The endoplasmic reticulum (ER) plays a pivotal role in the synthesis and proper folding of most proteins, including almost all secreted proteins (Oakes and Papa, 2015). When the accumulation of unfolded proteins in the ER exceeds a certain limit due to various intracellular and extracellular stimuli, a signal transduction pathway, called the unfolded protein response (UPR), is initiated to respond to this disturbance in ER proteostasis (Hetz et al., 2015). The ability of cells to perceive ER stress is essential for cell survival under adverse conditions (Tabas and Ron, 2011). UPR reaction is mainly mediated by three primary sensors: protein kinase R (PKR)-like endoplasmic reticulum kinase [PERK, also known as eukaryotic initiation factor 2-alpha kinase 3 (EIF2AK3)], inositol-requiring gene 1 (IRE1), and activating transcription factor 6 (ATF6).

Cancer cells usually invade into surrounding tissues. The conditions in these environments are usually unfavorable (hypoxia, lack of glucose, lactic acidosis, oxidative stress, insufficient amino acid supply, etc.), which will hinder protein folding in the ER (Lee et al., 2003; Ma and Hendershot, 2004; Lee and Hendershot, 2006; Moenner et al., 2007). In addition, many cancer cells have to overcome similar internal stresses, including oncogene activation, increased glycolysis, etc., which may cause overwhelming protein synthesis and a large demand for secretory pathways (Tollefsbol and Cohen, 1990; Ruggero, 2013; Dejeans et al., 2014). Accordingly, many studies have reported the activation of the UPR in various primary human solid tumors, including glioblastoma, rhabdomyosarcoma, and carcinomas of the breast, stomach, esophagus, liver, colon, and pancreas (Fernandez et al., 2000; Shuda et al., 2003; Moenner et al., 2007; Shi et al., 2019; Wang et al., 2019; McCarthy et al., 2020). The elevated UPR tends to alleviate the stress damage and promote cancer cell survival. The UPR pathway can also regulate cell survival by modulating apoptosis. When cells are exposed to prolonged ER stress conditions, UPR signaling will eventually induce cell death (Mori, 2000; Ron and Walter, 2007; Tabas and Ron, 2011).

PERK is a type I ER transmembrane protein containing a stress-sensing luminal domain and a cytosolic kinase domain (Shi et al., 1998; Wang et al., 2018). Under normal conditions, the ER chaperone GRP78/BiP associates with the luminal domain, thus inhibiting its activation. Accumulation of unfolded proteins in the ER triggers GRP78/BiP titration and PERK is then activated (Bertolotti et al., 2000; Su et al., 2008). PERK phosphorylates the eukaryotic initiation factor 2α (eIF2α) at serine-51 (Marciniak et al., 2006). Phosphorylation of eIF2α hinders the assembly of the ribosome and consequently reduces the protein translation. However, translation of certain eIF2α downstream effectors, ATF4, and CAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), which modulate cellular survival pathways, is increased upon ER stress. The other PERK substrate, transcription factor NF-E2-related factor 2 (NRF2), regulates cellular redox potential and contributes to cell adaptation (Cullinan and Diehl, 2004).

PERK functions as a mediator in UPR-related disease in humans, including tumorigenesis and neurodegenerative disorders. PERK has been shown to support tumor growth, metastasis, autophagy, and radiation resistance and was therefore proposed as a future therapy target to overcome therapy failure (Bobrovnikova-Marjon et al., 2010; Avivar-Valderas et al., 2011; Rouschop et al., 2013; Liu et al., 2015; Feng et al., 2017; Salaroglio et al., 2017; Zhang et al., 2018). Small molecule inhibitors were designed to inhibit PERK phosphorylation and its downstream signaling, which had been tested in antitumor treatment but showed severe side effects in preclinical studies (Atkins et al., 2013; Axten et al., 2013; Yu et al., 2015). However, the mechanisms underlying the effects of PERK in tumorigenesis and development need further study.

In addition, the interaction between cancer cells and the immune system plays a significant role in the occurrence, development, and treatment of cancer. The tumor microenvironment (TME) is comprised of interacting cancer and stromal cells. Among them, infiltrating immune cells account for a large proportion (Bindea et al., 2013). Almost all types of immune cells, including B cells, CD8⁺ T cells, CD4⁺ T cells, neutrophils, natural killer (NK) cells, and dendritic cells (DC), are found in the TME (Bindea et al., 2013). Different from the canonical antitumor role played by immune cells, immune infiltration into the TME represents a strategy tumor cells use to avoid being killed (Gajewski et al., 2013; Quail and Joyce, 2013; Topalian et al., 2015). A recent study showed that PERK promotes the functionality of tumor myeloid-derived suppressor cells (MDSCs) through stimulation of NRF2, which restricts the immunostimulatory axis of cytosolic mitochondrial DNA-STING-type I IFN (Mohamed et al., 2020). Immunotherapy targeting interactions between immune cells and tumor cells has been developed in recent years to reactivate adaptive and innate immune systems and create a robust antitumoral immune response. Inhibitors augment T-cell activity by blocking programmed cell death protein 1 (PD-1) and PD-1 ligand (PD-L1) and show remarkable clinical effects (Topalian et al., 2015; Gordon et al., 2017). However, there are still too few potential targets for immunotherapies. It is still necessary to further discover more specific or general immune biomarkers in cancer therapy.

In this study, we visualized the prognostic landscape of PERK in pan-cancer using databases, including Oncomine, PrognoScan, Kaplan–Meier plotter, and UALCAN. We further investigated the link between PERK and immune cell infiltration of tumors using the Tumor Immunoassay Resource (TIMER). The relationship of PERK with marker genes of immune cells was evaluated by searching the CellMarker database. The workflow of this study design and analysis is summarized in Figure 1. The findings indicate that PERK influences the prognosis of patients with cancers, probably via its interaction with infiltrating immune cells.

Previous studies have proven that PERK has the function of supporting tumor growth, metastasis, autophagy, and radiation resistance (Bobrovnikova-Marjon et al., 2010; Avivar-Valderas et al., 2011; Rouschop et al., 2013; Liu et al., 2015; Salaroglio et al., 2017). Herein, we report the correlation between PERK expression level and the prognosis of various cancers. The high-expression level of PERK correlates with a poorer prognosis in KIRP, LGG, BRCA, and THCA and with a favorable prognosis in HNSC. Moreover, our analyses show that in BRCA, HNSC, and THCA, the immune infiltration levels and diverse immune marker sets are correlated with the levels of PERK expression. Thus, our study provides insights into understanding the potential role of PERK in tumor immunology and its potential application as a cancer biomarker.

In this study, we examined the expression levels of PERK and the systematic prognostic landscape in different types of cancers. The differential expression of PERK between cancer and normal tissues was observed in many types of cancers. Based on the Oncomine database, we found that PERK was upregulated in brain and CNS cancer, head and neck cancer, and breast cancer and downregulated in lymphoma and sarcoma (Figure 2A). However, in the TIMER database, the expression of PERK mRNA was upregulated in BRCA, CHOL, LIHC, LUAD, LUSC, and STAD and downregulated in COAD, READ, and THCA, compared with normal adjacent tissues (Figure 2B). Nevertheless, in these databases, we found consistent prognostic correlations between PERK expression in KIRP, LGG, BRCA, THCA, and HNSC. The PrognoScan database search revealed that increased PERK expression correlated with poor prognosis in brain cancer and soft tissue cancer. Lung cancer was an exception where high levels of PERK expression showed a better prognosis. Furthermore, data analysis from the Kaplan–Meier plotter showed that a high level of PERK expression was correlated with favorable prognosis in bladder carcinoma, esophageal squamous cell carcinoma, lung adenocarcinoma, rectum adenocarcinoma, and thymoma and related to a poor prognosis in kidney renal papillary cell carcinoma, liver hepatocellular carcinoma, and thyroid carcinoma (Figure 4). The UALCAN database analysis demonstrated that the high expression of PERK was associated with a poor prognosis in KIRP, LGG, THCA, and BRCA (Figure 5). These results are consistent with previous findings that PERK signaling in cancer contributes to adaptive pathways rather than to cancer cell death, as demonstrated by the fact that pharmacologic inhibition of PERK attenuates tumor growth (Wang et al., 2017, 2019; Bagratuni et al., 2020). PERK's prometastatic functions in breast cancer were mediated by its downstream transcription factor CREB3L1. Inhibition of CREB3L1 by genetic or pharmacological methods suppresses cancer cell invasion and metastasis (Feng et al., 2017). Another report showed that inhibition of the PERK–eIF2α-GRP94 signaling pathway silenced the epidermal growth factor receptor (EGFR) and then increased the radiosensitivity of both radiosensitive and radioresistant oropharyngeal squamous cell carcinoma (OSCC) cells (Zhang et al., 2018). Also, PERK's expression increased the expression of phosphorylated eIF2α (p-eIF2α) and promoted G0–G1 arrest and survival of cancer cells in vitro, suggesting that eIF2α phosphorylation can initiate cytoprotective effects (Ranganathan et al., 2008). Taken together, these findings strongly suggest that PERK is a prognostic biomarker in BRCA, HNSC, and THCA.

We then investigated the potential functions of PERK in certain cancers and found that PERK was significantly enriched in the immune-related signaling pathways in BRCA, HNSC, and THCA, including B-cell differentiation, T-cell differentiation, T-cell activation, etc. This finding is consistent with previous studies showing that PERK has an essential function in the differentiation of naive B cells into plasma cells (Gass et al., 2008; Ma et al., 2010; Zhu et al., 2019). PERK has also been reported to play an important role in T-cell development, albeit in an ATF4-independent manner (Solanki et al., 2016). Further analysis demonstrated the strong associations between PERK expression and tumor-infiltrating immune cells, especially B cells, CD8⁺ T cells, macrophages, neutrophils, and dendritic cells, in BRCA, HNSC, and THCA (Figure 7). Moreover, the correlation between PERK expression and the marker genes of immune cells implicates the role of PERK in regulating tumor immunology in BRCA, HNSC, and THCA. Gene markers of M1 macrophages such as INOS and CXCL10 and M2 macrophage markers such as CD163 and IL10 showed strong correlations with PERK expression (Table 2). These results prove that PERK could regulate the polarization of tumor-associated macrophages. Similarly, PERK's function in regulating the phenotypic polarization of macrophages has been reported during the pathological progression of nonalcoholic fatty liver disease (NAFLD) (Yang et al., 2019). Additionally, our results also indicated that PERK is capable of activating Tregs and inducing T-cell exhaustion. The elevation of PERK expression positively correlates with the expression of Treg and T-cell exhaustion marker genes (FOXP3, CCR8, STST5B, CD25PD-1, CTLA4, LAG3, and GZMB in HNSC and THCA, Table 3). These results are consistent with a recent study revealing that the PERK signaling pathway contributes to mitochondrial exhaustion of T effector cells (Hurst et al., 2019). Furthermore, significant correlations were observed between PERK expression and the regulation of several markers of T-helper cells (Th17 and Th1) in HNSC and THCA. These correlations suggested a potential mechanism where PERK regulates T-cell functions in these cancers. In sum, our results initially indicate that PERK plays critical roles in recruiting infiltrating immune cells and attenuating the tumor immune system in BRCA, HNSC, and THCA.

Although there are few studies reporting the function of PERK in tumor immunoregulation, a recent report provides a possible mechanism which explains why PERK expression correlates with immune infiltration and poor prognosis. The myelopoiesis process that protects against tumors is drastically damaged in most cancers that block protective antitumor T-cell immunity and promote cancer cell progression (Singhal et al., 2016). Expansion of myeloid-derived suppressor cells (MDSCs) has emerged as a key mechanism of antitumor immune evasion and correlates with a poor clinical outcome and resistance to cancer immunotherapy (Lu et al., 2017). It was determined that PERK signaling increased in tumor-infiltrating MDSCs, mediating the immunosuppression pathway in tumors through inhibition of STING signaling. PERK deletion transformed MDSCs into myeloid cells that activated CD8⁺ T-cell-mediated immunity against cancer (Mohamed et al., 2020). Another study showed that inhibition of PERK in CD8⁺ T cells abrogates mitochondrial ROS generation in PD-1⁺ CD8⁺ tumor-infiltrating lymphocytes (TILs), which boosts CD8⁺ TIL viability and enhances antitumor immunity (Hurst et al., 2019). Therefore, PERK seems to be expressed predominantly in immune cells rather than cancer cells, and its function relates to promote tumor immune escape in BRCA, HNSC, and THCA.

In this study, we found that increased PERK expression correlates with poor prognosis and increased immune infiltration levels in B cells, CD8⁺ T cells, macrophages, neutrophils, and dendritic cells of multiple cancers, especially in breast and thyroid cancers. In addition, PERK expression potentially contributes to the regulation of tumor-associated macrophages, T-cell exhaustion, and Tregs. Therefore, PERK likely plays a pivotal role in immune cell infiltration and as a prognosis biomarker in patients with BRCA, HNSC, and THCA. However, our study lacks the in vitro and animal experiments to confirm the role of PERK in the growth and progression of BRCA, HNSC, and THCA and its relationship with the infiltration of immune cells in the tumor microenvironment. Therefore, further research is needed to verify the role of PERK in cancers using these models.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

PW and GZ designed this study, analyzed the data, and supervised the entire study. PW, LH, MY, XL, and YS extracted the information from the databases. PW wrote the manuscript. GZ revised the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.