Biochimica et Biophysica Acta (BBA) - Gene Regulatory Mechanisms
Epigenetic modification facilitates proline synthase PYCR1 aberrant expression in gastric cancer
Introduction
Metabolic reprogramming is one of the hallmarks of cancer [1]. Targeting cancer metabolism and related metabolic enzymes may provide new insights for cancer treatment. Recently, increasing evidence has demonstrated that rewiring of mitochondrial proline metabolism is emerging as a key pathway in sustaining cancer cell proliferation, survival, and metastasis [2]. Increased proline levels in tumor tissues or body fluid samples have been confirmed by metabolomic technology in gastric cancer (GC), and higher level of proline was also an indicator of GC metastasis [3]. Thus, targeting proline metabolism and related enzymes could be beneficial for cancer treatment [4], [5].
The key enzyme catalyzing the last step of proline biosynthesis, pyrroline-5-carboxylate reductase 1 (PYCR1), is overexpressed in several malignancies including GC . Our previous study has confirmed that PYCR1 upregulation contributes to GC progression and might be a potential biomarker for predicting poor prognosis [6]. We also identified that PYCR1 expression varied according to WHO (World Health Organization) histopathological classification of GC. In addition to histological classification, the Lauren classification, which distinguishes intestinal-type, diffuse-type and mixed-type GC, is the most commonly used [7]. More importantly, molecular subtypes proposed by The Cancer Genome Atlas (TCGA) and Asian Cancer Research Group (ACRG) in recent years that associate molecular features with histological phenotypes and clinical characteristics have deepened our understanding of GC heterogeneity [8], [9]. In this regard, we seek to know whether reprogramming of proline metabolism via PYCR1 in GC is cancer-subtype specific; however, there is little evidence.
On the other hand, the mechanism underlying PYCR1 overexpression in GC remains unknown. Studies including ours have reported that PYCR1 could be regulated by the oncogenic transcription factor c-MYC [10] and PI3K signaling [6], [11]. c-MYC amplification and PIK3CA (coding PI3K) mutation are common molecular events in gastric carcinogenesis [12], [13], which might partially account for PYCR1 upregulation. Apart from transcriptional factors regulation, epigenetic modifications, such as methylation and acetylation, play a critical role in gene expression regulation. Several studies have confirmed that epigenetic alterations, such as aberrant DNA methylation, histone modification, non-coding RNAs and RNA editing are common in gastric malignancies [14]. Therefore, it is necessary to study whether epigenetic modification affects PYCR1 upregulation in GC.
The current study applied bioinformatics analysis combined with histologic and molecular biological experiments to investigate PYCR1 expression patterns in GC subtypes and potential mechanisms underlying PYCR1 overexpression. The findings of this study might contribute to a better understanding of the expression profile and epigenetic regulation of the key enzyme in proline synthesis in carcinogenesis.
Section snippets
Cell lines, antibodies and chemical reagents
The human gastric cancer cell lines AGS (adenocarcinoma/primary, Lauren intestinal type), HGC27 (undifferentiated carcinoma/metastatic lymph node), MKN45 (poorly-differentiated carcinoma/liver metastasis, Lauren diffuse type), and NCI-N87 (well-differentiated adenocarcinoma/liver metastasis, Lauren intestinal type) were purchased from National Infrastructure of Cell Line Resource (Beijing, China). KATO III (signet cell carcinoma/pleural effusion, Lauren diffuse type) was obtained from Procell
PYCR1 is overexpressed in intestinal-type gastric adenocarcinoma and associated molecular subtypes
Using a bioinformatics approach based on patient-derived data, we found that according to Lauren classification, intestinal-type gastric adenocarcinoma presented higher PYCR1 mRNA expression than normal tissues either in TCGA cohort or ACRG cohort (Fig. 1a–b). IHC staining with GC tissues indicated that PYCR1 staining score was higher in intestinal-type GC compared to diffuse-type GC (Fig. 1c). The representative images of PYCR1 expression pattern in three subtypes of GC in Lauren
Discussion
Evidence showed that metabolic shift towards proline synthesis might be a general metabolic rewiring signature for most cancer types [2], [21]. PYCR1, being one of the key enzymes catalyzing proline synthesis, has recently received high emphasis in terms of tumor development and progression [22]. Based on our previous study [6], the current study provides new information on the expression profile of PYCR1 during gastric carcinogenesis and the epigenetic mechanism behind PYCR1 aberrant
Conclusions
Overall, this study found that PYCR1 expression is heterogeneous across GC subtypes, with intestinal-type GC and associated molecular subtypes having the highest expression. The coordinated effects of CpG sites hypomethylation and p300-induced H3K27ac modification occurring within the PYCR1 gene promoter partially contribute to maintaining the higher PYCR1 expression in GC (Fig. 6). The current study combined our previous work to further highlight the importance of proline metabolism catalyzed
Ethics approval and consent to participate
All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1964 and later versions. Informed consent to be included in the study, or the equivalent, was obtained from all patients.
Availability of data and materials
All data supporting the findings of this study are available within the article and its supplementary information files. Raw data can be obtained from the corresponding author on reasonable request.
Funding
This study was funded by Special funds for Beijing Key laboratory of Helicobacter pylori infection and upper gastrointestinal diseases from Peking University Third Hospital (Y57405-32) and National Natural Science Foundation of China (81672410).
CRediT authorship contribution statement
Conception and design: LYZ, SYX; Experiment, analysis and interpretation of the data: SYX, XYY, JXY, XLT, ZHY; Acquisition of data: SYX, XYY, JXY; Statistical analysis: SYX, XYY; Drafting of the article: SYX; Manuscript revision and supervision: LYZ, SYX.
Declaration of competing interest
The authors declare that they have no competing interests.
Acknowledgments
We thank the whole staff in the Medical Research Center of Peking University Third Hospital for their excellent technical assistance on experiments. We thank all anonymous reviewers helping us improve the quality of our manuscript.
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