Notch signaling via Wnt regulates the proliferation of alternative, CCR2-independent tumor-associated macrophages in hepatocellular carcinoma

Tumor-associated macrophages (TAMs) play pivotal roles in tumor progression and metastasis, but the contribution and regulation of different macrophage populations remain unclear. Here we show that Notch signaling plays distinct roles in regulating different TAM subsets in hepatocellular carcinoma (HCC). Myeloid-specific Notch blockade by conditional disruption of recombination signal binding protein Jϰ (RBPj cKO) significantly delayed the growth of subcutaneously inoculated Lewis lung carcinoma (LLC), but accelerated orthotopically inoculated hepatic Hepa1-6 tumors in mice. In contrast to subcutaneous LLC, RBPj cKO significantly increased the number of TAMs in hepatic Hepa1-6 tumors despite impeded differentiation of monocyte-derived TAMs (moTAMs). The dominating TAMs in orthotopic HCC manifested properties of Kupffer cells (KCs) and hence tentatively named KC-like TAMs (kclTAMs). The increased proliferation of RBPj cKO kclTAMs was maintained even in CCR2-/- mice in which moTAMs were genetically blocked. Notch signaling blockade accelerated proliferation of kclTAMs via enhanced β-catenin-dependent Wnt signaling, which also downregulated interleukin (IL)-12 and upregulated IL-10 expression by kclTAMs likely through c-Myc. Additionally, myeloid-specific RBPj cKO facilitated hepatic metastasis of colorectal cancer (CRC) but suppressed lung metastasis in mice, suggesting that the phenotype of RBPj cKO in promoting tumor growth was liver-specific. In patient-derived HCC biopsies, Notch signaling negatively correlated with Wnt activation in CD68+ macrophages, which positively correlated with advanced HCC stages. Therefore, Notch blockade impedes the differentiation of moTAMs, but upregulates Wnt/β-catenin signaling to promote the proliferation and pro-tumor cytokine production of kclTAMs, facilitating HCC progression and hepatic metastasis of CRC.