Structural characterization of a pectin polysaccharide from Phyllanthus emblica fruits and their antitumor effect via macrophage polarization in the cold immune microenvironment

The polarization of tumor-associated macrophages (TAMs) from M2 to M1 phenotype is an effective strategy for tumor immunotherapy. In this study, a pectic polysaccharide (PEP-1) was isolated from Phyllanthus emblica fruits via hot water extraction, ethanol precipitation, and purified by anion exchange and gel permeation chromatography. PEP-1 was identified as a homogalacturonan (molecular weight, 156.8 kDa) via characterization analysis. The structural attributes of PEP-1 were elucidated through GC–MS and NMR analyses, confirming that its repeating sugar units consist of GalA connected by α-1,4-glycosidic linkage. Functional assays demonstrated that PEP-1, at concentrations of 50, 100, and 200 μg/mL could strongly induce the transformation of M2 macrophages into M1 phenotype in vitro, and the conditioned medium (CM) of M2 pretreated with PEP-1 significantly promoted apoptosis of Hepa1-6 cells. At the cellular level, PEP-1 shifted tumor-promoting M2 macrophages to a tumor-inhibiting M1 phenotype by increasing the phosphorylation of NF-κB and MAPK. Additionally, PEP-1 (200 mg/kg) was capable of reaching the tumor microenvironment (TME), directly binding to TAMs, promoting their polarization towards M1 phenotype, and significantly inhibiting the tumor growth in Hepa1-6 tumor-bearing mice. These findings reveal the structural characteristics of PEP-1 and its potential to treat hepatocellular carcinoma immunotherapy via regulating TAMs.