Impact of the histone deacetylase inhibitor 4-phenylbutyrate on the clearance of apoptotic pancreatic carcinoma cells by human macrophages

Histone deacetylase inhibitors have been found to have potent anticancer activities, partly induced by tumour cell apoptosis. The clearance of apoptotic tumour cells is an important mechanism of antitumour immune surveillance. The aim of this study was to assess the impact of 4-phenylbutyrate (4-PB) and its immunological effects on the macrophage clearance of apoptotic pancreatic ductal adenocarcinoma (PDAC) cells. To this end, a co-culture system of human macrophages from donors and PDAC patients, and PDAC cell lines (T3M4, PANC-1 and AsPC-1) was established to study the effect of 4-PB. Apoptosis and phagocytic activity were analysed using flow cytometry, and phagocytosis was confirmed by confocal microscopy. Further, p21 expression was quantified by immunoblot analysis. 4-PB treatment (0-10 mM) resulted in a dose-dependent induction of tumour cell apoptosis in two of the cell lines (T3M4 and PANC-1), but it also induced human macrophage apoptosis. The apoptotic effect of gemcitabine on PDAC cells was further enhanced by 4-PB. Moreover, 4-PB led to a dose-dependent overexpression of the cell cycle regulator p21 in tumour cells. In co-culture, apoptotic PDAC cells were phagocytosed by donor macrophages and phagocytosis was increased through tumour cell exposure to 4-PB and/or gemcitabine, whereas phagocytosis of PANC-1 cells was reduced using macrophages of PDAC patients treated with 4-PB. The 4-PB treatment induced human macrophage expression of the pro-angiogenic IL-8 and simultaneously inhibited inflammatory cytokine release through modulation of IL-10 and TNFα after phagocytosis of apoptotic PDAC cells. In conclusion, the 4-PB treatment activated tumour cell death in PDAC cells, resulting in tumour cell phagocytosis by macrophages. The latter were characterized by an anti-inflammatory and pro-angiogenic cytokine response demonstrating adverse, tumour-promoting effects of macrophages on tumour cells. Thus, the potential of 4-PB as an anticancer agent against PDAC cannot be reliably assessed without taking into account the complex tumour microenvironment.