A novel and robust prognostic signature for patients with ccRCC has been in high demand. Different from previous prognostic systems, either comprehensive or simply based on clinicopathologic characteristics and the current TNM staging method, ideas about the prognostic implications of hypoxia-related gene signatures are the focus of this study.
Since the 1990s, hypoxia-induced pathways have received great attention from researchers. Rapidly-proliferating tumor cells, incremental oxygen consumption and structurally and functionally abnormal vasculature inside tumor tissue co-formed a unique hypoxia micro-environment of solid tumors, distinguishing cancer from normal tissue and also partly contributing to a poor clinical outcome (16). Interestingly, the hypoxia signaling pathway can sometimes also be activated by genetic alterations in cancers; for example, the disfunction of VHL can be observed in ccRCC (17).
Increasing evidence demonstrates that there are direct or indirect interactions between the hypoxia signaling pathway and tumorigenesis or tumor progression. However, adaptive responses triggered by hypoxia containing hundreds of related proteins and genes can be both diverse and specific. Besides, intersecting with other signaling mechanisms also makes the hypoxia signaling pathway intricate and obscure. It is impractical to figure out all the steps involved in the hypoxia signaling pathway relying on present theory. Accordingly, we focused our attention to the hypoxia-related genes of the hypoxia signaling pathway.
Hypoxia-induced up-transcription of genes has shown a demonstrable influence on a series of tumor biological processes, including cell uncontrollable proliferation and immortalization (18), angiogenesis (18), glucose metabolism (19), immune escape (20), tumor invasion and metastasis (21) and radiation resistance (22). This agrees with our assumption that the differential expression of hypoxia activating genes is a considerable risk feature for patient survival.
Here, we tried to investigate the correlation between the survival of patients with ccRCC and hypoxia-related genes and subsequently identify genes significantly contributing to clinical outcome. After a series of biological and statistical analyses, ANKZF1, ETS1, PLAUR, SERPINE1, FBP1 and PFKP were selected, and we hypothesized that they could be used together to generate an integrated hypoxia-related RiskScore for optimizing the previous prediction model.
Subsequently, a series of analyses including ROC curve, risk plot and survival analysis were performed to evaluate RiskScore, resultantly suggesting that RiskScore had ideal overall diagnosis and prognosis performance. In addition, results of univariate Cox regression analysis and multivariate Cox proportional hazards regression analyses revealed that RiskScore based on hypoxia-related genetic signatures was an independent risk factor beyond TNM staging and grading. A multiple risk factors ROC curve also suggested that RiskScore was an excellent independent prognosis predictor for the five-year survival rate of patients compared with other risk factors such as TNM staging, grading and age. A novel prognosis nomogram optimized via RiskScore, a new independent prognosis predictor, is feasible.
Notably, some genetic signatures chosen for RiskScore calculation seemed to be expressed more in high staging tumors, whereas others were expressed more in low staging tumors. Whether negative or positive, certain correlations between the gene signatures and the tumor clinical grades and TNM stages could be observed in the present study, indicating that those prognostic hypoxia-related gene signatures play anti-tumor or pro-tumor roles in the progression of ccRCC.
Differential expression of the abovementioned six gene signatures and their impacts in cancer have been reported previously. Over-expression of ETS1 (E26 transformation specific-1), which belongs to the large family of transcription factors with ETS domain, was identified in a variety of solid tumors, including breast cancer, lung cancer and, specifically, renal carcinoma (23). Further research provided evidence that ETS1 played a part through regulating the expression of HIF-related genes, consistent with the results of GO and KEGG enrichment analyses in our research (24). ETS1 was considered as an oncogene that was involved in many biological processes, such as angiogenesis, drug resistance and tumorigenesis, and thus was linked to unfavorable survival (23). However, our research came to the conflicting conclusion that the elevated expression of ETS1 was a protective feature for prognosis. Similar experimental conclusions have also been reported for colon cancer cells and invasive breast cancer (25, 26). This may be related to the heterogeneity of the tumor. The apoptosis-inducing activity of ETS1 may also account for that. Pro-apoptotic genes, including CDKN1A (encoding p21 protein), CDKN1B (encoding p27 protein) and caspase I (27), and tumor suppressor protein genes, including p16INK4A (28), were validated as targets of ETS1 (29).
Plasminogen activator urokinase receptor (PLAUR) was another gene among those identified as having prognostic features and over-expressing in a number of tumors. However, we inferred that what really caused poor survival was not PLAUR but its activator, urokinase type plasminogen activator (uPA). A series of integrins and proteases were identified as having the capacity to enable tumor tissues to penetrate the basement membrane (BM) and extracellular matrix (ECM), surround them and metastasize to other anatomical sites and form secondary tumor foci [18]. One of those proteases mediating invasion and metastasis-related molecular processes was uPA, making disseminated tumors hard to eradicate successfully and causing serious clinical problems (30, 31). Serpin Family E Member 1 (SERPINE 1), also known as plasminogen activator inhibitor type 1 (PAI-1), is the main inhibitor of the uPA/uPAR complex, regulating the adhesion balance of ECM in both tumor and normal tissue. Similar to PLAUR, SERPINE 1 has been validated to play a critical role in a variety of tumors (32, 33). Fructose-bisphosphatase1 (FBP1), a rate-limiting enzyme involved in gluconeogenesis, and phosphofructokinase-platelet (PFKP), an isoform of phosphofructokinase-1(PFK-1), are both involved in glucose metabolism regulation and were reported to be associated with the Warburg effect, which describes a trend to generate energy mainly via glycolysis in tumors, initially proposed by Otto Warburg (34–36). Last but not least, ANKZF1 is a cofactor that binds to p97 and regulates its cellular biological functions, such as protein quality control, apoptosis, autophagy, DNA damage repair and transcriptional activation (37). Therefore, the abovementioned six gene signatures have the potential to be reliable biomarkers for prognosis.
Necessarily, we compared the optimized prognosis nomogram and the previous one with several evaluations, including C-index, calibration curves, NRI, IDI and, especially, decision curve analysis (DCA), which was more practical than the typical ROC curve analysis since it took the consequences of clinical strategies into consideration (38). The optimized nomogram was clearly the superior one, with better accuracy and discrimination. Unsurprisingly, subsequent external validation in the ICGC database showed the promising performance of the optimized nomogram. We believe that the findings of our research could provide robust prognostic indicators and underlying therapeutic targets for patients with ccRCC and, more importantly, provide insight into the correlation between hypoxia-related gene expression and clinical outcomes. We believe that patients may benefit from our study.
Although we made several accomplishments in the investigation of novel prognostic biomarkers, there were several limitations in this study. For example, those components participating in the hypoxia signaling pathway and influencing the regulation of hypoxia-inducible genes, whether directly or indirectly, such as pVHL and PHD, were not involved in the construction of the prognostic system, meaning their impact and underlying biological process information were ignored. Further studies on the hypoxia-related signaling pathway and ccRCC are ongoing, and well-designed and prospective studies for enriching relevant theories are warranted.