Integrin-interacting proteins in human cancer progression

Abstract

Integrins are the major transmembrane receptors for the extracellular matrix (ECM) which regulate a diverse array of cellular functions crucial to tumor cell migration, invasion, proliferation and survival. Integrins contact the ECM via their N-terminal extracellular domains and connect to the intracellular environment via the C-terminal cytoplasmic domains. Therefore, studies on the integrins cytoplasmic domain binding proteins will help to better understand the mechanisms of tumor progression and make them appealing targets for cancer therapy. Kindlin and PAK (p21-activated kinase) family proteins have been identified as integrin-interacting proteins. The following studies in this thesis aimed to investigate the role of Kindlin-2 and PAK5 in human cancer progression.

In paper I, we investigated the expression of Kindlin-2 in a series of malignant mesothelioma (MM) and found it to be highly expressed and correlated to tumor cell proliferation. To evaluate the biological relevance of Kindlin-2 in MM, we also evaluated ILK (integrin-linked kinase) and Kindlin-1 expression levels. Notably, in vitro depletion of Kindlin-2 impaired tumor cell adhesion and migration. Our findings provide new evidence that Kindlin-2 contributes to MM progression and may therefore be a potential target for anti-cancer therapy in MM.

In paper II, we demonstrated a novel role of Kindlin-2 as a signaling molecule that controls a Wnt-paralleling signaling pathway. We showed that Kindlin-2 specifically activates small GTPase Cdc42, but not Rac1 and RhoA, and regulates beta-catenin activation via a Cdc42 -PAR 6 -PKCzeta -GSK-3beta cascade. Overexpression of Kindlin-2 in zebrafish embryo xenograft promotes tumor growth, invasion and dissemination. Importantly, overexpression of Kindlin-2 correlates to a poor prognosis in malignant mesothelioma patients, suggesting an important role of Kindlin-2 in cancer progression. Our data indicates that Kindlin-2 controls a signaling pathway that regulates tumor cell invasive growth.

In paper III, we used the human prostate cancer cell line PC-3 as a working model and to analyze the role of Kindlin-2 in cell cycle regulation by a loss-of-function approach. We found that depletion of Kindlin-2 causes mitotic arrest during metaphase, with cyclin B1 accumulation, mitotic spindle disruption, γ tubulin mislocation and abnormal chromosome formation. In addition, we demonstrated that Kindlin-2 is involved in Cdc42 mediated functions at metaphase. Our results identify a novel role of Kindlin-2 in the regulation of cell cycle progression in mitosis.

In paper IV, we showed that PAK5 was overexpressed in colorectal carcinoma (CRC) and associated with CRCs progression from adenoma to carcinoma. Overexpression of PAK5 also correlated to CRC development from lower Duke s grades to higher grades and correlated to CRC cell differentiation. Depletion of PAK5 reduced CRC cell adhesion but promoted their migration. Our study demonstrated that PAK5 expression correlates to CRC progression and that PAK5 promotes CRC metastasis by regulating CRC cell adhesion and migration.

Taken together, our studies highlight the importance of Kindlin-2 and PAK5 association with human cancer. This work also strengthens the link between Kindlin-2 and PAK5 expression and tumor malignancy in general, and therefore, promotes Kindlin-2 and PAK5 as novel putative targets for anti-cancer therapies.