STAD, prevalent and with a poor prognosis in Asian populations, requires specific prognostic biomarkers and therapeutic targets to enhance patient survival[26]. The complex biological functions of PXDN are indispensable for maintaining the normal physiological function of cells. Previous research has shown that PXDN promotes various tumors, including oral squamous cell carcinoma (OSCC), melanoma, prostate cancer (PCa), and ovarian cancer (OC)[18, 21, 27]. Although it has been reported that PXDN is highly expressed in STAD and associated with poor prognosis, the specific mechanism has not been thoroughly elucidated[28].
In the present study, both PXDN protein and transcript expression levels were significantly upregulated in STAD tissues, and patients with high expression of PXDN had a worse prognosis compared to patients with low expression. These findings strongly suggested the concept that PXDN may act as a tumor promoter in STAD. Previous studies have shown that the expression levels of oncogenes have a tendency to gradually increase in pathological information[29–31]. However, our preliminary data showed no significant correlation between PXDN and pathological information, possibly due to uneven sample distribution and tumor heterogeneity.
The specific role of PXDN in STAD is unknown, thus we performed functional enrichment analyses to reveal the underlying mechanisms, including KEGG, GO, and GSEA. Surprisingly, we found that STAD patients with high expression of PXDN were significantly enriched in multiple functional signaling pathways, which included extracellular matrix structure, PI3K/AKT, EMT, angiogenesis, inflammatory response and KRAS signaling pathways. It is well known that EMT, PI3K/AKT, angiogenesis, and the KRAS signaling pathway play an integral role in maintaining tumor development and metastasis[32–35]. Significantly, to corroborate our aforementioned results, we conducted PXDN knockdown in STAD cells followed by a series of in vitro experiments, including immunoblotting, Cell Counting Kit-8 (CCK-8), Transwell assays, colony formation, and angiogenesis assays. Collectively, these results demonstrate the crucial role of PXDN in sustaining tumor proliferation, invasion, migration, and angiogenesis. These results provided strong evidence in support of the oncogene function of PXDN in STAD.
Accumulating previous studies have shown that the structure of extracellular matrix and inflammatory response can regulate the infiltration of immune cells in tumor, thus reshaping the TIM[36]. To investigate whether PXDN participate in the regulation of immune cell infiltration in STAD, we applied the TISIDB database and CIBERSORT algorithm to analyze the correlation between PXDN and the abundance of various immune cells. The results showed that PXDN was significantly negatively correlated with the abundance of CD8 cells, and negatively correlated with M2 macrophages. CD8 T cells are recognized as key players in anti-tumor immunity and the primary immune cells responsible for tumor destruction, as established in prior research[37]. Infiltration and polarization of tumor macrophages are considered to be important events in carcinogenesis, and M2 polarization of macrophages has the ability to produce tumor-promoting effects on tumor progression[38]. We applied immunohistochemistry to analyze the association of PXDN with CD8 cells and M2 macrophages, and the results obtained were consistent with the database. These findings collectively emphasized that PXDN can influence STAD progression not only by promoting tumor value-addition and metastasis, but also by modulating immune cell infiltration and thus reconstitution of the TIM. However, this study's limitation lies in not elaborating on the specific mechanisms of how PXDN regulates CD8 and M2 cell infiltration.