The targeting of endoglin on vascular endothelial cells affects the infiltration of M2 macrophages into the breast cancer microenvironment by modulating the interleukin-6 (IL-6) level

Abstract

Background: The effect and underlying mechanisms of endoglin on angiogenesis and immunity during tumour progression were investigated. Methods: Differences in the growth of established EO771 breast tumours between endoglin-knockout (Eng-iKOe) and control mice were evaluated. The tumour tissues were harvested on the day 7th and 14th after engraftment, at which times the growth differences between the groups were significant. The expression of markers of angiogenesis (CD31), endoglin (CD105), tumour associated macrophages (TAMs) (F4/80) and M2 macrophages (CD206) and of interleukin-6 (IL-6), IL-10, and IL-6/Janus kinase 2 (JAK2)/signal transducer and activators of transcription 3 (STAT3) proteins in tumour tissues of Eng-iKOe and control mice were evaluated. Results: We found that tumour angiogenesis and growth were both inhibited in the Eng-iKOe group relative to the control group on day 7, whereas, no significant between-group difference was observed on day 14. Moreover, in breast cancer tissues of the Eng-iKOe group, the numbers of M2 macrophages were significantly decreased on day 7 and increased on day 14, and the expression of IL-6 and IL-10 were lower on day 7 and higher on day 14. Positively strong correlations were found between the IL-6 level and the number of M2 macrophages both on days 7 (P=0.017, r=0.80) and 14 (P < 0.01, r=0.94). However, there was just a moderate correlation between IL-10 expression and the number of M2 macrophages on day 14 (P=0.043, r=0.682), not day 7 (P=0.055). Furthermore, the protein levels of IL-6, p-JAK2, and p-STAT3 were significantly lower on day 7, and higher on day 14 in the Eng-iKOe group compared with the control group, which was in accordance with the changes of M2 macrophage numbers. Conclusions: The effect of endoglin-targeted anti-angiogenic therapy was weakened by affecting the infiltration of M2 macrophages in breast cancer through modulation of the IL-6 level. This indicated that IL-6 could not only predict the efficacy after anti-angiogenic therapies, but also acts as a potential target to improve efficacy of anti-angiogenic therapies.