Due to the diverse molecular oncogenes that have not been fully explored, CRC still poses a major challenge to human economic development and quality of life [18]. Biomarkers have been shown in many reports to improve the diagnostic level of colon cancer [19–20], and their application in clinical treatment remains highly promising. Data mining and expansion contribute to the discovery of new therapeutic targets and accelerate the implementation and development of precision oncology [21]. Therefore, in this study, we screened for the differentially expressed factor PDZD4 in CRC from the UCSC Xena, TCGA, and GEO databases. Differential expression analysis revealed that PDZD4 was downregulated in multiple tumors, including colon cancer, which was consistent with our immunohistochemical staining results. Clinical pathological analysis showed that PDZD4 expression gradually decreased with the progression of colon cancer grade and stage, indicating a potential correlation between the low expression of PDZD4 and poor prognosis in colon cancer patients. Univariate and multivariate Cox analyses also suggested that PDZD4 could serve as an independent prognostic factor in colon cancer.
In the enrichment analysis, GO analysis showed that PDZD4 was positively correlated with the biological process of cell-cell adhesion via plasma membrane adhesion molecules. The involvement of cell-cell adhesion via plasma membrane adhesion molecules can inhibit tumor cell invasion and metastasis. Adhesion between normal cells can restrict the migration of tumor cells and maintain the integrity of normal tissue structure [22]. Abnormal expression or mutation of cell membrane adhesion molecules, such as E-cadherin and N-cadherin, in tumors may lead to the loss of cell-cell adhesion, thereby promoting tumor cell invasion and metastasis [23]. This suggests that the abnormal expression of PDZD4 may lead to the functional loss of cell membrane adhesion molecules, thus promoting the progression of colon cancer. KEGG analysis showed a negative correlation between PDZD4 and the calcium signaling pathway. Abnormal activation of the calcium signaling pathway is associated with tumor development and proliferation. In certain tumor cells, increased calcium ion influx leads to sustained activation of the calcium signaling pathway, which may be related to enhanced cell proliferation, survival, and invasion capabilities. For example, abnormal expression of calcium ion channels, calcium pumps, and calcium regulatory factors in some tumor cells may result in uncontrolled cell cycle progression and inhibition of apoptosis, promoting tumor development [24, 25]. This suggests that the abnormal expression of PDZD4 may lead to sustained activation of the calcium signaling pathway, thereby promoting the progression of colon cancer.
Immunohistochemical staining experiments showed that PDZD4 was downregulated in colon cancer tissues compared to adjacent normal tissues. The expression level of PDZD4 was significantly higher in well-differentiated colon cancer tissues compared to poorly differentiated colon cancer tissues, and higher in stage I-II colon cancer tissues compared to stage III-IV colon cancer tissues. This is consistent with our previous clinical pathological analysis of bioinformatics results. qRT-PCR experiments showed that PDZD4 expression was also lower in colon cancer cell lines compared to normal cells.
Although the exact mechanism of PDZD4 in cancer is not yet elucidated, the functional roles of homologous proteins have been widely reported in various cancers. LNX1, as one of the family members of PDZD4, inhibits cell proliferation by mediating the ubiquitination and degradation of PBK [26]. Another family member, PDZRN4 (LNX4), has been shown to be an inhibitor of tumor cell proliferation in prostate, breast, and liver cancers [27–29]. In our study, CCK-8 assays demonstrated that PDZD4 (also known as LNX5 or PDZRN4L) overexpression could suppress the proliferation capacity of CRC cell lines SW620 and SW480, providing the first evidence of the impact of PDZD4 on cell proliferation in colon cancer. The family member PDZRN4 has been found to inhibit cell migration and invasion in prostate and breast cancers [27, 28]. In our study, scratch and Transwell assays showed that PDZD4 could suppress migration and invasion of SW620 and SW480 colon cancer cell lines, providing the first evidence of the impact of PDZD4 on cell migration and invasion in colon cancer.
The PI3K-AKT pathway is an important signaling pathway involved in various biological processes such as cell growth, survival, proliferation, and metabolism. Abnormal activation of the PI3K-AKT pathway is associated with tumor cell growth and proliferation. In many tumors, mutations, gene amplification, or excessive activation of the PI3K-AKT pathway can lead to abnormal cell proliferation. Activated AKT can promote cell cycle progression, inhibit apoptosis, and enhance cell growth and proliferation through multiple pathways. Activated AKT can also regulate the expression and activity of extracellular matrix-degrading enzymes in tumor cells, promoting cell invasion and metastasis. Moreover, activation of the PI3K-AKT pathway can affect cell cytoskeleton remodeling, epithelial-mesenchymal transition, and further promote tumor cell invasion and metastasis [30–31]. We found that overexpression of PDZD4 led to a significant downregulation of the expression levels of P-PI3K/PI3K and P-AKT/AKT in both SW620 and SW480 CRC cell lines, suggesting that PDZD4 could inhibit activation of the PI3K-AKT pathway, thereby suppressing proliferation, invasion, and migration of colon cancer cells.
PDZD4 has been identified as a biological target in various tumors. For instance, a study on hepatocellular carcinoma demonstrated that PDZD4, as a neutrophil-related gene, is a reliable predictor for hepatocellular carcinoma and its expression levels are positively correlated with patient survival [32]. In a study on gastric cancer, PDZD4 was identified as one of the genes in an early diagnostic model for gastric cancer [14]. Additionally, in bioinformatics analysis of rectal cancer, PDZD4 was found to be one of the prognostic factors associated with m6A modification genes [17]. However, the specific functions of PDZD4 in cancer have yet to be validated, and our research contributes to understanding its biological effects in colorectal cancer.
In conclusion, PDZD4 can inhibit the progression of colon cancer, and its low expression is often associated with poor prognosis in colon cancer patients, providing new targets and insights for clinical treatment. However, our study still has certain limitations, and the related pathways and specific molecular mechanisms of PDZD4 action require further investigation.