Zinc plays a crucial role in human biology, being integral to more than 10% of the human proteome and participating in over 300 cellular functions, including the regulation of gene expression by approximately 3000 zinc finger transcription factors, many of which are implicated in cancer development [11]. Among the members of zinc transporters family, SLC30A6 demonstrates unique expression patterns, notably enriched in tissues such as the brain, lung, and intestine [6]. The irregular expression of zinc transporters has been linked to the advancement of cancer [9, 11]. Therefore, our study aimed to comprehensively analyze the gene expression patterns of the SLC30A6 gene and its methylation and mutation profile across diverse types of cancer along with its impacts within the tumor microenvironment.

SLC30A6 plays a multifaceted role in cancer by impacting critical genes across various biological pathways. It regulates zinc homeostasis, influencing genes like MT1 and ZIP1, and controls cell cycle progression through CDK1 and E2F1 [27]. SLC30A6 also affects mitochondrial function via genes like MT-CO1 and contributes to chromosome stability by interacting with TP53 and BRCA1 [28]. In the immune microenvironment, it modulates immune responses through PD-L1 and CTLA4, while also influencing apoptosis and cell survival via BCL2 and CASP3 [29]. Additionally, SLC30A6 impacts key signaling pathways involving AKT1 and NF-κB, and plays a role in epigenetic regulation by affecting DNMT1 and EZH2. Overall, SLC30A6 is a critical regulator in cancer, influencing numerous genes and pathways essential for tumor progression [30]. In Fig. 9, SLC30A6's diverse functional roles in cancer are illustrated through its impact on key genes across various biological pathways.

SLC30A6 plays a pivotal role in cancer by regulating key genes involved in zinc homeostasis, cell cycle progression, mitochondrial function, and chromosome stability. It also modulates immune responses, apoptosis, signaling pathways, and epigenetic regulation. These interactions underscore SLC30A6's critical influence on tumor development and progression

In our investigation, we initially assessed the expression levels of SLC30A6 in tumor tissues compared to corresponding normal tissues using data from TCGA and GTEx databases. Our analysis revealed significantly elevated expression of SLC30A6 across various cancer types, indicating its crucial role in maintaining zinc homeostasis. Given that SLC30A6, primarily located in the trans-Golgi network and vesicular compartments, functions as a zinc exporter, its upregulation likely contributes to maintaining lower levels of cytoplasmic zinc in these malignancies. Consequently, SLC30A6 emerges as a potential therapeutic target for these cancers. This observation is consistent with prior studies reporting increased expression of SLC30A6 in normal human colonic mucosa and the upregulation of SLC30A6 transcripts encoding zinc efflux transporters on ER membranes in patients with CRC and gastric cancer [14]. Additionally, upregulation of SLC30A6 has been documented in prostate cancer cells, and under mild cytotoxic zinc exposure, SLC30A6 overexpression was noted. Conversely, decreased expression of SLC30A6 was observed in normal breast epithelial cells [31].

According to our analysis, the expression levels of SLC30A6 may serve as prognostic indicators for OS in certain cancer types, including PDAC, UCEC, LUAD, LIHC, and KIRP. Elevated expression of SLC30A6 in these patients was associated with poorer OS, whereas lower expression was linked to ESCC KIRC. Moreover, it has shown that upregulation of SLC30A6 expression in gastric malignancy was positively associated with better OS, first-progression survival (FPS), and post-progression survival (PPS) [32], nevertheless, increased expression of that predicts poor prognosis in pancreatic cancer with the possible contribution in various cancer-related signaling pathways such as p38 MAPK and NFkB pathways [33]. In this regard, it has shown that modulation of SLC30A6 expression through zinc supplementation in cancer patients presents a promising approach for addressing zinc deficiencies and regulating cancer-associated inflammation.

In another part of study, our findings revealed a correlation between SLC30A6 levels and distinct gene expression profiles across various cell lines. Positive associations were observed between SLC30A6 expression and heightened expression in several cell lines, including breast, thyroid, myeloma, and certain subtypes of lung cancer cells. Conversely, negative correlations were noted between SLC30A6 expression and the expression levels of several other cell lines, including leukemia, tongue, head and neck, melanoma, and a subtype-specific lung cancer cell line. However, a conflicting observation was reported in a study by Barman et al. [31], which documented elevated SLC30A6 expression in a prostate cancer cell line (PC3) while observing reduced expression in certain breast cancer cell lines (MDA-MB-231 and MCF7). Thus, although the molecular mechanisms underlying SLC30A6-related processes remain unclear, the differential expression of SLC30A6 may serve as a potential biomarker for the mentioned cancers. Furthermore, the association of SLC30A6 expression with specific subtype-specific cell lines, such as lung cancer, suggests that the sequestration of zinc into the trans-Golgi network and vesicular compartments may contribute to the malignant phenotype. Regarding SLC30 family, Liu et al. investigated the roles of zinc transmembrane transporters, in different cancers. The research focused on SLC30A5 and its significance in hepatocellular carcinoma (HCC). The findings revealed that SLC30A family genes are overexpressed across various cancers, with SLC30A5 being particularly associated with poor prognosis in HCC. High SLC30A5 expression correlated with increased immune cell infiltration, immune checkpoints, and key molecules involved in angiogenesis and epithelial-mesenchymal transition (EMT). Functional assays demonstrated that knocking down SLC30A5 in HCC cells reduced their proliferation, migration, and invasion capabilities, highlighting SLC30A5 as a potential prognostic marker and therapeutic target in HCC [34]. More recently, Iwabuchi et al. also explored the role of zinc transmembrane transporters, particularly ZnT5 and ZnT6, in breast cancer. The authors note that zinc levels and the expression of these transporters are elevated in breast cancer tissues compared to normal tissues. Despite this, the specific functions of ZnT5 and ZnT6 heterodimers in cancer remain unclear. The study reveals that ZnT5 may play a role in reducing cancer aggressiveness by inhibiting cell migration. Knockdown of ZnT5 in breast cancer cells led to increased migration, reduced E-cadherin expression (associated with cell adhesion), and increased markers of epithelial-mesenchymal transition (EMT), such as vimentin and matrix metalloproteinase 9. These findings suggest that ZnT5 could be involved in moderating breast cancer progression by restraining cell migration, making it a potential target for therapeutic strategies [35].

The functional enrichment analysis of SLC30A6 and its correlated genes across various cancer types revealed a notable enrichment of pathways closely linked to cancer progression and fundamental cellular functions. Moreover, it highlights the potential involvement of SLC30A6 in nuclear processes crucial for cancer cell growth and proliferation [36]. Additionally, the identification of molecular functions such as nucleic acid binding and catalytic activity further implicates SLC30A6 in the regulation of gene expression and cellular signaling pathways [7].

Pan-cancer analysis of key genes correlated with SLC30A6 in our study also revealed the probable role of SLC30A6 in cancer development and progression. Several key genes involved in numerous cellular processes were detected, among which SMEK2 is a pivotal gene that affected five essential cancer signaling pathways by the phosphorylation of the key proteins involved in these pathways including AKT, MAPK3, JAK/STAT, TGFß, and NFkB [37]. Studies have underscored SMEK2's pivotal role in gene expression regulation, alongside its dual nature as both an oncogene and tumor suppressor. Moreover, research has highlighted SMEK2's significance in leukemia development and progression [38], as well as its involvement in apoptosis and proliferation in human breast cancer cells [39]. Moreover, heightened expression of BIRC6 has been associated with an unfavorable prognosis across a spectrum of malignancies, including colorectal, epithelial ovarian, non-small-cell lung, prostate, and hepatocellular carcinomas [40,41,42,43]. Meanwhile, STRNs have been implicated in the regulation of critical cellular functions such as cell cycle control, cell growth, and motility [44]. Dysregulation of these processes has been linked to the initiation and progression of various malignancies.

Furthermore, we examined the relationship between the expression levels of SLC30A6 and the prognosis of malignant tumors. SLC30A6 showed potential prognostic significance in several carcinomas, including CSCC, KRPCC, LHC, LA, and PDA. These findings align with previous research indicating that higher expression of SLC30A6 predicts poorer DFS in pancreatic cancer patients [33]. The underlying mechanism behind the prognostic role of SLC30A6 may involve networks of co-expressed genes associated with the metabolism of DNA, RNA, or protein. Furthermore, the knockdown of SLC30A6 in Capan-1 pancreatic cancer cells suppressed the phosphorylation of ERK1/2, NF-κB p65, and p38, suggesting a potential link between SLC30A6 and aberrant activation of nuclear factor-kappa B (NF-κB) contributing to the rapid proliferation of malignant cells [33]. In another study by Liu et al., they explore the critical role of zinc transmembrane transporters, particularly the SLC30A/ZNT and SLC39A/ZIP families, in cancer development and progression. Their study highlights the disruption of zinc homeostasis as a key factor in cancer, with specific focus on the expression patterns of these transporters across various cancer types. The findings have shown significant dysregulation of three SLC39A genes (SLC39A1, SLC39A4, and SLC39A8) in cancers such as cervical, liver, pancreatic, and kidney cancers. This dysregulation was closely linked to patient prognosis, with SLC39A8 showing low mutation frequency in kidney cancer and SLC39A4 mutations significantly affecting survival outcomes in pancreatic cancer. The study also suggests that these transporters may act as immune regulators within the tumor microenvironment, offering potential targets for therapeutic strategies and advancing the understanding of zinc's role in cancer progression [45].

Moreover, we investigated how SLC30A6 may influence cancer patients' response to immunotherapy by analyzing immune-related data using various algorithms. Our results consistently revealed a positive association between SLC30A6 expression and the infiltration of multiple immune components, including TAMs, NK cells, CD4 + T cells, CD8 + T cells, B cells, macrophages, mast cells, and dendritic cells, among others, across various cancers. In contrast, several other immune cell types displayed a negative correlation with SLC30A6 expression levels and their infiltration.

It has revealed that SLC30 family members, such as SLC30A6, exhibit distinct expression patterns in various immune cells and play crucial roles in inflammatory and infectious responses. Zinc homeostasis is essential for DC maturation, with a key step being the reduction of intracellular free zinc induced by lipopolysaccharide (LPS). This reduction is facilitated by alterations in the expression of zinc transporters. Previous research has shown that LPS stimulation increases SLC30A6 expression in DCs, and it has been suggested that LPS-mediated reductions in intracellular free zinc in DCs occur predominantly through TRIF-mediated Toll-like receptor (TLR) signaling [46]. The decline in intracellular zinc levels during DC maturation leads to increased expression of major histocompatibility complex (MHC) II, differentiation of monocytes, enhanced antigen presentation, adaptive immune cell activation, and granulopoiesis [46]. Moreover, intracellular zinc deficiency impairs various cellular activities, including neutrophil functions such as phagocytosis and neutrophil extracellular trap (NET) formation, cytokine production, reactive oxygen species (ROS) production, and NK cell functions such as recognition of MHC-I on target cells and lytic activity [47].

We also identified distinctive patterns of SLC30A6 methylation alterations across different cancer types. Specifically, hypermethylation of SLC30A6 was evident in KIRC patients, while hypomethylation was observed in several other carcinomas. These variations may arise from differences in methylation levels at various CpG sites within the promoter region of SLC30A6, potentially influenced by transcription factor binding. However, it is important to note that our analysis did not determine the methylation status of specific CpG sites within the SLC30A6 promoter. It is plausible that methylation of CpG sites in regions less sensitive to transcription factor binding may have a limited impact on SLC30A6 expression.

In the final stage of our study, we examined the mutation profile of SLC30A6 and identified missense mutations across multiple cancer types. However, another study utilizing the COSMIC database demonstrated an elevated frequency of loss of function mutations in SLC30A6 within cancer tissues compared to healthy samples [10]. Missense mutations represent somatic driver mutations that confer a growth advantage to tumor cells. Mutant forms of SLC30A6 may fail to efficiently export zinc, leading to the accumulation of zinc within intracellular vesicles and disrupting zinc homeostasis. This dysregulation could potentially contribute to the initiation and/or progression of tumorigenesis.

This study provides the first comprehensive analysis of SLC30A6 expression in PAAD, filling a significant gap in the current literature, where no prior research has specifically examined this gene's role in PAAD. Our findings, based on data from the TCGA and GTEx databases, reveal that SLC30A6 is significantly overexpressed in multiple cancer types, including PAAD, when compared to normal tissues. This overexpression suggests that SLC30A6 may play a crucial role in tumorigenesis and cancer progression across various cancers.

In our study, SLC30A6 expression was also correlated with several key genes across different cancers, implicating it in broader oncogenic processes. Notably, in PAAD, high SLC30A6 expression was linked to significant genomic alterations, including the loss of function in chromosome segment 12q24.33. This association suggests that SLC30A6 might contribute to chromosomal instability, a common feature in cancer development and progression.

Functional enrichment analysis further highlighted the involvement of SLC30A6 in critical biological processes and pathways, such as cell cycle regulation, DNA replication, and mitochondrial function, all of which are essential for cancer cell survival and proliferation. The enrichment of these pathways in PAAD underscores the potential importance of SLC30A6 in driving pancreatic cancer progression. Moreover, the prognostic analysis revealed that elevated SLC30A6 expression is associated with poorer outcomes in several cancers, including PAAD. This suggests that SLC30A6 could serve as a valuable prognostic biomarker, helping to identify patients with more aggressive disease and potentially guiding treatment strategies. Finally, the analysis of the immune microenvironment in PAAD indicated that SLC30A6 expression is associated with both immunostimulatory and immunosuppressive effects. This dual role could have significant implications for the development of immunotherapeutic strategies, particularly in targeting SLC30A6 as a way to modulate the immune response in pancreatic cancer.

In conclusion, our thorough pan-cancer analysis of SLC30A6 revealed different expression patterns across various tumors, with significant correlations observed with clinical prognosis, immune infiltration levels, and mutation as well as methylation profile in multiple human cancers. Overall, these findings underscore the crucial involvement of SLC30A6 in tumorigenesis and tumor progression, warranting further research to uncover underlying mechanisms and explore therapeutic avenues targeting SLC30A6 in cancer treatment strategies.