Acid-induced autophagy protects human gastric cancer cells from apoptosis by activating Erk1/2 pathway

Acidic microenvironments exist widely in tumors. However, the specific mechanism of cancer cell survival under an acidic microenvironment remains unknown. This study aims to investigate whether acid can induce autophagy and examine the mechanism of autophagy in gastric cancer cells.

Human gastric adenocarcinoma (AGS) cells were cultured in media with different pH values in vitro and then subjected to autophagy detection under different conditions. To determine the effect of an acidic microenvironment on autophagy, we employed real-time quantitative polymerase chain reaction (PCR), Western blot, mRFP-GFP-LC3 immunofluorescence, and transmission electron microscopy (TEM) to detect the expression of various autophagy indicators. We also performed cell counting kit 8 (CCK8) and cell invasion and migration assays to examine cell viability and invasion, respectively.

We found that the protein expression of autophagy markers such as LC3II/I and Beclin1 was higher in AGS cells treated with an acidic microenvironment than in control cells. The protein expression level of P62 was obviously decreased in acid-treated cells compared to that in control cells. Furthermore, the expression of Erk1/2 pathway markers, including p-Erk1/2, was also increased in response to acidic pH. Dense LC3 puncta were observed in cells cultured under acidic conditions, whereas untreated cells exhibited diffuse and weak LC3 puncta; an increased autophagy flux could also be observed. The presence of autophagosomes was observed by TEM in AGS cells subjected to low pH. Additionally, autophagy was inhibited by the autophagy inhibitor Bafilomycin A1 (Baf) and apoptosis was obviously increased. Moreover, cells exposed to an acidic microenvironment displayed facilitated growth compared with that in control cells.

Taken together, these results indicate that the acidic microenvironment promotes AGS cell growth by upregulating autophagy through the Erk1/2 pathway, which acts as a survival adaptation.