Immunosuppression and Epigenetic Therapy: Amyloid Precursor-Like Protein 2-Mediated Macrophage Dynamics and Histone Deacetylase Inhibition in Neuroblastoma

Amyloid precursor-like protein 2 (APLP2) has been previously associated with pro-tumor phenotypes in cancer cells, and these studies investigate the expression and functions of this protein in macrophages. Collective findings demonstrated increased APLP2 expression in U937 monocyte-like cells following cytokine-induced differentiation to macrophage-like cells. Analysis of human mRNA data revealed elevated APLP2 levels in M2/anti-inflammatory (pro-tumor) macrophages compared to M1/pro-inflammatory (anti-tumor) macrophages. Consistent with the mRNA data, immunoblotting confirmed higher APLP2 protein expression in mouse M2 macrophages. Through comparison of macrophages from wild-type and APLP2-knockout mice, we correlated alterations in inflammation-associated markers with the presence of APLP2, which suggests that APLP2 influences macrophage polarization dynamics between M0/unpolarized and pro- and anti- inflammatory states. Notably, intratumoral infiltration of M2/anti-inflammatory macrophages has been reported in several cancers, including the pediatric cancer neuroblastoma (NB), and NB-conditioned media induced macrophage polarization resembling APLP2-mediated profiles. These findings implicate APLP2 in shaping macrophage dynamics, potentially contributing to tumor-associated immunosuppression. The immunosuppressive tumor microenvironment in NB is a contributor to poor outcomes for patients, with high-risk patients facing a five-year survival rate of ~50%. New therapeutic strategies are needed for NB, and histone deacetylase (HDAC) inhibitors have emerged as promising anti-cancer agents due to their ability to modulate gene expression and tumor phenotypes. These studies assessed M344, an HDAC inhibitor, for its therapeutic efficacy in NB. Analysis of clinical NB data showed higher HDAC transcript expression in advanced-stage tumors relative to early-stage samples, and M344 treatment increased histone acetylation. M344 also exerted anti-proliferative and pro-apoptotic effects on NB cells through G0/G1 cell cycle arrest and pro-apoptotic caspase activation. Compared to vorinostat, an HDAC inhibitor in clinical use for lymphoma and clinical trials for NB, M344 demonstrated superior cytostatic and cytotoxic effects, as well as reduction of NB cell migration. In vivo, metronomic M344 dosing significantly suppressed tumor growth and extended survival in NB-bearing mice. Additionally, M344 enhanced tolerability of topotecan and prevented tumor rebound when combined with cyclophosphamide. These findings position M344 as a potent candidate for NB therapy, with improved tumor suppression, reduced toxicities, and potential to enhance long-term outcomes in patients.