Matrix stiffness-mediated effects on macrophages polarization and their LOXL2 expression

Previously, we reported that the secreted lysyl oxidase like 2 (LOXL2) from hepatocellular carcinoma (HCC) cells under higher stiffness stimulation contributed to the formation of lung premetastatic niche. To further clarify whether matrix stiffness also alters LOXL2 expression in other cells within tumor microenvironment, we developed a gel-based culture system combined with a model of macrophage polarization to evaluate the effects of matrix stiffness on the polarization of M2 macrophages and their LOXL2 expression. THP-1 cells cultured on 6KPa, 10KPa, and 16KPa stiffness substrates were first incubated with 100nM phorbol 12-myristate 13-acetate (PMA) for 24 hours and subsequently treated with 20nM interleukin-4 (IL-4) and 20nM interleukin-13 (IL-13) for 48 hours. The polarization states of M2 macrophages under different stiffness stimulation were comparatively analyzed, and their LOXL2 expressions as well as the underlying molecular mechanism were further explored. Our results demonstrated that increased matrix stiffness remarkably strengthened M2 macrophage polarization and promoted their LOXL2 expression. Activation of integrin β5-FAK-MEK1/2-ERK1/2 pathway participated in matrix stiffness-mediated HIF-1α upregulation, and HIF-1α upregulation resulted in a significant improvement in LOXL2 expression. Additionally, M2 macrophage polarization state and LOXL2 expression in HCC tissues with COL1^High /LOX^High were consistent with the results in vitro, further confirming the regulation roles of matrix stiffness in macrophage polarization and LOXL2 expression. The findings about LOXL2 upregulation in the polarized macrophages under higher stiffness stimulation will be helpful to better understand the underlying mechanism of matrix stiffness-induced premetastatic niche formation in HCC.