Abstract
We have shown previously that cytoskeletal reorganization (CSR) induced by pharmacological reagents such as colchicine or cytochalasins can up-regulate the urokinase-type plasminogen activator (uPA) gene via the Ras/Erk signaling pathway. In this present study using the small interfering RNA technique, we have found that ShcA adapter proteins play a rather active role in CSR-induced Erk activation, contrary to their mostly redundant role in other signaling pathways, e.g. growth factor-induced Erk activation, where Grb2 can bind directly to the receptor tyrosine kinase and activate Erk in the absence of ShcA. ShcA knockdown abolished CSR-induced activation of both Erk and the uPA promoter. Expression of small interfering RNA-escaping silent mutants of p52 or p46 but not p66 ShcA isoform efficiently rescued CSR-induced Erk activation. Moreover, we have shown that phosphorylation of either Tyr-239/Tyr-240 or Tyr-313 in p52(ShcA) can mediate CSR-induced Erk activation equally well. In a quest for molecules upstream of ShcA in this signaling, we found that CSR-induced ShcA tyrosine phosphorylation, its association with Grb2, Erk activation, and uPA gene expression were all dependent on Rho kinase, p38 mitogen-activated protein kinase, and Src. In summary, we have found a novel, non-redundant role for ShcA in contrast to its redundant role in many other signaling pathways.
Highlights
There are three members of the Shc family, ShcA, ShcB, and ShcC, encoded by three different genes [7]
In this present study using the small interfering RNA technique, we have found that ShcA adapter proteins play a rather active role in cytoskeletal reorganization (CSR)-induced Erk activation, contrary to their mostly redundant role in other signaling pathways, e.g. growth factor-induced Erk activation, where Grb2 can bind directly to the receptor tyrosine kinase and activate Erk in the absence of ShcA
In a quest for molecules upstream of ShcA in this signaling, we found that CSR-induced ShcA tyrosine phosphorylation, its association with Grb2, Erk activation, and urokinase-type plasminogen activator (uPA) gene expression were all dependent on Rho kinase, p38 mitogen-activated protein kinase, and Src
Summary
There are three members of the Shc family, ShcA, ShcB, and ShcC, encoded by three different genes [7]. Expression of the largest isoform is not essential for development [16], the benefit to the organism of this particular isoform remains to be elucidated Likewise, it is not clear how important ShcA is in the regulation of growth factorinduced Ras/Erk signaling. ShcA seemed to be required for sensitizing cells and giving full induction of Erk activity at low concentrations of growth factor [14] Based on these observations, it has been suggested that p52Shc/p46Shc act as amplifiers of receptor tyrosine kinase-mediated signaling in pathways leading to Ras activation and involving a Grb2-Sos complex [10]. ShcA tyrosine phosphorylation and its association with focal adhesion kinase (FAK) and Src after CSR were observed, suggesting a role for these proteins in CSR-induced Erk activation and subsequent uPA induction [28]. Undetermined is the possible relative contribution of each ShcA isoform toward Erk activation
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