The patients of this study demonstrated that patients with small-round-cell sarcomas and patients with metastases at the first visit showed high LDH levels. LDH is a ubiquitous enzyme among vertebrae organisms and catalyzes the interconversion of pyruvate and lactate concurrently with the interconversion of NAD+ and NADH.[2] LDH has five major isoenzymes, numbered LDH-1 through LDH-5, formed by the association between two different types of subunits, M and H, and encoded by two different genes: ldh-a and ldh-b. LDH-1 and LDH-5 are commonly known as LDHB and LDHA, respectively. The profile ratio of LDH isozyme is tissue-specific. Tumor tissues showing an anaerobic condition express LDH-4 and LDH-5 [17].
The Warburg effect has been shown as the role of LDH in cancer metabolism [3]. In most cells, glucose is metabolized to pyruvate via glycolysis, and then most pyruvate are completely oxidized into CO2 in the mitochondria under the condition of abundant oxygen, a process known as oxidative phosphorylation. When oxygen shortage occurs, the pyruvate is redirected from the mitochondrial oxidative phosphorylation by generating lactate, a process known as anaerobic glycolysis. In normal cells, lactate generation by anaerobic glycolysis is limited to the condition of oxygen shortage; however, in cancer cells, most glucose is converted to lactate regardless of whether oxygen is present. This aerobic glycolysis is known as the Warburg effect [18].
According to the Warburg effect theory, the serum LDH level is increased in patients with metastatic tumor, comprising several active tumor cells. In this study, the serum LDH level was not associated with the tumor size. The serum LDH level may be more strongly associated with tumor activity and metastatic potential than mere tumor size.
Several studies have investigated the prognostic factors associated with soft tissue sarcoma. Distant metastasis at the first visit is evidently a poor prognostic factor [15, 16]. According to recent review papers on the prognostic factors in soft tissue sarcoma, age as a continuous nonlinear variable has an important effect on disease-specific mortality. Tumor size, grade, and margin were prognostic factors for both local recurrence and disease-specific mortality, and compartmentalization and anatomical location of tumors are only associated with disease-specific mortality [1]. Regarding the biomarkers, the pretreatment serum C-reactive protein level was correlated with prognosis [19–21], high neutrophil-lymphocyte ratio was also associated with poor prognosis in soft tissue sarcoma [20, 21], although available reports were limited. Furthermore, serum albumin [22] and hemoglobin [23] levels were also reported as prognostic biomarkers of soft tissue sarcomas. Among them, this study showed that the presence of distant metastases at the first visit was a poor prognostic factor for DSS in patients of all histologic types, and tumor size was a poor prognostic factor for EFS in both patients of all histologic types and those with non-small-round cell sarcomas. Several studies have reported the relationship between distant metastasis and poor DSS [11, 12], and relationship between large tumor size and local relapse risk [24] or distant recurrence [25]. Although some reports showed inconsistent results, our results are generally consistent with those of previous reports.
In addition, this study newly showed that high LDH level is associated with poor DSS rate in patients of all histologic types. Moreover, even in the cohort of patients with non-small-round cell sarcoma without distant metastasis, high LDH level is associated with poor DSS rate. LDH levels are reportedly diagnostic, prognostic, and a predictive markers of therapeutic response in many cancers [4, 6, 17], including renal cell carcinoma [26], nasopharyngeal carcinoma[27], melanoma [28], prostate cancer [29], colorectal cancer [30], and lung cancer [31]. In terms of bone and soft tissue sarcoma, high LDH level was shown to be a significant predictive factor for disease-free survival or overall survival in patients with osteosarcoma [7–9] and Ewing sarcoma [10–12]; however, the relationship between LDH level and prognosis in other sarcomas including non-small-round cell sarcoma, a major component of soft tissue sarcomas, has not been reported. The serum LDH level is a prognostic factor in both patients of all histologic types and those with non-small-round cell sarcomas.
As mentioned above, if the serum LDH level is associated with tumor cell activity and metastatic potential, these results can be reported in the same mechanism. Patients with soft tissue sarcoma showing high serum LDH levels are highly at risk of metastasis, which has a major impact on the patient’s prognosis; therefore, patients with soft tissue sarcoma with high serum LDH level show poor prognosis.
This study has some limitations. First, the number of patients was relatively less compared with the article that examined other prognostic factors. Accordingly, multivariate analysis was considered to be somewhat inappropriate and has not been performed in this study. When multivariate analysis was performed using the LDH level and metastasis at the first visit as variables, high LDH level was an independent risk factor for DSS (hazard ratio, 3.89; 95% confidence interval, 1.15-13.6; p = 0.029, Cox regression proportional hazard model). Second, high- and low-grade histologic tumors were analyzed together. Although analyzing the prognostic factors for high-grade sarcomas only may be interesting, it was not performed due to the small number of patients. Moreover, this analysis method was reasonable to investigate the prognostic factor for all histologic types of soft tissue sarcomas. Third, LDH isozyme was not distinguished. Not only LDH-4 and LDH-5 were expressed in tumor tissues, but also other LDH isoenzymes should have been evaluated together in this study. Therefore, the possibility that other isoenzyme levels were elevated due to other mechanisms cannot be entirely negated. However, in routine blood tests, LDH isozyme was not usually tested, and special examination is required to identify only LDH-4 and LDH-5. This problem should be addressed in a future research.