Abstract
Tumour metastasis initially requires invasion through the basement membrane and tissue migration towards the vascular system. During these stages cancer cells are thought to use actin rich protrusions called invadopodia to facilitate matrix and basement membrane degradation. p21-activated kinases (PAKs), a family of six serine/threonine protein kinases, have been linked to cell invasion and are found to be amplified/overexpressed in a wide variety of human invasive cancer cell lines and tissue. PAK1 localises in invadopodia, and PAK4 in podosomes (a closely related structure). This study looks at the role of these two PAK isoforms in invadopodia formation and cellular invasion in melanoma.In this study a significant increase in the level of PAK1 and PAK4 expression in melanoma cell lines was demonstrated and both proteins were expressed in cell strains derived from patient samples. PAK1 overexpression correlated with melanoma cell line invasive potential, while both PAK1 and PAK4 overexpression occurred in cell lines and cell strains that formed invadopodia. Depletion of PAK1 and/or PAK4 expression resulted in a significant decrease in invasive capacity in melanoma cell lines and in an in vivo zebrafish invasion assay.
Protein depletion experiments point to differential roles for PAK1 and PAK4 during the invadopodia lifecycle. The data suggest that PAK1 promotes the formation of this protrusion while PAK4 is more important for the maturation and matrix degradation activity of invadopodia. This is the first time that PAK4 has been shown to be involved in invadopodia activity. Furthermore, this study has identified a signalling pathway, unique to PAK4, which suggests that this protein inhibits the function of PDZ-RhoGEF to promote the formation and degradation of invadopodia.
In conclusion, the data obtained from this study indicate that PAK1 and PAK4 play an important role in invadopodia activity and cellular invasion in melanoma. Distinct functions for each of these PAK isoforms in the invadopodia lifecycle has been established, with the identification of a novel PAK4 pathway involving the localised inhibition of PDZ-RhoGEF. PAK1 and PAK4 therefore present interesting therapeutic targets for the prevention/reduction of metastatic melanoma progression.
| Date of Award | 2014 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Simon Ameer-Beg (Supervisor) & Claire Wells (Supervisor) |