Cell signaling. A sophisticated scaffold wields a new trick

Signal inhibition by a dynamically regulated pool of monophosphorylated MAPK

MAPKs are activated by dual phosphorylation. Much of the MAPK Fus3 is monophosphorylated and acts to inhibit signaling in vivo. Computational models reveal how a kinase scaffold and phosphatase act together to dynamically regulate dual-phosphorylated and monophosphorylated MAPKs and the downstream signal.

Protein kinases regulate a broad array of cellular processes and do so through the phosphorylation of one or more sites within a given substrate. Many protein kinases are themselves regulated through multisite phosphorylation, and the addition or removal of phosphates can occur in a sequential (processive) or a stepwise (distributive) manner. Here we measured the relative abundance of the monophosphorylated and dual-phosphorylated forms of Fus3, a member of the mitogen-activated protein kinase (MAPK) family in yeast. We found that upon activation with pheromone, a substantial proportion of Fus3 accumulates in the monophosphorylated state. Introduction of an additional copy of Fus3 lacking either phosphorylation site leads to dampened signaling. Conversely, cells lacking the dual-specificity phosphatase (msg5Δ) or that are deficient in docking to the MAPK-scaffold (Ste5^ND) accumulate a greater proportion of dual-phosphorylated Fus3. The double mutant exhibits a synergistic, or “synthetic,” supersensitivity to pheromone. Finally, we present a predictive computational model that combines MAPK scaffold and phosphatase activities and is sufficient to account for the observed MAPK profiles. These results indicate that the monophosphorylated and dual-phosphorylated forms of the MAPK act in opposition to one another. Moreover, they reveal a new mechanism by which the MAPK scaffold acts dynamically to regulate signaling.

Optimal ratio of scaffold complex to free Fus3 to maximise the accumulation of phosphorylated Fus3 in yeast pheromone signalling pathway

In this study, the author considers the design rule of the intracellular signalling pathway. In yeast pheromone signalling pathway, scaffold Ste5 tethers the components of signalling pathway, Ste11, Ste7 and Fus3. Even though scaffold complex is independently produced before stimuli, excessively expressed Fus3 as compared with scaffold exists in cytoplasm as free kinase. How the ratio of scaffold complex to the free Fus3 is determined is not clear yet. First, the contribution of free Fus3 to signal transduction is theoretically shown by using a simplified model of pheromone signalling pathway. Next, the optimum expression levels of Ste5, Ste11, Ste7 and Fus3 are systematically explored by using the detailed model and genetic algorithm under the constraint that the total expression level of these four genes is limited. Excessive expression of Fus3 is advantageous for the efficient signalling without stall of the signal transduction. The result suggests that the component of signalling pathway is optimally expressed to maximise the accumulation of phosphorylated Fus3 at a fixed time point under the constraint that the total gene expression is limited. The proposed model provides further insight into the signalling network from the point of view of not only its function but also its optimality.

Targeting HSP90 in ovarian cancers with multiple receptor tyrosine kinase coactivation

BACKGROUND

Ovarian cancer has the highest mortality rate of all gynecologic malignancy. The receptor tyrosine kinases (RTKs), including EGFR, ERBB2, PDGFR, VEGFR and MET, are activated in subsets of ovarian cancer, suggesting that these kinases might represent novel therapeutic targets. However, clinical trials have not or just partially shown benefit to ovarian cancers treated with EGFR, ERBB2, or PDGFR inhibitors. Despite multiple RTK activation in ovarian cancer pathogenesis, it is unclear whether transforming activity is dependent on an individual kinase oncoprotein or the coordinated activity of multiple kinases. We hypothesized that a coordinated network of multi-RTK activation is important for the tumorigenesis of ovarian cancers.

RESULTS

Herein, we demonstrate co-activation of multiple RTKs (EGFR, ERBB2, ERBB4, MET and/or AXL) in individual ovarian cancer cell lines and primary tumors. We also show that coordinate inhibition of this multi-kinase signaling has substantially greater effect on ovarian cancer proliferation and survival, compared to inhibition of individual activated kinases. The inhibition of this multi-RTK signaling by HSP90 suppression results in profound pro-apoptotic and anti-proliferative effects, and is associated with the inactivation of RTK downstream PI3-K/AKT/mTOR and RAF/MAPK signaling.

CONCLUSION

These studies suggest that anti-multiple RTK strategy could be useful in the treatment of ovarian cancer.