SummaryHere we show that Rab25 permits the sorting of ligand-occupied, active-conformation α5β1 integrin to late endosomes/lysosomes. Photoactivation and biochemical approaches show that lysosomally targeted integrins are not degraded but are retrogradely transported and recycled to the plasma membrane at the back of invading cells. This requires CLIC3, a protein upregulated in Rab25-expressing cells and tumors, which colocalizes with active α5β1 in late endosomes/lysosomes. CLIC3 is necessary for release of the cell rear during migration on 3D matrices and is required for invasion and maintenance of active Src signaling in organotypic microenvironments. CLIC3 expression predicts lymph node metastasis and poor prognosis in operable cases of pancreatic ductal adenocarcinoma (PDAC). The identification of CLIC3 as a regulator of a recycling pathway and as an independent prognostic indicator in PDAC highlights the importance of active integrin trafficking as a potential drive to cancer progression in vivo.

Chloride intracellular channel 3 (CLIC3) drives invasiveness of pancreatic and ovarian cancer by acting in concert with Rab25 to regulate the recycling of a5b1 integrin from late endosomes to the plasma membrane. Here, we show that in two estrogen receptor (ER)-negative breast cancer cell lines, CLIC3 has little influence on integrin recycling, but controls trafficking of the pro-invasive matrix metalloproteinase MT1-MMP (also known as MMP14). In MDA-MB-231 cells, MT1-MMP and CLIC3 are localized primarily to late endosomal/lysosomal compartments located above the plane of adhesion and near the nucleus. MT1-MMP is transferred from these late endosomes to sites of cell-matrix adhesion in a CLIC3-dependent fashion. Correspondingly, CLIC3-knockdown opposes MT1-MMP-dependent invasive processes. These include the disruption of the basement membrane as acini formed from MCF10DCIS.com cells acquire invasive characteristics in 3D culture, and the invasion of MDA-MB-231 cells into Matrigel or organotypic plugs of type I collagen. Consistent with this, expression of CLIC3 predicts poor prognosis in ER-negative breast cancer. The identification of MT1-MMP as a cargo of a CLIC3-regulated pathway that drives invasion highlights the importance of late endosomal sorting and trafficking in breast cancer.

Background: We previously described a Chloride Intracellular Channel-3 (CLIC3)-dependent recycling pathway which trafficked active integrins from late endosomes to the cell surface and which was required for the migratory and invasive behaviour of A2780 ovarian cancer cells in vitro. We also demonstrated that elevated expression of CLIC3 was associated with poor prognosis in pancreatic cancer. We therefore set out to investigate the role of CLIC3 in breast cancer. Materials and Methods: CLIC3 expression was assessed by immunohistochemistry and the weighted histoscore method in a tissue microarray (TMA) consisting of triplicate cores from 141 patients diagnosed with invasive estrogen receptor (ER)-negative early breast carcinoma between 1995 and 1998. Full clinicopathological and follow-up data were available. Further data were obtained from publicly available gene expression datasets (Desmedt, GSE7390) and Oncomine™. Knockdown of CLIC3 in the ER-ve MDA-MB231 breast cancer cell line was achieved by nucleofection of 2 different siRNA sequences (Dharmacon). Inverse invasion assays measured invasion into a plug of matrigel supplemented with fibronectin over 72 hours whilst organotypic invasion assays measured invasion into a fibroblast-containing collagen matrix over 6 days. Results: CLIC3 mRNA expression was significantly elevated in breast cancer in comparison to normal breast tissue in two independent data sets. High CLIC3 protein levels were associated with significantly shorter breast cancer specific survival (p = 0.026) in a TMA of 141 ER-ve patients. This finding was corroborated by the observation that high CLIC3 mRNA levels were associated with shorter overall survival in 198 patients in the Desmedt dataset (p = 0.038). Transient silencing of CLIC3 expression using two independent siRNA sequences had no effect on proliferation of MDA-MB231 cells but significantly reduced their invasiveness by 46% and 93% respectively in an inverse invasion assay and by 36% and 42% respectively in an organotypic invasion assay. CLIC3 was found to co-localise with Rab7 and LAMP1 in late endosomes/lysosomes in MDA-MB231 cells. Conclusions: Our clinical and in vitro data indicate an important role for CLIC3 in the invasive and metastatic behaviour of some breast cancers. Further work to elucidate molecular mechanisms is underway and will be presented. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P1-05-13.