Cortactin Expression Is Tightly Connected to B-Cell Chronic Lymphocytic Leukemia Aggressiveness

Cortactin is an ubiquitous actin-binding protein, encoded by EMS1 gene and localized in chromosome 11q13 region. This protein is expressed in nearly all mammalian tissues and mostly localizes to dynamic actin structures. As a consequence, cortactin is involved in regulating several actin-dependent processes, including lamellipodium protrusion, trafficking of the key invadopodia proteases and intracellular transducer downstream to kinase-mediated cell signalling upon phosphorylation by Src and Syk tyrosine kinase families. Cortactin is over-expressed in several tumors, such as esophageal squamous cell carcinoma, head and neck squamous cell carcinoma and gastric carcinoma, and so tied to tumor aggressiveness by the promotion of cell invasion and metastasis. We previously showed that cortactin is over-expressed also in neoplastic B cells of patients with B-Cell Chronic Lymphocytic Leukemia (B-CLL). In the present study we investigated the regulation of cortactin activity by the Src kinase Lyn in neoplastic B-CLL cells and the biological implications of cortactin overexpression in this leukemia.

Twenty patients and ten normal controls were enrolled. Informed consent was obtained from all patients according to the Declaration of Helsinki. CD19+ lymphocytes were purified from peripheral blood by negative selection using the RosetteSep cells isolation kit (StemCell Technologies). Cortactin localization was performed by Confocal Microscope analysis at basal condition, in presence of the chemotactic stimulus CXCL12 and Src kinase inhibitor PP2. Lyn kinase and cortactin phosphorylation levels were evaluated by western blotting analysis. Migration assay was performed using 3 μm pore filters (Transwell Permeable Supports) in the presence of CXCL12, Src kinase inhibitor PP2 and after cortactin silencing.

We found that cortactin localization is regulated by Lyn kinase through its phosphorylation. In CLL cells, cortactin was over-phosphorylated and it did not co-localize with actin, as compared to normal cells. PP2-inhibition of Lyn decreased cortactin phosphorylation and triggered its co-localization with actin. Cortactin over-expression proved to be associated to the increased B-CLL spreading through Lyn activity. In fact, not only cortactin over-expression correlated with leukemic cell increased response to CXCL12 (r = 0.9), but also the inhibition of cortactin activity, through PP2 inhibitor or cortactin silencing, drastically reduced neoplastic cell migration after chemotactic triggering.

These results suggest that cortactin is involved in aggressiveness and spreading of B-CLL cells and that Lyn-cortactin axis could represent an alternative target for the development of new therapeutic strategies.

No relevant conflicts of interest to declare.