Abstract:
Background Arsenic exposure has been demonstrated to induce apoptosis. The fibronectin type III structural domain 3B protein (FNDC3B) has been shown to promote cancer cell proliferation; however, its role in arsenic-induced apoptosis remains to be elucidated.
Objective To investigate the effects of sodium arsenite (NaAsO2) and its metabolites on the expression of FNDC3B gene in A549 cells and to understand the function of FNDC3B gene in A549 cells.
Methods (1) A549 cells were exposed to varying final concentrations of NaAsO2 and their optical density at 450 nm values were measured by Cell Counting Kit-8 (CCK-8) after 48 h. Survival curves were plotted, and a final exposure dose was selected according to the survival rate. Total protein and RNA were extracted by exposing A549 cells to high (30 µmol·L−1), medium (20 µmol·L−1), and low (10 µmol·L−1) NaAsO2 concentrations, high (30 µmol·L−1) monomethylarsinic acid (MMA), and high (30 µmol·L−1) dimethylarsinic acid for a period of 48 h. mRNA expression and the protein expression of the FNDC3B gene was detected by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot (WB), while the protein ubiquitination expression of the FNDC3B gene was detected by co-immunoprecipitation (CO-IP) and WB assay. (2) Knockdown of FNDC3B gene expression was achieved in A549 cells by siRNA interference. The si-FNDC3B fragment was transfected in A549 cells for 48 h. The mRNA and protein expression of FNDC3B gene was then detected by qRT-PCR and WB assay. Cell viability was determined through CCK-8 assay. Hoechst 33342/propidium iodide (PI) double staining and JC-1 mitochondrial membrane potential assay were employed to detect both early and late apoptosis, while cleaved caspase3 protein and P53 signalling pathway related protein expressions were evaluated by WB.
Results (1) The CCK-8 results demonstrated a decline in the viability of A549 cells with an increase in NaAsO2 concentration, with an inhibitory concentration at 50% of 38.12 µmol·L−1. The qRT-PCR results demonstrated that compared to the control group, varying concentrations of NaAsO₂ (10, 20, and 30 µmol·L⁻¹) significantly upregulated the mRNA expression of FNDC3B gene (P<0.01). In contrast, MMA and DMA showed no significant effect on FNDC3B mRNA expression (P>0.05). The WB analysis revealed that the protein expression of FNDC3B was reduced in the NaAsO₂-treated group compared to the control, accompanied by elevated ubiquitination levels of FNDC3B protein, particularly at the K48 ubiquitination site. MMA and DMA exhibited no impact on FNDC3B protein expression. (2) Following the specific knockdown of FNDC3B expression in A549 cells, the CCK-8 assay demonstrated a significant reduction in cell viability in the silenced FNDC3B group (si-FNDC3B) compared to the control group. The JC-1 assay demonstrated that the mitochondrial membrane potential was diminished in the si-FNDC3B group relative to the control group. The Hoechst 33342/PI staining assay revealed that the si-FNDC3B group exhibited a notable degree of apoptosis. The si-FNDC3B group also displayed substantial apoptosis. The WB analysis indicated that the relative expressions of cleaved caspase3, P53, MDM2, Bad, and Bax proteins were elevated in the si-FNDC3B group in comparison to the control group.
Conclusion The presence of NaAsO2 is observed to promote the ubiquitination expression of the FNDC3B protein, which in turn reduces the expression of FNDC3B protein. However, the main metabolites DMA and MMA have no effect on the expression of FNDC3B. Furthermore, the silencing of FNDC3B is observed to inhibit the viability of A549 cells and promote apoptosis, a phenomenon related to the activation of P53 signaling pathway.
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