The role of adhesion proteins in regulating tumor cell invasion and metastasis

Abstract

Tumor cell metastasis is a multistep process that involves breaching the basement membrane, stromal invasion, intravasation and dissemination. Invadopodia are actin-rich protrusions that degrade the ECM and are required for each of these stages in metastasis. Although invadopodial actin regulators have been studied extensively, the role of adhesion proteins in regulating invadopodium function and the question of whether invadopodia adhere to ECM are unclear. Since a number of studies have linked the adhesion proteins beta1 integrin and talin to tumor invasion, I hypothesized that they may promote invadopodium-mediated matrix degradation.;My work demonstrated that invadopodia indeed adhere to ECM through the adhesion receptor beta1 integrin and that beta1 integrin promotes invadopodium maturation, specifically by interacting with the tyrosine kinase Arg. Arg binding to beta1 integrin is thought to disrupt its autoinhibited conformation to allow Arg autophosphorylation and Src-mediated Arg phosphorylation, resulting in full activation. Arg then phosphorylates cortactin to disrupt its inhibitory interaction with cofilin, culminating in cofilin-dependent actin polymerization at invadopodia, a key step in invadopodium maturation.;Cortactin binds cofilin in a pH-dependent manner, and NHE-1 increases the intracellular pH at invadopodia to disrupt this interaction. The mechanism of NHE-1 recruitment to invadopodia, however, is not known. In fibroblasts, NHE-1 is linked to the cytoskeleton by ERM proteins, and it interacts with multiple adhesion proteins, including alpha5beta1 integrin and talin. I hypothesized that talin recruits NHE-1 to invadopodia to regulate actin polymerization. Talindepletion indeed blocks invadopodium matrix degradation as well as cofilin-dependent actin polymerization. I demonstrated that talin coimmunoprecipitates with NHE-1 and moesin. Furthermore, purified talin binds directly to moesin in vitro, and talin knockdown attenuates moesin and NHE-1 enrichment at invadopodia, suggesting that talin recruits a complex of moesin-NHE-1 to invadopodia to promote their maturation. Finally, talin shRNA tumors have impaired metastasis in vivo, further supporting a role for talin in tumor cell metastasis.;Collectively, my work elucidates a role for adhesion proteins in regulating invadopodial actin polymerization through the NHE-1-cortactin-cofilin pathway. These novel mechanisms of invadopodial regulation improve our understanding of tumor metastasis and suggest that adhesion proteins may be attractive targets for anti-metastatic drug therapies.