Modulation of the activity of multiple transcriptional activation domains by the DNA binding domains mediates the synergistic action of Sox2 and Oct-3 on the fibroblast growth factor-4 enhancer.

Fibroblast growth factor (FGF)-4 gene expression in the inner cell mass of the blastocyst and in EC cells requires the combined activity of two transcriptional regulators, Sox2 and Oct-3, which bind to adjacent sites on the FGF-4 enhancer DNA and synergistically activate transcription. Sox2 and Oct-3 bind cooperatively to the enhancer DNA through their DNA-binding, high mobility group and POU domains, respectively. These two domains, however, are not sufficient to activate transcription. We have analyzed a number of Sox2 and Oct-3 deletion mutants to identify the domains within each protein that contribute to the activity of the Sox2 x Oct-3 complex. Within Oct-3, we have identified two activation domains, the N-terminal AD1 and the C-terminal AD2, that play a role in the activity of the Sox2 x Oct-3 complex. AD1 also displays transcriptional activation functions in the absence of Sox2 while AD2 function was only detected within the Sox2 x Oct-3 complex. In Sox2, we have identified three activation domains within its C terminus: R1, R2, and R3. R1 and R2 can potentiate weak activation by Sox2 in the absence of Oct-3 but their deletion has no effect on the Sox2 x Oct-3 complex. In contrast, R3 function is only observed when Sox2 is complexed with Oct-3. In addition, analysis of Oct-1/Oct-3 chimeras indicates that the Oct-3 homeodomain also plays a critical role in the formation of a functional Sox2 x Oct-3 complex. Our results are consistent with a model in which the synergistic action of Sox2 and Oct-3 results from two major processes. Cooperative binding of the factors to the enhancer DNA, mediated by their binding domains, stably tethers each factor to DNA and increases the activity of intrinsic activation domains within each protein. Protein-protein and protein-DNA interactions then may lead to reciprocal conformational changes that expose latent activation domains within each protein. These findings define a mechanism that may also be utilized by other Sox x POU protein complexes in gene activation.

FGF signaling inhibits chondrocyte proliferation and regulates bone development through the STAT-1 pathway.

Several genetic forms of human dwarfism have been linked to activating mutations in FGF receptor 3, indicating that FGF signaling has a critical role in chondrocyte maturation and skeletal development. However, the mechanisms through which FGFs affect chondrocyte proliferation and differentiation remain poorly understood. We show here that activation of FGF signaling inhibits chondrocyte proliferation both in a rat chondrosarcoma (RCS) cell line and in primary murine chondrocytes. FGF treatment of RCS cells induces phosphorylation of STAT-1, its translocation to the nucleus, and an increase in the expression of the cell-cycle inhibitor p21WAF1/CIP1. We have used primary chondrocytes from STAT-1 knock-out mice to provide genetic evidence that STAT-1 function is required for the FGF mediated growth inhibition. Furthermore, FGF treatment of metatarsal rudiments from wild-type and STAT-1(-/-) murine embryos produces a drastic impairment of chondrocyte proliferation and bone development in wild-type, but not in STAT-1(-/-) rudiments. We propose that STAT-1 mediated down regulation of chondrocyte proliferation by FGF signaling is an homeostatic mechanism which ensures harmonious bone development and morphogenesis.

Synergistic activation of the fibroblast growth factor 4 enhancer by Sox2 and Oct-3 depends on protein-protein interactions facilitated by a specific spatial arrangement of factor binding sites.

Octamer binding and Sox factors are thought to play important roles in development by potentiating the transcriptional activation of specific gene subsets. The proteins within these factor families are related by the presence of highly conserved DNA binding domains, the octamer binding protein POU domain or the Sox factors HMG domain. We have previously shown that fibroblast growth factor 4 (FGF-4) gene expression in embryonal carcinoma cells requires a synergistic interaction between Oct-3 and Sox2 on the FGF-4 enhancer. Sox2 and Oct-3 bind to adjacent sites within this enhancer to form a ternary protein-DNA complex (Oct-3*) whose assembly correlates with enhancer activity. We now demonstrate that increasing the distance between the octamer and Sox binding sites by base pair insertion results in a loss of enhancer function. Significantly, those enhancer "spacing mutants" which failed to activate transcription were also compromised in their ability to form the Oct* complexes even though they could still bind both Sox2 and the octamer binding proteins, suggesting that a direct interaction between Sox2 and Oct-3 is necessary for enhancer function. Consistent with this hypothesis, Oct-3 and Sox2 can participate in a direct protein-protein interaction in vitro in the absence of DNA, and both this interaction and assembly of the ternary Oct* complexes require only the octamer protein POU and Sox2 HMG domains. Assembly of the ternary complex by these two protein domains occurs in a cooperative manner on FGF-4 enhancer DNA, and the loss of this cooperative interaction contributes to the defect in Oct-3* formation observed for the enhancer spacing mutants. These observations indicate that Oct-3* assembly results from protein-protein interactions between the domains of Sox2 and Oct-3 that mediate their binding to DNA, but it also requires a specific arrangement of the binding sites within the FGF-4 enhancer DNA. Thus, these results define one parameter that is fundamental to synergistic activation by Sox2 and Oct-3 and further emphasize the critical role of enhancer DNA sequences in the proper assembly of functional activation complexes.

Synthetic alleles at position 121 define a functional domain of human interleukin-1 beta.

The non-conservative substitution of the tyrosine residue at position 121 of human interleukin-1 beta (IL-1 beta) generates protein mutants showing strong reduction of the capacity to induce (a) prostaglandin E2 (PGE2) release from fibroblasts and smooth muscle cells, (b) murine T-cells proliferation and (c) activation of interleukin-6 (IL-6) gene expression. It is generally accepted that these functions are mediated by the type-I interleukin-1 receptor (IL-1RI). However, the mutant proteins maintain the binding affinity to the types-I and II IL-1 receptors, which is the same as the control IL-1 beta, suggesting that this amino acid substitution does not alter the structure of the molecule, except locally. Thus we have identified a new functional site of IL-1 beta different from the known receptor binding region, responsible for fundamental IL-1 beta functions. Moreover, we show that the same mutants maintain at least two hypothalamic functions, that is, the in vitro short-term PGE2 release from rat hypothalamus and the induction of fever in rabbits. This result suggests that there is yet another site of the molecule responsible for the hypothalamic functions, implying that multiple active sites on the IL-1 beta molecule, possibly binding to more than one receptor chain, trigger different signals.