Significant histological therapeutic resistance to NACRT was detected in groups with high expression of Nrf2, TXNRD1, and HO-1 among the ESCC cases examined in this study. In the pre-therapeutic biopsy specimens, the tumor reduction effect was significantly attenuated in the high Nrf2, TXNRD1, and HO-1 expression groups. TXNRD1Δ and HO-1Δ were significantly higher, while 8-OHdGΔ was significantly lower in the NACRT-ineffective groups. In surgical specimens, the OS was significantly lower in the high Nrf2, TXNRD1, and HO-1 expression groups. DFS was also significantly lower in the high Nrf2, TXNRD1, HO-1, and Ki-67 expression groups.
The results indicate that the therapeutic efficacy of NACRT could be predicted by examining the expression status of antioxidant proteins in pre-therapeutic endoscopic biopsy specimens. In addition, the results of recent studies indicate that the elimination of ROS is related to the development of radioresistance in ESCC [40-42]. Generally, ROS generation is considered an essential mediator of the cytotoxic effects of cisplatin [9]. A significant correlation between unfavorable histological responses to NACRT and high Nrf2, TXNRD1, and HO-1 expression in pre-NACRT endoscopic biopsy specimens indicated that the cytotoxic effects of NACRT in could be impaired significantly in this group. Kawasaki et al. reported the possibility of the conversion of non-responder patients to responders by Nrf2 knockdown before subjecting patients with ESCC with high-Nrf2 expression to CRT, which may lead to a more favorable prognosis [29]. In contrast, targeting the transcription factor Nrf2 was considered significantly challenging in clinical settings [43]. To include curative resection as a viable treatment option for ESCC, surgical resection without NAC or NACRT might be a potential treatment strategy for patients with ESCC who exhibit high Nrf2, TXNRD1, and HO-1 expression, considering the highly aggressive malignant tendencies of tumors and the probable ineffectiveness of neoadjuvant therapy [17, 18, 24, 25, 44]. The expression of antioxidant proteins in the endoscopic biopsy specimens could also predict the effects of definitive CRT (dCRT) and NACRT, which could help avoid esophagectomy, a potentially invasive procedure, in patients with ESCC. In previously reported studies on dCRT, high Nrf2 expression status in biopsy specimens was reported to be correlated with the development of therapeutic resistance to dCRT [37, 45]. Therefore, patients with ESCC exhibiting low expression of antioxidant proteins in the Nrf2 signaling pathway in pre-therapeutic endoscopic biopsy specimens are considered reasonably suitable candidates for dCRT. In addition, results of several studies have also suggested that Nrf2 and TXNRD1 are useful markers for predicting the efficacy of chemotherapy or irradiation therapy in other neoplasms, such as non-small cell lung cancer or ovarian cancer [46, 47]. However, further investigation is necessary for confirming these propositions.
The status of Nrf2, TXNRD1, and HO-1 expression in the surgically resected specimens correlated significantly with the 5-year OS and DFS rates of patients with ESCC participating in this study. In addition, increased HO-1Δ correlated significantly with unfavorable 5-year OS and DFS rates, as well as with a high Ki-67Δ value and DFS rate in the patients examined. HO-1 is well known to be associated with therapeutic resistance to cisplatin-based therapy [26-28]. Therefore, increased HO-1Δ could predict the clinical outcomes in patients undergoing cisplatin-based chemotherapy or CRT. However, none of these markers were found to be independent predictive factors, as shown in the multivariate analysis in this study. This discrepancy is considered to result from the intrinsic correlation among the multiple factors examined. For instance, the upregulation of the Nrf2 pathway is commonly known to enhance antioxidant response and cell proliferation, which would result in increased expression of TXNRD1, HO-1, and Ki-67 [12, 13, 19-21, 38]. In addition, TXNRD1 and HO-1 expression were significantly correlated with established clinicopathological factors such as pT and pN in the present study.
The significant upregulation of the Nrf2 pathway has also been reported to contribute to increased tumor cell proliferation and angiogenesis in cancer [18, 24, 25, 38], which possibly led to a significantly lower DFS in high Nrf2, TXNRD1, HO-1, and Ki-67 expression groups in this study. Therefore, the abundance of these proteins could help predict the recurrence of ESCC in patients. In addition, upregulation of the Nrf2 pathway was reported to reduce the clinical benefits obtained from cisplatin-based adjuvant chemotherapy in lung squamous cell carcinoma [48]. As in lung carcinoma, the increased expression of Nrf2, TXNRD1, and HO-1 could potentially suppress the effects of NACRT in patients with ESCC; however, further clinical studies are warranted.
Significant differences were observed in the expression status of Nrf2 and Ki-67 and the levels of 8-OHdG between endoscopic biopsy and surgical specimens. In addition, TXNRD1Δ, HO-1Δ, and 8-OhdGΔ were significantly associated with the histological therapeutic efficacy in the cases examined in this study. The significant increase in Nrf2 expression in ESCC was considered to be attributable to the antioxidant response, which is consistent with results of a previous study by Kawasaki et al. [29]. In addition, Ki-67 was also reported to be upregulated in specimens with increased Nrf2 expression, which is indicative of high levels of cell proliferation [38]. However, the expression of TXNRD1 and HO-1 did not increase significantly in response to NACRT, possibly because the expression levels of TXNRD1 and HO-1 were high even before NACRT was administered. Increased Nrf2 expression owing to genetic alterations in the NRF2 (known as NFE2L2) have been reported [49]. Therefore, high levels of TXNRD1 and HO-1 expression could be associated with these genetic abnormalities upstream of the Nrf2 pathway. Furthermore, the activation of Kras, Braf, and Myc oncogene activation and PTEN anti-oncogene disruption, which are detected frequently in ESCC, could also lead to the upregulation of the transcription of Nrf2 [50-52]. Therefore, TXNRD1 and HO-1 overexpression could be induced because of diverse Nrf2 upregulation patterns in carcinogenesis. A significant positive correlation was also observed between Nrf2 and TXNRD1 expression in pre-NACRT endoscopic biopsy specimens in the present study (Additional file 3). In addition, TXNRD1 expression was reported to be inhibited efficiently by both cisplatin and its monohydrated complex [53] therefore, the upregulation of TXNRD1 expression by Nrf2 can be opposed. An alteration in HO-1 expression in response to chemotherapy or irradiation therapy has not been reported yet; however, the upregulation of HO-1 as well as TXNRD1 expression by Nrf2 could be countered by treatment with cisplatin or by irradiation. The significant difference in TXNRD1Δ, HO-1Δ, and 8-OHdGΔ values between the ineffective and effective groups in the present study also suggested that the ineffective group elicited a greater antioxidant response. However, no significant differences were observed in Nrf2Δ values between the two groups. These results indicate that patients with ESCC in the NACRT-ineffective group could exhibit an antioxidant response that involves the selective and stronger upregulation of TXNRD1 and HO-1 expression, which contributes to ROS detoxification by Nrf2 pathway proteins [15, 16, 54]. In addition, in the effective group, TXNRD1 expression was significantly suppressed upon cisplatin treatment, which could occur independent of the Nrf2 pathway. The difference in TXNRD1 expression between the ineffective and effective groups suggested that the suppression mechanism undertaken by cisplatin could vary.
It is also important to note the limitations of this study. First, ESCC is characterized by intertumoral heterogeneity; therefore, the sites of endoscopic biopsy could have considerably influenced the results of the expression of antioxidant proteins in these specimens. Second, as carcinoma cells completely disappeared after NACRT in some cases, the changes in protein expression in these cells remained unevaluated. Third, besides pT, no other factor was determined to be an independent prognostic factor, possibly owing to the intrinsic correlation among the factors studied. Further investigations are necessary to elucidate the clinicopathological significance of the results of our present study.