The ErbB2 receptor and breast carcinoma cell migration : memo is a novel mediator of cell motility

The ErbB family of receptor tyrosine kinases play important role in normal physiological processes occurring during development; moreover, their deregulated expression has been implicated in human cancer. Cancer patients, whose tumors have alterations in ErbB1 or ErbB2, tend to have a more aggressive disease associated with parameters predicting a poor clinical outcome, including tumor metastases. For tumors to metastasize, the cells have to possess specific characteristics, including the ability to migrate and to invade the surrounding basal membrane. The role of the Neu/ErbB2 receptor in cancer cell migration is the major topic of this thesis. The Neu/ErbB2 receptor is often overexpressed in different human tumors, including breast and ovarian tumors. Clinical and in vitro studies revealed that Neu/ErbB2 plays important functions in tumor cell motility. Upon ErbB receptors activation via ligand-induced dimerization, receptors autophosphorylate specific tyrosines in the carboxy domain leading to activation of downstream signaling cascades, including the mitogen-activated protein kinase (MAPK) and the phosphatidylinositol-3-kinase (PI3K) pathways. These pathways, which are known to be important for cell migration, are involved in actin cytoskeleton remodeling, leading to formation of lamellipodia and actin stress fibers. In this work we used T47D breast carcinoma cells expressing Neu/ErbB2 add-back mutants harboring none or only one of the five major autophosphorylation sites, to study the contribution of individual Neu/ErbB2 tyrosine autophosphorylation sites in cell migration. We showed that activation of MAPK and PI3K in T47D cell failed to induce efficient cell motility in the absence of the Neu/ErbB2 tyrosines 1201 or 1227 phosphorylation. Moreover, we present evidence that efficient, long-term cell migration depends upon ongoing transcription and translation. Signaling downstream of tyrosine 1201 and 1227 is required for de novo synthesis of RNA and protein involved in cell migration. Further investigation of the function of these two tyrosines led to the identification of a novel protein that specifically interacts with the phosphorylated tyrosine 1227. We called this new protein Memo for mediator of ErbB2-driven cell motility. Memo does not bind directly to the phosphorylated tyrosine 1227 of the Neu/ErbB2 receptor, but very likely via the adaptor molecule Shc. Memo is required for ErbB2-driven breast carcinoma cell migration, because its downregulation leads to decreased motility of cells expressing the receptor with the tyrosine 1227. Interestingly, we found that Memo is not only required for migration downstream of the ErbB receptors, but it may be a general mediator of growth factor-induced breast carcinoma cell migration. Cell migration is a multistep process and we further defined at which step Memo is required. We found that upon Neu/ErbB2 activation, wild type cells, but interestingly also Memo-deficient cells form actin stress fibers and extend lamellipodia. However, Memo-deficient cells are not able to extend microtubules toward the cell cortex. There is increasing evidence that not only the actin cytoskeleton but also the microtubule cytoskeleton plays a crucial role for cell migration. For instance, microtubules are required for the polarization of the cells and also for the transport of proteins required for motility to the cell leading edge. Further studies have to be done to understand the exact role of Memo in microtubule outgrowth and its contribution to cell motility.
In summary, the work presented in the thesis shows the identification of a novel protein, Memo, which is required for breast carcinoma cell migration. We propose that Memo controls cell motility by transmitting extracellular chemotactic signals to the microtuble cytoskeleton.