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    2020-08-28


    melanoma 108084-47-5 [52,53]. Indeed, silencing of IL32 in CAFs partly in-creased tumour growth in mice in the present study. Nevertheless, some evidence indicates that IL32 closely correlates with tumour malignancy (e.g. oesophageal cancer) [54], but the underlining mechanism is 
    unclear. In the present report, we show that CAF-secreted IL32 can promote breast cancer cell invasion through interaction with integrin β3 at the plasma membrane of breast cancer cells. Treatment of these cells with rIL32 caused marked invasion of breast cancer cells in vitro
    (caption on next page)
    Fig. 7. IL32 promotes lung metastasis of breast cancer xenografts via integrin β3. (A) The tumour sizes of each group. (B) The curves of tumour growth in mice (*P < 0.05). (C) Representative images of pulmonary metastases examined by H&E staining in lung sections. The arrows show metastatic foci; the histogram shows the metastatic foci per lung section from each mouse group (*P < 0.05; **P < 0.01). (D) Representative images (left panel) of IHC staining of p-p38 (left panel) and quantitation of p-p38 levels (right panel) in each group (*P < 0.05, **P < 0.01). (E) Protein levels of phosphorylated and total p38 in tumours were determined by western blotting. (F) A schematic model illustrating that the secreted protein IL32 derived from CAFs binds to integrin β3 at the breast cancer cell membrane to activate downstream p38 MAPK signal transduction, thus promoting breast cancer cell invasion. Magnification, × 200 in (C) and (D).
    and yielded a greater number of metastatic nodules in mouse lungs; neutralisation of IL32 in the supernatant of CAFs by means of a specific antibody attenuated CAF-induced cancer aggressiveness. Moreover, suppression of the connection between IL32 and integrin β3 (such as a knockdown of IL32 in CAFs or silencing of integrin β3 in BT549 cancer cells) notably reduced the number of metastatic foci in mouse lungs. Thus, IL32, a new secreted protein, serves as an important mediator between stromal fibroblasts and cancer cells by contributing to tumour invasion and metastasis.
    Our previous work suggests that the WNT-GSK3β, PI3K-AKT, and ERK-MAPK are the downstream networks of integrin β1 through FAK-ILK signalling [39]. Nonetheless, FAK-ILK signalling was unchanged during the interaction of integrin β3 with IL32 (data not shown). Moreover, in breast cancer cells, only p38 MAPK, a typical cytokine pathway, was activated after IL32 binding to integrin β3. This result is consistent with the finding that IL32 can activate p38 MAPK pathway in mouse macrophages and human oesophageal cancer cells [23,54], and that p38 MAPK is activated after integrin β3 interacts with an extra-cellular ligand (TN-c) [21]. Thus, it is worth noting that p38 MAPK may be a specific signalling protein that is stimulated by the interaction of integrin β3 with extracellular components (e.g. cytokines). Function-ally, p38 MAPK signalling may contribute to EMT and cancer cell in-vasion. Indeed, our current work shows that fibronectin, N-cadherin, and vimentin are up-regulated in breast tumour cells, and tumour cell invasion and metastasis are strengthened only by p38 MAPK signalling.
    In summary, our study shows that integrin β3 in breast tumour cells acts as a functional receptor of IL32, a crucial CAF-derived cytokine, by promoting breast cancer invasiveness. These findings have significant implications for the understanding of CAFs’ molecular mechanisms of action regulating cancer behaviour. Our study offers a new possible therapeutic strategy for targeting stromal components of a breast tu-mour.
    5. Conflicts of interest
    The authors declare that no conflicts of interest exist.
    Acknowledgements
    Appendix A. Supplementary data
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