As the main enzymes controlling glycolysis and glycogen synthesis in cancer cells, PGMs may play an all-purpose role of specific inhibition of glucose metabolism in cancer cells. So far, we have known that PGM1 is involved in glycogen metabolism, catalysing the mutual conversion between 1-phosphate glucose and 6-phosphate glucose and playing a vital role in regulating glucose homeostasis[12]. PGM2 has phosphopentose mutase activity. It is more than 10 times more effective as a phosphopentose mutase than as a glucose phosphate mutase. PGM2L1 has high glucose 1,6-bisphosphate (G16P) synthase activity but low phosphopentose mutase and glucose phosphate mutase activities (< 5%)[13]. PGM3 is mainly involved in the biosynthesis pathway of hexosamine, catalysing the mutual conversion of N-acetylglucosamine 6-phosphate and N-acetylglucosamine 1-phosphate[14]. PGM5 (also called apiculin) is a molecular chaperone of muscular dystrophy proteins[15]. PGM5 lacks enzymatic activity but plays a significant role in the formation, maintenance, and repair of myofibrils[16]. However, previous PGM studies were often limited to a single member in a single tumour or a specific pathway[6, 8–10]. With its multidisciplinary analysis employing transcriptomics, proteomics, and cytomics, this study is the first to give a systematic and comprehensive overview of the expression, genetic variation, and activation pathways of the PGM family and their roles in immune cell infiltration and prognosis in human tumours. The results showed that the PGMs are differentially expressed between various carcinoma and corresponding normal tissues in multilevel data analysis. The expression of PGMs is associated with various signal transduction pathways and immune cell infiltration status. The expression of PGMs can influence the prognosis of many cancers, and mutation or CNV of PGMs can have an impact on their expression in cancers and the prognosis of the patients.
In this study, multi-tier data from TCGA, Oncomine, the Human Protein Atlas, and CCLE were used to perform multidimensional analysis on the mRNA and protein expression levels of PGMs in 33 tumours. In terms of mRNA expression level, the results of our TCGA data analysis and Oncomine database validation showed that the expression of PGMs differs between cancers. PGM1 and PGM5 are significantly downregulated in a variety of tumours, including breast cancer and colon cancer. In addition, we verified the higher expression of PGM2L1 and the lower expression of PGM5 in gastric cancer, and the lower expression of PGM1 and PGM5 in colorectal cancer using the qRT-PCR experiments. At the protein expression level, immunohistochemistry showed that the staining intensity of PGMs mostly showed moderate or low intensity in all 20 examined tumour tissues. PGM3 is the most abundantly expressed PGM, with a certain amount of expression observed in all 20 tissues. At the cellular level, PGM2L1 expression is low in five cell lines but not the colorectal cancer cell line, PGM5 expression is low in all six cancer cell lines, but the expression of the other PGMs in all six cancer cell lines is high. Existing studies support these results. For example, PGM1 has been downregulated in lung cancer cells and hepatic cancer cells[6, 7, 17]. PGM5 is a downregulated tumour suppressor gene in colorectal cancer, suggesting that PGM5 has the potential to be a new predictor and treatment target for colorectal cancer[10]. Our study is the first to describe the expression profiles of PGMs in a variety of cancers, which provides a new perspective for studying PGMs as biomarkers and therapeutic targets of cancer.
To explore the molecular significance of PGMs in carcinogenicity, we analysed the correlation between the expression of PGMs and cancer signal transduction pathways. The results showed that the expression of PGMs is significantly correlated with the inhibition and activation of multiple carcinogenic pathways, such as protein secretion, DNA repair, UV response, the G2/M checkpoint, and the mitotic spindle. PGM1 is mainly involved in haemoglobin metabolism and the hypoxic pathway. PGM2 is mainly involved in pathways including the mitotic spindle, PI3K-AKT-mTOR signalling, the G2/M checkpoint, and protein secretion. PGM5 is mainly involved in pathways including myogenesis, mTORC1 signalling, the G2/M checkpoint, and DNA repair. The pathways most associated with PGM family expression include protein secretion, UV response, and DNA repair. In addition, PGM1, PGM2, PGM2L1, and PGM3 are more closely related to the activation of oncogenic pathways, while PGM5 has more inhibitory effects on oncogenic pathways. In summary, our study showed that the PGM family are involved in various cancer-related signal transduction pathways and play different roles in the occurrence and progression of cancer.
PGMs has been reported to be involved in immune cell responses[18–20], so we analysed the correlation between the expression level of PGMs and the level of immune cell infiltration in pan-cancer. The results showed that the expression of PGMs is significantly associated with the infiltration of multiple immune cells in different cancers, the most relevant of which are naive B cells, activated natural killer cells, resting mast cells, M0 macrophages, and activated dendritic cells. Up to now, few studies have been conducted on the relationship between immune cell infiltration and the PGM gene family, and only the mutation of PGM3 weakens human immunity and increase serum IgE, causing severe combined immunodeficiency syndrome[21–23]. Our study found that the expression of PGM family is closely associated with the level of immune cell infiltration in various cancers. Therefore, the relationship between the PGM family and immune cell infiltration is worth further exploration.
This study analyses the relationship between PGMs and the prognosis of pan-cancer. The results show that the expression of different PGMs has significantly different prognostic significance in a variety of cancers. For example, the expression of the PGM1 gene is correlated with a high survival rate from mesothelioma and cervical cancer and a low survival rate from lung squamous-cell carcinoma, lower-grade brain glioma, uveal melanoma, and pancreatic cancer, while PGM5 gene expression is associated with a high survival rate from renal clear cell carcinoma and cervical cancer and a low survival rate from lung squamous carcinoma, hepatocellular carcinoma, adrenocortical carcinoma, and lung adenocarcinoma. Our findings as mentioned above suggest that both the positive and negative effects of PGMs on prognosis in different cancers may be modified by different cancer tissues,and the mechanism needs further study.
Glycogen metabolism plays an important part in the survival and growth of cancer cells under glucose-depleted or hypoxic conditions[24, 25]. PGM1 is associated with hypoglycaemia-induced accumulation of glycogen in cancer cells[26] and is an essential factor through which cells maintain growth under sustained glucose depletion[27]. Therefore, the expression of PGM1 is closely associated with the development of cancer. The existing literature validates this idea. For example, PGM1 can inhibit the progression of hepatic cancer by regulating the transport of glucose[6]; PGM1 is closely associated with the development of gliomas[28]; endometrial cancer patients are more sensitive to different levels of PGM1 expression[29]; glucose deprivation–induced AMPK activation can induce PGM1 expression, which is associated with poor prognosis in lung cancer patients[7]. In addition, PGM3 is involved in the induction of prostate cancer cell death and may be a potential molecular target for prostate cancer therapeutic agents[30], and Targeting PGM3 to inhibit the hexosamine biosynthetic pathway can lead to growth arrest and apoptosis in breast cancer[8]. Low expression of hepatic PGM5 is associated with poor prognosis in hepatic cancer patients; PGM5 has the potential to be a prognostic and diagnostic biomarker of hepatic cancer[9]; low expression of PGM5 is also associated with poor prognosis in colorectal cancer patients; and the upregulation of PGM5 inhibits the progression of proliferation, migration, and invasion of colorectal cancer cells, indicating that PGM5 can serve as a potential novel prediction and therapeutic target for colorectal cancer[10]. Our results indicated that,as a group of important enzymes in glucose metabolism, PGMs play significant roles in various cancer types. It suggested that metabolic changes are important hallmarks of cancer cells. The potential of PGMs as prognostic predictors and therapeutic targets in different cancers is worth further investigation.
This study also analysed the mutation and CNV frequencies of PGMs in pan-cancer. The results indicated that the overall average mutation rate of PGMs ranged from 0 to 10.6%, in which the mutation frequencies of PGM1 and PGM5 are relatively higher. The mutation frequency of all PGMs is relatively high in endometrial cancer, which is a cancer with a high global mutation burden[31]. In contrast, mutations of PGMs are very rare in thymoma, pheochromocytoma, paraganglioma, chromophobe renal cell carcinoma, and cholangiocarcinoma. The CNV frequencies of PGMs are relatively low. PGM2L1 exhibits more copy number amplification in cancers such as breast cancer, oesophageal cancer, head and neck cancer, squamous-cell carcinoma, and adenocarcinoma of the cervix, while PGM3 shows a high frequency of copy number reduction in prostate cancer. No CNV in PGMs is observed in renal clear cell carcinoma, papillary renal cell carcinoma, thyroid carcinoma, or thymoma. We also analysed the mutations of PGMs in human cancer cell lines which indicated that PGM mutations are more common in lung cancer and colorectal cancer cell lines. These results revealed the genetic variation characteristics of PGMs across cancer types, suggesting that variations in PGMs may be associated with the genesis and progression of certain cancers.
When we analysed the effect of PGM variations on the expression of PGMs and the pan-cancer prognosis, we found that PGM mutations do not affect the expression of PGMs in most cancers; PGM1 mutation only affects its expression in endometrial carcinoma, and PGM3 mutation only affects its expression in hepatocellular carcinoma and endometrial cancer. The CNVs of PGMs can widely affect their expression in a variety of cancers, including breast cancer, cervical cancer, and colon cancer, and PGM expression is usually upregulated with increasing copy number. The burden of tumour copy number change is an important prognostic factor related to the recurrence and death of pan-cancer[32]. Therefore, we analysed the effects of PGM variations on the prognosis of pan-cancer. The results showed that mutation and CNV of PGMs could affect the prognosis of some cancers, and most of them have adverse effects. Among them, mutation of PGM3 is a protective factor against endometrial cancer. The increase in the copy number of PGM2L1 is a risk factor for squamous-cell carcinoma and adenocarcinoma of the cervix. According to the above findings, the mutation frequency of PGM3 in endometrial cancer is high, and the copy number amplification frequency of PGM2L1 in squamous-cell carcinoma and adenocarcinoma of the cervix is relatively high. Therefore, we may conclude that mutations in PGM3 and copy number amplification of PGM2L1 could serve as prognostic predictors for endometrial cancer and squamous-cell carcinoma and adenocarcinoma of the cervix, respectively.