Analysis of Base Excision Repair proteins in A549 lung cancer cell line upon induction of cancer cell senescence by genotoxicants treatments

Senescence is a cellular process characterized by an irreversible arrest of the cell cycle following various stressors potentially harmful to cells, such as DNA damage, oxidative stress, or oncogene expression. The p53-p21CIP1 and p16-Rb pathways were activated in response to these stimuli and promote cell cycle arrest. One of the most important hallmarks of senescence is the "senescence-associated secretory phenotype" (SASP), in which chemokines, cytokines, and proteases are secreted in an autocrine and paracrine manner. Among cytokines, IL-6 and IL-8 are mainly involved in regulating the tissue microenvironment. Several studies have shown that cellular senescence is related to tumor progression and chemoresistance. When damaged DNA is not properly repaired, cell clearance leads to a chronic pro-inflammatory state. The Base Excision Repair pathway (BER), specifically the protein Apurinic/Apyrimidinic endodeoxyribonuclease 1 (APE1), has been shown to be involved in the induction of intrinsic cell senescence. Currently, involvement of BER in extrinsic cancer cell senescence is still unknown. In recent years, a new pharmacological strategy has been developed to selectively eliminate senescent cells by using senolytic compounds. In the present work, we investigated a possible involvement of BER enzymes in the onset of extrinsic cell senescence of A549 cancer cells. To this end, we first established a model of extrinsic cancer cell senescence by using cisplatin (CDDP) and doxorubicin (DOXO) as genotoxic agents. We analysed the expression of the majority of the BER enzymes: APE1, XRCC-1, POL-β, POL-δ. We found that APE1 was downregulated in a proteasomal-dependent manner during the onset of cancer cell senescence, whereas no significant changes were detected for the other BER enzymes. Of interest, APE1 controls the expression of miRNAs involved in cell senescence. We observed that miR regulated by APE1, i.e., miR-130b, which regulates the expression of p21CIP1, is downregulated in line with APE1 expression, leading to upregulation of its target p21CIP1 and promotion of the senescence process. Moreover, APE1 contributed to the onset of senescence by inducing SASP factors (IL-6 and IL-8) in the early phase of the process. Interestingly, treatment of senescent cells with APE1 inhibitors sensitizes cancer cells to CDDP treatment. These data suggest that APE1 is involved in the early stages of senescence through induction of SASP factors and deregulation of miRNAs. We also demonstrated the senolytic activity of APE1 inhibitors. Overall, this work suggests that APE1 plays a role in the development of extrinsic cell senescence through several mechanisms and that inhibition of APE1 activity could be a promising target for the development of new senolytic agents.