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Li, Ling

University of British Columbia

High expression of α6-integrins in breast cancers is correlated with poorer clinical outcomes including reduced survival, chemoresistance, radioresistance, and metastasis. The α6 subunit can heterodimerize to generate α6β1- or α6β4-integrins that bind the extracellular matrix protein, laminin. Alternative splicing of α6 generates two protein isoforms, α6A and α6B, that share common extracellular ligand-binding domains but with distinct cytoplasmic domains predicted to mediate differential outside-in integrin signaling. Previous work showed that β1-integrins function in invadopodia maturation, actin-rich protrusions that facilitate tumour cell invasion. However, the role of α6-integrins in invadopodia formation and function is largely unknown. I characterized the roles of α6-integrins in triple negative breast cancer cell lines by generating α6⁻⧸⁻ derivatives lacking α6 expression, and derivatives expressing only α6A or α6B. Compared to wildtype cells with endogenous α6A and α6B expression, α6⁻⧸⁻ cells exhibited significant defects in cell adhesion, migration and proliferation. Furthermore, α6⁻⧸⁻ cells exhibited reduced Matrigel-invasion and punctate gelatin-degradation, concomitant with loss of Tks5-positive invadopodia. Integrin α6 localizes to and is required for maturation of invadopodia, partly by regulating cortactin phosphorylation, and expression of Tks5 and MMP 14. α6⁻⧸⁻ cells re-expressing only α6A or α6B revealed previously unknown isoform-specific functions. Integrin α6A was found to mediate heterodimerization with and cell surface expression of β4. Furthermore, α6A restored the adhesion, migration, invasion and invadopodia formation deficiencies of α6⁻⧸⁻ cells, while α6B restored the adhesion and proliferation deficiencies. Despite the preferential pairing of α6A with β4, β4⁻⧸⁻ cells exhibited no defect in tumour cell invasion. In vivo, orthotopic xenografts of α6⁻⧸⁻ cells exhibited significantly reduced local tumour invasion when compared to wildtype xenografts. Local invasion was preferentially restored with α6A expression, whereas α6B contributed to the α6-mediated growth advantage of orthotopic xenografts and of subsequent metastasis. In sum, my dissertation delineates the in vitro and in vivo roles of α6-integrins and their isoforms in breast tumour cell adhesion, migration and proliferation, with particular emphasis on tumour cell invasion mediated by invadopodia. Herein, I provide empirical evidence that the unique cytoplasmic domains of integrin α6 can differentially interpret common extracellular cues to mediate an aggressive tumour phenotype leading to cancer progression.

Text

2025-01-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/87279

Pathology and Laboratory Medicine

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/