In this investigation, we have substantiated the upregulation of UPR (unfolded protein response) stress pathways within glioblastoma tumors. We have also pinpointed UPR-related genes whose expression levels exhibit significant associations with patient prognosis. Furthermore, we have unraveled a compelling correlation between the expression levels of two pivotal UPR genes and the enrichment of immune cells within the TME.
Our analysis has unveiled an increase in the expression of ER stress-related genes in glioblastoma when compared to normal tissue and low-grade gliomas. This upregulation stems not only from tumor cells but also from non-tumor cells within the TME, primarily macrophages. Importantly, we observed pronounced intra- and inter-tumor heterogeneity in the expression of ER stress signatures and markers. This heightened expression of ER stress-related genes encompassed nearly all the UPR-related genes we examined, with one noteworthy exception being MAPK8.
Crucially, we have identified certain UPR-related genes, including HSPA5, P4HB, and PDIA4, as robust risk factors, demonstrating their significant prognostic relevance across all three survival analyses conducted. Additionally, OS9 and HSP90B1 emerged as prognostically relevant in two out of the three analyses. These findings align with previous research, which has highlighted the overexpression of HSPA5 and the PDI (protein disulfide isomerase) family in glioblastoma models. This consistency reinforces the notion that UPR's cytoprotective branches play a pivotal role in facilitating the rapid growth of tumors and their ability to adapt to the challenging conditions of the microenvironment [9, 10].
While several ERAD genes exhibited upregulation in glioblastoma tumors and were designated as risk factors, a notable exception was SEL1L. In contrast to previous findings that indicated an escalation in SEL1L protein expression with increasing malignancy in glioma cells and tissues [35, 36], we observed a downregulation of SEL1L in glioblastoma tissue when compared to normal brain and low-grade glioma samples. Notably, SEL1L presents multiple alternative transcripts encoding putative protein isoforms, suggesting that its role may oscillate between oncogenic and tumor-suppressive, contingent upon the cellular context and the prevailing isoform predominance [35].
Another gene exhibiting downregulation in glioblastoma, MAPK8, emerged as a protective factor in all three survival analyses conducted. Previous studies have alluded to the prognostic significance of the MAPK8 gene, postulating that this relevance is rooted in its involvement in programmed cell death [37, 38]. ERN1 (IRE1α), which serves as the activator of MAPK8 within the context of UPR signaling, was also identified as a protective factor in two of the analyses. This finding diverges from a prior report that associated high ERN1 expression with poor prognosis in glioblastoma patients [18]. Notably, in our Cox regression analysis, ERN1 did not emerge as a prognostic factor when adjusted for MAPK8 as a covariate (data not shown), implying that its prognostic relevance may be contingent upon the protective role of MAPK8.
Regarding the PERK/eIF2α/ATF4 branch, our analysis revealed that all the scrutinized genes exhibited upregulation in glioblastoma compared to normal tissue. This finding agrees with previous reports that showed elevated protein and transcript levels of both ATF4 and its target CHOP (DDIT3) in xenograft tumors. Additionally, these findings align with the observed positive correlation between PERK activation and tumor grade in glioma [9, 39].
Although none of the analyzed genes of the PERK/eIF2α/ATF4 branch had a significant impact on prognosis across all three survival analyses, EIF2A and TRIB3, the latter being an apoptosis-related gene activated by CHOP [40], emerged as protective factors based on Kaplan-Meier curve and Cox analysis, respectively. In contrast, ATF3 and ATF5, which are induced by the eIF2α/ATF4 pathway, were identified as risk factors in one and two of the survival analyses, respectively. These findings are consistent with previous reports in other cancer types [41, 42]. These results underscore the complexity of this pathway, that leads to an intricate balance between cytoprotective and cytotoxic outcomes.
Intrigued by the contrasting functions of the IRE1α branch, which can either induce apoptosis via JNK1 or promote survival through the XBP1-dependent activation of adaptive responses, we conducted a survival analysis involving these two genes. In line with their antagonistic roles within the UPR signaling pathway, we observed that patients with elevated XBP1 expression and reduced MAPK8 expression experienced shorter survival compared to patients exhibiting low XBP1 and high MAPK8 expression. However, it is important to note that our standard transcriptomic methods did not provide access to crucial details such as XBP1 mRNA splicing or JNK1 (MAPK8) phosphorylation levels. Additionally, multiple pathways distinct from the UPR can converge on the phosphorylation of JNK1, making it an indirect indicator of IRE1α activation. Therefore, a comprehensive understanding of how these pathways interplay and contribute to either cell survival or cell death is still lacking.
While the precise mechanisms underpinning the connection between IRE1α downstream pathways and immune infiltration remain elusive, prior studies have indicated that IRE1α can either promote immunosurveillance or facilitate immune system evasion through the regulation of damage-associated molecular patterns, cytokines, and the propagation of ER stress signals [20, 34]. We observed that heightened XBP1 and diminished MAPK8 expression correlated with elevated expression of immune cell meta-signatures, which have previously been associated with unfavorable prognosis in glioblastoma. These meta-signatures encompassed cell types such as macrophages, MDSC, Th1, and Treg [29]. This result aligns with recent studies proposing a link between ER stress and the expression of immune cell markers in glioma patient samples [43–45].
Furthermore, when analyzing single cell data, we noted that, although both XBP1 and MAPK8 were highly expressed in immune cells, the XBP1/MAPK8 expression ratio within tumor cells correlated strongly with the abundance of immune cells in each sample. This suggests a potential role for the IRE1α pathways in shaping the TME and influencing the infiltration of immune cell types that may potentiate immunosuppression and are typically associated with worse patient outcomes. However, it is important to emphasize that a more in-depth investigation is warranted to elucidate the intricate mechanisms governing this relationship.