From a follow-up period of up to 130 months, we collected consecutive colorectal NET patients by pathological diagnosis from 2011-2017 and found that FFA ≥ 0.52 mml/L can be considered a cut-off point for predicting LNM and poor OS, HDL-C≤1.0 mmol/L can be considered another cut-off point for predicting tumour size ≥2 cm and poor OS, and the combined detection of FFA and HDL-C complementarily predicted OS in our study.
Tumour grade in colorectal NETs refers to the proliferative activity of neoplastic cells, as measured by the mitotic rate and/or the Ki-67 index, and differentiation refers to the extent to which tumour cells resemble their normal counterparts (Klimstra et al. 2010). Some studies have suggested that worse survival was found for advanced tumour grade, consistent with our results(Wu et al. 2020). The risk of metastases is verylow (<3%) in rectal NETs< 10 mm and very high (30%-80%) in rectal NETs ≥ 20 mm in size, while betweenthese two extremes, 4%-20% of patients with rectal NETs measuring 10-19 mm have synchronous or metachronous metastases(de Mestier et al. 2019). In addition, tumour size and lymph node metastasis have also been shown to be predictors of poor prognosis in colorectal NETs (Fields et al. 2019; Sohn et al. 2017; Wu et al. 2020), consistent with our study.
Traditionally, the level of FFA has been detected to assess lipid metabolism and has been associated with hypertension, cardiovascular disease, type 2 diabetes, and obesity (Ghosh et al. 2017).Similarly, HDL-C, LDL-C, TG, CHOL, ApoA1) and ApoB, all widely used, were analysed to assesslipid metabolismin the body as combined biochemical indicators. Recently, an increasing number of researchers have indicated that FFAs and HDL-C are involved in colorectal cancer. FFAs are overexpressed in colorectal cancer (Zhu et al. 2021), dietary palmitic acid promotes cancer metastasis (Pascual et al. 2021), and the reprogramming of fatty acid metabolism plays an important role in LNM of various cancers by the fatty acid-binding protein 5 (FABP5) pathway (Kawaguchi et al. 2016; Zhang et al. 2020).Interestingly, lymph node metastasis and poor prognosis were significantly associated with the pretreatment FFA level of colorectal NETs, consistent with our results. Oxidative modification of HDL results in compositional and functional changes, and following increased cholesterol ester transfer protein (CETP) activity in parallel with decreased lecithin–cholesterol acyltransferase (LCAT) activity, HDL particles become larger, and changes in HDL composition, such as enrichment with TG and reduced ApoA1, paraoxonase-1 (PON1) and apoM and increased serum amyloid A (SAA) proteins, occur. The interaction of overexpressed SAA with TLR2 in cancer cells leads to cancer progression through the NF-κB-mediated pathway (Ganjali et al. 2021). In addition, HDL-C was related to poor prognosis inpatients with colorectal cancer, and very low levels of HDL-C (<30 mg/dL) in women were significantly associated with cancer mortality (Penson et al. 2019; Wang et al. 2019). In our study, tumour size (≥2 cm) was related to pretreatment HDL-C level, and worse overall survival was found in colorectal NETs with larger tumour size, consistent with the above researchers.
CgA, Syn and CD56 are three neuroendocrine differentiation (NED) immunohistochemistry markers frequently used in NETs (MacIntosh et al. 2015; Modlin et al. 2010; Wiedenmann 1991). The results of immunohistochemistry are usually marked by semiquantitative scores to show positive cell percentages and positive cell staining intensities but are limited to qualification and by pathologist experience. Reports of CgA, Syn and CD56 are difficult to standardize, and it isdifficult to predict prognosis by immunohistochemistry results directly. CgA in serum is an important biomarker in advanced pancreatic cancer and metastatic neuroendocrine tumours (Malaguarnera et al. 2009; Warner et al. 2011), but due to the lower incidence of NETs (Cives and Strosberg 2018) and the high cost of detecting reagents, the serological CgA test has not been widely performed. FFA and HDL-C are common biochemical biomarkers detected in clinical laboratories, and they have promising applications in predicting LNM and tumour size and predicting the OS of colorectal NETs.
In our study, the patient numbers of FFA ≥0.52 mmol/L and HDL-C ≤1.0 mmol/L were very low, and thus there may be some statistical bias associated with their combined detection. It is worth noting that patients who had FFA ≥0.52 mmol/L accounted for almost half of the total colorectal patients, and those with HDL-C ≤1.0 mmol/L accounted for almost one-fifth of the total; interestingly, patients with FFA ≥0.52 mmol/L and HDL-C ≤1.0 mmol/L also made up one-fifth of the patients with FFA ≥0.52 mmol/L. The median survival of patients with FFA ≥0.52 mmol/L was 67.8±7.3 months, the median survival of those with HDL-C ≤1.0 mmol/L was 53.8±9.7 months, and the median survival of those with FFA ≥0.52 mmol/L and HDL-C ≤1.0 mmol/L was only 45.9±14.3 months. The 5-year survival of patients with FFA ≥0.52 mmol/L was 50.9%, that of patients with HDL-C ≤1.0 mmol/L was 47.4%, and that of patients with FFA ≥0.52 mmol/L in combination with HDL-C ≤1.0 mmol/L was only 40.0%. Therefore, patient with high FFA and low HDL-C had a worse prognosis than patients with only high FFA, only low HDL-C and patients with low FFA or high HDL-C. Some researchers have suggested that FFA and HDL-C may promote cancer progression by different signalling pathways (Ganjali et al. 2021; Kawaguchi et al. 2016; Pascual et al. 2021; Zhang et al. 2020), so the combined detection of FFAs and HDL-C may have a complementary effect, consistent with our findings.
Mixed adenoneuroendocrine carcinoma of the colon and rectumare rare cancers; they are characterized by the presence of a combination of epithelial and neuroendocrine elements, where each component represents at least 30% of the tumour (Qiu et al. 2018), and are anunmet area where NETs need to be described and defined (Ramage et al. 2019). In our study, there were more colorectal NET patents with positive LNM than with negative LNM, which may be due to not excluding mixed adenoneuroendocrine carcinoma among the patient groups, which is composed of poorly differentiated neuroendocrine carcinoma and easily metastasizes(Milione et al. 2018; Qiu et al. 2018).
Reduced plasma levels of HDL-C are a hallmark of obesity and cardiovascular diseases (CVDs); similarly, reduced ApoA1has also been associated with cardiovascular risk (Raitakari et al. 2013; Su and Peng 2020). Due to component differences between ApoA1 and HDL-C, a similar association was not found for LNM, tumour size or poor survival with HDL-C. However, the level of ApoA1 in colorectal NETs was significantly lower than that in controls, and the area under the receiver operating characteristic (ROC) curve was 82.2%. Interestingly, the potential diagnostic ability is worth analysing.
There were some limitations in our study. First, it was of a retrospective design and included a relatively small number of patients. However, we believe the results are reliable. Because this study lasted more than 130months, we could investigate the long-term survival outcomes and prognostic factors after different treatments, even with the small number of patients. Second, progression-free survival (PFS) data were not collected, and prognostic results could not be predicted comprehensively. Finally, further studies should be performed to validate our main conclusions.