Analysis of the role of liprins protein in breast cancer cell invasion.

The metastatic process requires the ability of cancer cells to break the basement mem¬brane and migrate through a complex three-dimensional environment. The laboratory has recently identified the protein liprin-α1 as an important regulator of integrin me¬diated focal adhesion dynamics and cell motility in non-neuronal cells (Asperti et al., 2009, Asperti et al., 2010). Liprins are a family of cytosolic scaffold proteins including the liprin-α and liprin-β subfamilies based on sequence similarities (Serra Pagés et al., 1998). The human genome encodes four liprin-α (liprin-α1-4) and two liprin-β proteins (liprin-β1 and liprin-β2). Interestingly, the gene PPFIA1 for liprin-α1 is frequently am-plified in tumors. Moreover, the levels of expression of the liprin-α1 protein are often increased in human breast cancers (Astro et al., 2011). Functional analysis has revealed that liprin-α1 is specifically required for migration and invasion in vitro of highly invasi¬ve MDA-MB-231 human breast cancer cells. The analysis of lamellipodia dynamics has revealed a decrease of the stability of these protrusions in cells depleted of endogenous liprin-α1, which are defective in cell motility. Furthermore, liprin-α1 silencing causes a reduction of tumor cell invasion through Matrigel. The examination of the invasive po¬tential has demonstrated that liprin-α1 is important also for the degradation of the extra¬cellular matrix (ECM) (Astro et al., 2011). Starting from these observations, the first aim of my project has been to investigate the function of liprin-α1 in vivo. I have generated MDA-MB-231-derived cell lines with either stable overexpression or stable depletion of liprin-α1, and I have used these cells for injection or transplantation in mice, to determine their invasive potential. The characterization of these cell lines in vitro has confirmed that liprin-α1 overexpression causes an increase of both cell migration on FN and invasion through Matrigel, by promoting the stability of the lamellipodia. On the other hand, li¬prin-α1-depleted cells have reduced ability to both migrate and invade in vitro. However, all the cell lines with altered liprin-α1 levels have shown similar proliferation rates and viability compared to the control MDA-MB-231 cells. To investigate the involvement of liprin-α1 in invasion in vivo, experimental metastasis assays and spontaneous metastasis assays were performed. In both assays, the formation of lung metastases by the modified and control breast cancer cell lines has been evaluated. The results indicated that liprin-α1 overexpression did not affect lung colonization. Considering the high invasive ability of MDA-MB-231 wild type cells, increase in lung colonization by liprin-α1 overexpression may be irrelevant in vivo. On the contrary, injection of liprin-α1-depleted cells resulted in the reduction of the formation of lung metastases compared to control cells. This is the first evidence that liprin-α1 is not involved in primary tumor growth, while it is important for tumor cell invasion. Being a scaffold protein, liprin-α1 is unlikely to act alone as a regulator of tumor cell invasion. Previous studies have described the interaction between liprin-α1 and liprin-β1 (Serra-Pages et al., 1998), and have suggested a possible role of liprin-β2 in migration and invasion (von Thun et al., 2012). However, the available data on the functions of li¬prin-β proteins and their relationship with liprin-α1 are not exhaustive. As the second aim of my PhD, I have addressed the biochemical interaction of liprin-α1 with either liprin-β1 or liprin-β2, and I have tried to elucidate the role of the two proteins in cell motility and invasion. While liprin-α1 interacts with liprin-β1, it does not interact with liprin-β2. This is the first evidence of the different ability of the two liprin-β proteins to interact with liprin−α1. The biochemical analysis has shown that the interaction between liprin-α1 and liprin-β1 occurs via the C-terminus of liprin-α1, and that two of the three SAM domains of liprin-α1 are sufficient to mediate this interaction. The study of the subcellular localization has indicated that liprin-β1 colocalizes with liprin-α1 at the cell edge, whereas liprin-β2 partially colocalizes with cortactin-positive invadopodia. Functional analysis has shown that liprin-β1 silencing did not affect cell invasion through matrigel, whereas liprin-β2 silencing led to an increase of cell invasion, and enhanced ECM degradation, supporting the hypothesis of the different role of this protein in regulating the function of invadopodia with respect to liprin-α1 and liprin-β1. Analysis of the involvement of liprin-β1 and liprin-β2 in cell migration underlined the different effects of the two proteins. As previously observed for liprin-α1 (Astro et al, 2011), silencing of liprin-β1 led to a decrease of the speed of the cells in random migra¬tion assays. On the contrary, liprin-β2 silencing did not significantly affect cell motility. These data support the hypothesis of a cooperation between liprin-α1 and liprin-β1 in regulating cell motility, while they indicate that liprin-β2 does not have a relevant role in this process. Altogether the work presented in my thesis sustains a key role of liprin-α1 as a positive regulator of the invasive apparatus of tumour cells in vivo, and has highlighted for the first time first time distinct roles of liprin-β1 and liprin-β2 in tumor cell motility