Regulation of immediate early gene expression.

From this brief overview of the regulation of the c-fos promoter, it can be seen that the regulation of early-response genes is a complex affair. Therefore, it is not easy to predict from the upstream sequence of a given early-response gene exactly which elements are responsible for responding to what signals in a given cell type. However, from studies of other early-response genes, it is clear that several of the elements found upstream of c-fos appear frequently and are important in the regulation of other early-response genes. For instance, the zif/268 gene has four separate CArG boxes that are similar to fos. These CArG boxes can function in the serum and TPA response, but interestingly, they are not imbedded in a region of dyad symmetry as in c-fos (Christy and Nathans 1989). Upstream of the c-jun oncogene is an AP-1 site that can modulate the expression and induction of this gene and is responsive to TPA (Angel et al. 1988). Moreover, other studies suggest that cAMP-mediated signals can repress induction of c-jun through this element (de Groot et al. 1991; Mechta et al. 1989). This would explain why in certain circumstances, such as depolarization of PC12 cells or in the striatum in response to cocaine, there is an uncoupling of the induction of c-jun and jun-B. Depolarization induces c-fos and jun-B but not c-jun; however, growth factors such as NGF can induce all three genes in the same cell (Bartel et al. 1989). Upstream of the jun-B gene there does not appear to be an SRE, but there is a new element that can be responsive to both cAMP and phorbol esters (de Groot et al. 1991). Genes such as c-myc, JE, and KC have no consensus SREs upstream, and the regulatory elements responsible for the induction of these genes have not been clearly identified (Rollins et al. 1988). However, there is some evidence from the c-myc gene that the E2F binding sites are important for its regulation by serum (Mudryj et al. 1990; Sacca and Cochran 1990). In addition, there are two SIF sites upstream of the c-myc proto-oncogene (B.H. Cochran and T.E. Hayes, unpublished results). Upstream of the nur-77 gene there are no SREs, but there are four potential calcium/CRE-like elements (Watson and Milbrandt 1989).(ABSTRACT TRUNCATED AT 400 WORDS)