Regulation and function of tumor suppressor DAB2IP in the cellular response to mechanical inputs

The crosstalk between tumor cells and their microenvironment is a major determinant of cancer progression. Thus, understanding and eventually interfering with the dynamic interplay between external cues and cellular responses may lead to more efficient chemotherapeutic approaches. The tumor suppressor DAB2IP encodes a RasGAP and cytoplasmic adaptor that controls specificity, amplitude, and duration of intracellular signaling events in response to multiple extracellular inputs such as inflammatory cytokines, growth factors and hormones. Accordingly, loss of DAB2IP function amplifies multiple oncogenic pathways and fosters cancer aggressiveness. In addition to secreted factors, also mechanical forces from the extracellular environment significantly affect tumorigenesis, metastasis and chemoresistance; here I asked if DAB2IP may contribute to sensing and/or reacting to mechanical inputs, and if loss of function of DAB2IP would alter the response of cancer cells to mechanical cues. Using tumoral and normal mammary cell lines, I found that DAB2IP protein levels are reduced when cells are grown on a soft surface, suggesting that ECM rigidity positively affects its expression. I also observed that cell attachment sustains DAB2IP expression – possibly via Focal Adhesions (FAs). Similarly, I found that DAB2IP is downregulated in low density cultures, while upregulated at confluency, indicating that cell-cell contact controls DAB2IP levels - likely via Adherens Junctions (AJs). Finally, I verified that integrity and contractility of the actin cytoskeleton are required to maintain DAB2IP protein levels. Inspired by this evidence, I asked whether DAB2IP may in turn modulate the cell’s mechanical properties and/or the cell’s response to mechanical inputs. Preliminary experiments suggest that DAB2IP depleted cells may have larger nuclei and a higher elastic module (rigidity) when grown to confluency. Under the same conditions, I collected robust evidence that DAB2IP knockdown cells exhibit enhanced activation of YAP, a key transcriptional effector of mechanical inputs and powerful oncogene. Together, these observations suggest that DAB2IP might contribute to restrain YAP activity under conditions of contact-inhibition. The data collected in this Thesis provide the basis for further studies aimed to verify the above hypothesis, and its possible mechanism.