Beyond p53 inactivation is essential fro majority of human tumors, broading our view of p53 role in silicosis will extremely aid understanding as to how a disease caused by silica, the ubiquitous airborne contaminant can lead to EMT progress. p53 is one of the most extensively studied cancer and environment stress-related adaptive proteins in the past four decades[34]. Strategies that target mutant p53expression,focusing on degradation of mutated protein, restoring the wild type activity, forming p53 complexes with other proteins, or interfering with p53-related signaling pathways can potentially help prevent and control cancer [14, 34]. However, many related studies remain controversial. For instance, in lung cancer cells, p53 attenuated glucose uptake leading to glycosis inhibition, whereas, in muscle cells, p53 can induce glycolytic enzymes. The possible reason mainly depends on the research context which was pointed out by Edward et al. that the disparate p53 activities and functions can be interpreted in different contexts [14]. Hence, in this study, we tried to knockout p53-wt from HBE cells and investigated the resulting interference in the downstream metabolic pathways. Our study revealed that following silica exposure resulted in HBE cell dysfunction through 52 mRNAs that were putatively involved in p53-dependent pathways. Metabolomic profiling and bioinformatics analysis revealed that 42 metabolites were putatively involved in p53-dependent silica-mediated HBE cell dysfunction. Through integrated data analysis, we obtained three significant p53-dependent metabolic pathways, including phenylalanine, glyoxylate, dicarboxylate, and linoleic acid metabolism, and the TCA. In particular, we identified roles for benzeneacetic acid, a key regulation metabolite in the phenylalanine metabolic pathway, attenuated the silica-induced EMT in HBE cells in a p53-dependent manner.
Evidence shows that normal epithelial cells may be involved in controlling EMT in complicated mechanisms[35]. The classical function of p53 is to be a transcription cofactor for cells in response to various environmental stresses, including cell cycle arrest, apoptosis, and senescence[36].However, recent findings suggest that p53 restricts epithelial cell plasticity[37]. This partly occurs due to the initiation of the p53-MDM2-SLUG pathway to enhance the epithelial biomarker E-Cadherin expression[38]. p53 also negatively regulates EMT progression by enhancing miR-34a expression to inhibit its downstream Snail1 expression, a mesenchymal marker that induces EMT progression [39]. Additionally, p53 is expressed in mesenchymal cells, indicating that there may be additional molecular mechanisms involved in the maintenance of epithelial integrity. In normal epithelial cells, p53 protein expression is augmented upon different stresses, including DNA damage and hyperproliferative signals[35]. p53 is also involved in the regulation of silica induced EMT progression. Cho et al. indicated that activated TβR1 by silica induces the phosphorylation and degradation of RKIP, consequently resulting in Snail-mediated p53 suppression and the occurrence of EMT [40]. In this study, we compared the mRNA alterations and found that ANKRD22, GALNT16, LGR5, ITGA9, ELMOD1, SLC9B1, GOLGA8Q, FAM19A5, PLA2G2C, and LAIR2 were significantly changed post silica exposure when p53 was deficient. In p53-KO murine gastric epithelial (GIF-14) cells, EMT-induced plasticity is reflected in the expression of the embryonal proto-oncogene LGR5[41], whereas the relationship between p53 and other mRNAs in EMT progression has not been reported. Further studies are warranted to confirm the alternations of these mRNAs in silica-induced p53 deficiency.
p53 regulation may be involved in EMT-related metabolic pathways. Sabine et al. suggested that treatment with calcitriol, the active vitamin D3 metabolite, activates p53 expression, and increases the repression of SNAIL, inhibiting EMT progression[42]. Fu YM et al. showed that restriction of phenylalanine modulates p53 and glucose metabolism in prostate cancer cells[43]. Karin et al. indicated that phenylalanine hydroxylase (PAH) is involved in the regulation of tyrosinase via p53 through the transcription of hepatocyte nuclear factor 1 alpha, which regulates melanogenesis[44]. Indeed, p53 controls multiple metabolic pathways, including glycolysis and pentose phosphate pathway, lipid, sphingolipid, amino acid, ammonia, and iron metabolism, mitochondrial biogenesis, integrity and respiration, TCA cycle, and ferroptosis[45] in normal cells, suggesting that deregulation of any associated metabolic activities contributes to several cancer and non-cancer human pathologies. Through this, it is obvious that p53 functions in metabolic pathways and displays certain cell and tissue specificity [45]. Thus, despite the classical function of the “guardian of the genome”, p53 may be better characterized as a “guardian of homeostasis”. Here, we report that phenylalanine, glyoxylate, dicarboxylate, and linoleic acid metabolism, and TCA may be involved in silica-stimuli in a p53-dependent manner. Phenylalanine metabolism belongs to amino acid metabolism. A recent study demonstrated that the retention of L-arginine, a semi exponential amino acid, increased p53 expression, and reduced the contents of proinflammatory cytokines in hepatic cells[46]. For TCA, the oncometabolites stabilize the hypoxia inducible factor 1, inhibit p53 expression, and increase glutaminolysis, glycolysis, or dysregulation of EMT, suggesting that these changes in p53 and TCA-related metabolites are important driving forces of cancer pathogenesis and progression[47]. However, few studies have been reported on the p53-dependent changes in relative metabolites following silica exposure. We used integrative multiple-omics analysis to provide baseline information for further p53 studies.
Benzeneacetic acid, was one of the major compounds among the 31 substances found in the ethyl acetate extract[48]. A previous study has shown that paraquat exposure in rats increases benzeneacetic acid levels[49]. Benzeneacetic acid has also been considered as an anti-inflammatory compound with the function of inhibitinglipopolysaccharide-induced inflammation in vivo[50]. Asynchronized smooth muscle cells treated with S-diclofenac (2-[(2,6-dichlorophenyl) amino] benzene acetic acid 4-(3H-1,2,dithiol-3-thione-5-yl) phenyl ester) showed an increase in stabilization of p53[51]. In the KEGG database, benzeneacetic acid is involved in phenylalanine metabolism, and ALDH3, feaB, and paaI are thought to promote its levels, thereby increasing phenyl-acetylglutamine. Despite these reports, a mechanistic link between benzeneacetic acid and silica induced EMT has not yet been reported. The present study confirms the major role of p53-dependentbenzeneacetic acid generation in normal HBEs post the silica stimulus. Notably, increased benzeneacetic acid levels in p53-wt cells inhibited the silica-induced EMT progression. The present study suggests that certain clinical benefits of benzeneacetic acid observed with p53 existence status may be in part mediated via attenuation in silica-triggered EMT signaling events in vitro. Although the present study focuses only on the function of benzeneacetic acid in human HBE cells, one can speculate that benzeneacetic acid has the potential to modulate other EMT-related markers or pathways, and accordingly, may potentially influence multiple silica-linked physiological processes.
Research trends regarding p53 research and control provide a closer look at this field. Since its discovery 40 years ago, p53 remains heavily investigated in the global scientific community, specifically in the USA. China has rapidly advanced the understanding of p53 in recent years, with large government investments through NSFC foundations. However, the clinical translation of p53 research is still challenging, despite a few studies that have applied for invention patents. This reflects the lack of innovative research and innovation. Olivier et al. emphasized that p53 translational research for improving cancer detection, prognosis, prevention, and therapy based on p53 study is critical in qualitative terms. p53 research has been conducted for three periods: the discovery from 1980 to 1990, first considered an oncogene factor; the rapid development study period from 1990 to 2010, second mainly characterized as multi-functional (transcription factor and causing double-edge effects in the various contexts); the third period was from 2010 to present, where research lacks substantial findings and clinical translation has been stagnating. At this time, we should encourage and incentivize high-impact metabolic future studies that use rare human disease contexts and novel animal models that can be clinically translatable.
Although p53 research has been intensive and obtained extremely discoveries, there still remains much to explore for p53 origin and regulation. Challenge in the coming era of p53 research would be the first of the comprehensive p53 function in other diseases instead of cancers, the second is the urgent need to translate p53 research results into clinical application. And new technologies applied in p53 function explore is also a big challenge for more robust clinical advances.