To observe the expression levels of base excision repair (BER) pathway-related proteins in small cell lung cancer (SCLC) tissues, and analyze their relationship with the prognosis and tumor immune microenvironment.

A retrospective cohort study was conducted on 74 patients with limited-stage SCLC undergoing surgical treatment in our medical center from December 2018 to June 2023. Immunohistochemical staining was performed to analyze the protein expression of BER pathway components, apurinic/apyrimidinic endonuclease 1 (APE1), 8-oxoguanine DNA glycosylase 1 (OGG1), DNA polymerase β (POLβ), X-ray repair cross-complementing protein 1 (XRCC1), ATP-dependent DNA ligase I (LIG I), and immune cell infiltration markers of CD3⁺ T cells, CD8⁺ T cells, CD68⁺ macrophages in SCLC tissues. Chi-square test was applied to analyze the relationship of BER protein expression and clinicopathological features; Kaplan-Meier survival curve was plotted to evaluate the impacts of BER protein expression and immune cells on disease-free survival (DFS) and overall survival (OS), multivariate Cox regression analysis was utilized to identify DFS prognostic factors, and Spearman correlation analysis was performed to analyze the correlation of BER-immune cell infiltration. In in vitro experiments, transient transfection was applied in H196 cells to overexpress APE1/POLβ/LIG I , respectively. Thus, the cells were divided into negative control (NC, empty vector) and overexpression (OE, target plasmids) groups. CCK-8 and TUNEL assays were employed to determine the effects of OEAPE1, OEPOLβ and OELIG I on cell sensitivity to cisplatin. In in vivo experiments, nude mice bearing xenograft tumors were grouped into WT, E3330 (APE1 inhibitor), cisplatin, and cisplatin+E3330 groups to determine the effects of the combination therapy on tumor growth.

There were no significant correlations of the expression levels of key BER pathway proteins with clinicopathological characteristics, including gender, age, smoking history, tumor location, Ki67 index, or TNM stage (all P>0.05). The patients with low expression of APE1, POLβ, and LIG I had obviously higher DFS rates than those with high expression (P<0.05), and the patients with larger proportion of CD3⁺ T cells also had higher DFS rates than those with smaller proportion (P=0.043). Multivariate Cox regression analysis indicated that tumor TNM stage (HR=2.465) and APE1 expression (HR=2.730) were independent risk factors for the prognosis of SCLC patients (P<0.05). Spearman correlation analysis demonstrated a positive correlation between APE1 and CD8⁺ T cell proportion in the SCLC patients (r=0.27, P<0.05). In vitro experiments showed that the overexpression (OE) cells (OEAPE1 and OELIG I) exhibited reduced sensitivity to cisplatin than the NC group (P<0.05). Animal experiments indicated that cisplatin+E3330 significant inhibited xenograft tumor growth, indicating enhanced therapeutic efficacy (P<0.01).

High expression of APE1, POLβ, and LIG I in the BER pathway indicates poor prognosis and low DFS rate in SCLC patients. High expression of APE1 is positively correlated with CD8⁺T cells, and can be used as an auxiliary marker for SCLC immunotherapy.

To investigate the regulatory mechanism of apurnic/apyrimidinic endonuclease 1 (APE1) in the transformation of chronic intestinal inflammation to colitis-associated colorectal cancer (CAC).

C64S mutant (APE1^C64S) mice and APE1 wild type (APE1^WT) mice were randomly divided into experimental group and control group. In vivo CAC model was established by azoxymethane (AOM) and dextran sulfate sodium salt (DSS) solution. Immunohistochemistry (IHC) and multiple IHC (mIHC) assays were used to observe the expression of APE1 and immune cell infiltration in colon tissues of each group. A mouse colon cancer cell line MC38 with stable knockdown of APE1 was constructed by lentivirus transfection, and subcutaneous tumor bearing experiments were performed in APE1^WT and APE1^C64S mice to confirm that tumor cell-derived APE1 caused immunosuppressive tumor microenvironment. The expression of APE1 and CXCL1[chemokine (C-X-C motif) ligand 1]and the infiltration of immune cells in tumor-bearing specimens were analyzed by IHC and mIHC assays. The tumor specimens of a 28-year-old female patient with CAC from Army Medical Center of PLA were analyzed for the expression of APE1 and CXCL1 and the infiltration of immune cells in the tumor and adjacent inflammatory tissues by IHC and mIHC assays.

Compared with the control group and APE1^WT erperimental group, APE1^C64S erperimental group had significantly reduced disease activity index and tumor formation, polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) infiltration, and CD4⁺ and CD8⁺T cells (P<0.05). No significant differences were observed in tumor growth and immune cells between APE1^WT and APE1^C64S mice bearing subcutaneous tumors. However, in the tumor-bearing experiment using tumor cells with knockdown of APE1, the tumor growth was significantly lower and the number of infiltrated PMN-MDSCs was reduced, while those of CD4⁺ and CD8⁺T cells were significantly increased (P<0.05). Furthermore, high expression of APE1 and increased infiltration of PMN-MDSCs were found in the tumor tissues of the young CAC patient, and CD8⁺T cells were significantly reduced in the tumor tissues compared with the inflammatory tissues (P<0.05).

APE1-redox in tumor cells can promote the infiltration of PMN-MDSCs and reduce the number of T cells, thereby forming an immunosuppressive tumor microenvironment and mediating the occurrence and development of CAC.