun proteins are activators of gene expression.
Although jun and c-fos have transcription activation domains and activate gene expression in cotransfection experiments, they both can repress gene expression as well in the appropriate circumstance. The most well-known example of this is the auto-repression of the c-fos promoter by c-fos itself. This repression requires the C-terminus of c-fos specifically in order to act. C-jun inhibits differentiation of muscle cells by interfering with the action directly of the myoD transcription factor. The same is true for other members of the family,. For instance jun-b can heterodimerize with c-fos to up regulate some genes, but at the same time can inhibit the activation of c-jun by itself.
2. c-fos and jun control the cell cycle and drive differentiation.
Though antisense and antibody injection experiments indicate that c-fos at least is involved in cellular proliferation, its role here is not fully understood. Some experiments indicate that all family members must be inhibited to block cellular proliferation. In the case of the CSF-1 and IL-2 receptors, mutants which fail to induce c-fos and jun can still nevertheless proliferate, although perhaps not as efficiently. Moreover, knockouts of c-fos in embryonic stem cells have no clearly discernible phenotype either on the growth or differentiation of these cells.
1. c-fos/jun are completely understood.
Although much is known about c-fos and jun and their activities, we are far from a complete understanding of these two factors. Their significance in the brain or in any other tissue has not been conclusively demonstrated. Moreover, recent findings that ES cells and mice lacking c-fos have only mild phenotypes clouds the picture even further.