Apolipoprotein A1 and lipoprotein(a) are protective factors for male papillary thyroid cancer patients. A cross-sectional study of single academic center in China.

Background: Papillary thyroid cancer (PTC) is the most common type of thyroid cancer and the incidence of PTC continued to increase over the past decades. Many studies showed that obesity is an independent risk factor for PTC and obese PTC patients tend to have a relative larger tumor size and higher grade of tumor stage. Obesity is closely related with lipid metabolism, while the correlation between serum lipid and PTC remains unclear. Therefore, this study aimed to investigate the association between serum lipid level and PTC. Methods: We retrospectively analyzed 1018 PTC patients diagnosed and treated in our hospital, all these cases were first diagnosed with PTC and had complete clinical information including ultrasound reports before surgery, serum lipid (CHOL, TG, HDL-c, LDL-c, Apo-A1, Apo-B, Apo-E, Lp (a)) results, surgical records and pathological reports. Results: All these serum lipid biomarkers did not associated with tumor size in the female group. In the male group, on crude analysis, Apo-A1 and Lp (a) showed a marginally significant association with tumor size OR and 95% CI 0.158 (0.021-1.777), p=0.072; 0.997 (0.993-1.000) p=0.082 respectively. After adjusting for age and multifocality, Apo-A1 and Lp (a) showed a significant association with tumor size OR and 95% CI 0.126 (0.016-0.974) p=0.047; 0.996 (0.992-1.000) p=0.046 respectively. This association become more apparent in a young male subgroup, OR and 95% CI 0.051 (0.005-0.497), p=0.009; 0.926 (0.978-1.012), p=0.037 respectively. CHOL, TG, HDL-c, LDL-c, Apo-B, Apo-E did not show significant association with tumor size. As for LNM, neither in the male group nor in the female group were found to be associated with any serum lipid biomarkers.

This association become more apparent in a young male subgroup, OR and 95% CI 0.051 (0.005-0.497), p=0.009; 0.926 (0.978-1.012), p=0.037 respectively. CHOL, TG, HDL-c, LDL-c, Apo-B, Apo-E did not show significant association with tumor size. As for LNM, neither in the male group nor in the female group were found to be associated with any serum lipid biomarkers.
Conclusion: As PTC incidences continues to increase, our study demonstrated that Apo-A1 and Lp (a) are two protective factors for male PTC patients and provided us new clues about PTC and lipid metabolism, which may contribute to further investigation concerning diagnosing and preventing this most common type of thyroid cancer.

Background
The incidence of PTC increased drastically in the past few decades, meanwhile, many researchers had noticed a simultaneously increased morbidity of obesity. Many studies had been launched and completed to probe the correlation between these PTC and obesity which incidence rate zoomed in the same period [1]. A pooled analysis in 2014 revealed that body mass index (BMI) and body fat percentage were significantly associated with increased risk of PTC in a population composed of Americans, Italians and Germans [2]. Furthermore, a meta-analysis comprised 12199 thyroid cancer cases unveiled that a statistically significant greater risk of thyroid cancer (including papillary thyroid cancer, follicular thyroid cancer and anaplastic thyroid cancer) in overweight and obese individuals [3]. Besides higher risk of morbidity of PTC, obesity was associated with larger tumor size and marginally significantly associated with advanced tumor stage according to a population-based study from Nevada [4]. Serum lipid is closely related with obesity and BMI and abnormal lipid metabolism is a common feature in many cancers, such as breast cancer and clear-cell renal carcinoma [5,6]. While the correlation between serum lipid and PTC remains elusive.
The aim of this study is to elucidate the relationship between serum lipid and extend of PTC at diagnosis, through the use of a population-based samples. Expecting to provide more clinical evidence about the correlation between PTC and serum lipid. This study sets out to explore whether serum lipid is associated with tumor size, multiplicity and lymph node metastasis (LNM) of PTC in a Chinese population.

Study participants and data collection
Patients newly diagnosed with papillary thyroid cancer between June 2018 and June 2019 were retrospectively analyzed in this study. The inclusion criteria were as follows: (1) primary PTC verified by pathology; (2) age ≧ 18 years old; (3) without hypolipemic agents history; (4) did not merge with other kind of diseases; (5) complete Clinical and pathological data.
The main clinical data include serum lipid level when diagnosed with PTC, age, gender, ultrasound evaluation before surgery and pathological reports after surgery.
Serum lipid markers include CHOL, TG, HDL-c, LDL-c, Apo-A1, Apo-B, Apo-E, Lp (a). Age was classified as 55 versus 55 and older. Gender was male or female. Tumor size was classified as ≤ 2 cm versus 2 cm. Information about lymph node dissection was retrieved from surgical records and pathological reports, 5 or more lymph nodes presented in pathological reports were considered lymph node dissection had been performed in the previous surgery. Cancer stage was determined through the American Joint Committee on Cancer (AJCC) TNM Staging For Thyroid-Differentiated and Anaplastic Carcinoma (8th ed., 2017). The number of tumors was determined by ultrasound reports and pathological reports, cases with only one tumor were deemed as unifocal and with 2 or more tumors were considered as multifocal. We compared the ultrasound reports before surgery and pathological reports after surgery to find out the correct and false prediction rate of lymph node metastasis by ultrasound before surgery. Clinical and pathological data were collected from the database established by The First Affiliated Hospital of Sun Yat-Sen University. Data collection was performed by two independent researchers. Statistical analysis. SPSS version 23.0 was used to conduct all statistical analyses. Univariate and multivariate logistic regression models were applied to assess the influence of serum lipid level on clinical characteristics by calculating the odds ratios and their corresponding 95% confidence intervals (CIs). P value < 0.05 was considered to be statistically significant.

Results
A total of 1018 PTC patients were included in this study (Figure 1), necessary clinicopathological information are shown in Table 1 (at the end of this text). Among the 1018 PTC patients, 892 were under the age of 55 (87.6%), 126 (12.4%) were 55 years old or older. The ratio of women to men was 2.8:1. The tumor size of 905 (88.9%) cases were 2cm or smaller, 113 (11.1%) were larger than 2cm. 606 (59.5%) PTC patients were performed lymph node dissection during surgery, 398 (39.1%) cases with lymph node metastasis verified by pathological reports. As to the TNM stage, the majority of patients had stage Ⅰ cancers (96.4%), 35 and 2 patients had a stage Ⅱ and stage Ⅲ cancers (3.4% and 0.2% respectively), no patient included in this study had a stage Ⅳ cancer. 713 (70%) patients had only one tumor, 305 (30%) cases had two or more tumors. 262 patients were suggested with LNM and 756 patients were regarded without LNM by the ultrasound report before surgery. Among these 756 patients, 374 (49.5%) were performed prophylactic central lymph node dissection and 178 (23.5%) were shown with LNM actually based on the pathological reports. The inaccurately predicted rate may be higher because there were 382 patients did not go through prophylactic lymph node dissection during the surgery.
Logistic regression univariate analysis and multivariate analysis were used to analyze the association between each of the 8 serum lipid biomarkers and tumor size or LNM or multifocality or false negative prediction of ultrasound. No statistically significant association was found in terms of this analysis ( Table 2, additional file 1). After adjusting for age, we found that patients with high level of serum Apo-A1 were shown to have marginally significant higher odds of small tumor size relative to patients with lower level of serum Apo-A1, OR and 95% CI 0.514 (0.204-1.292).
Then we divided these patients into two groups by gender (265 men and 753 women) and analyzed the association between each of the 8 serum lipid biomarkers with tumor size or LNM respectively. As shown in Table 3 (additional file 2 (Table 4).
As for LNM, neither in the male group nor in the female group were found to be associated with any serum lipid biomarkers (

Discussion
Serum lipid profile has been shown to be a potential diagnostic biomarker for many cancers, such as head and neck squamous cell carcinoma, colorectal cancer and lung cancer [7][8][9]. The aberrant lipid biosynthesis was also showed to be associated with cancer cell migration, invasion and induction of tumor angiogenesis [10]. In addition, many studies had demonstrated that obesity is strongly related to lipid disturbances and abnormal metabolism [11,12]. Furthermore, obesity has been regarded as a risk factor for many cancers, including thyroid cancer [1,13], so it is natural to speculate the relationship between blood lipid and papillary thyroid cancer.
In this study, we found that patients with lower levels of serum Apo-A1 and Lp (a) are more likely to be diagnosed with larger tumor sizes of PTC in a male cohort, especially in a young male subgroup (< 55 years old), this correlation was not seen when it comes to the female cohort. Tumor size has been demonstrated as an independent predictor of recurrence in PTC in previous study (tumors > 2 cm associated with higher risk of recurrence than those ≤ 2 cm ), [14] which indicates Apo-A1 and Lp (a) are two protective biomarkers for male PTC patients.
Apo-A1 has been proved to be associated with many cancer, furthermore, it could be used as a potential biomarker for detection and diagnosis for many cancers such as bladder cancer [15,16]. In a recent study, researchers observed that lower serum levels of Apo-A1 in thyroid cancer patients compared to healthy controls, indicating that Apo-A1 may play an anti-tumor role in thyroid cancer [17]. Similarly, Lp (a) also manifested anti-neoplastic properties in other cancers [18][19][20]. One Japanese study enrolled 10413 participants revealed that a low Lp (a) level was a significant risk for cancer deaths [21]. Interestingly, both Apo-A1 and Lp (a) were showed association with tumor size only in the male group, but not in the female group. This phenomenon implies that sexual hormone may influence lipid metabolism and further affect PTC tumor growth. A study from Salford once reported that low-dose testosterone administration to women for two years would result in atherogenic effects on some parameters of lipid and lipoprotein metabolism, which include HDL-C, Apo-A1 and VLDL-C [22].
Though several studies had demonstrated that Apo-A1 can be a used as a prognostic parameter in many cancers, but the mechanism of the association between high serum Apo-A1 levels and favorable prognosis in several cancers are still unknown. There are increasing evidence manifested that systemic inflammation plays an important role in contributing the development and progression of malignancies [23]. Systemic inflammatory markers, such as CRP (C-reactive protein), was shown to be an independent predictor of poor outcome in patients suffered from various cancers [24][25][26]. A recent research unveiled that serum Apo-A1 levels showed strong negative correlation with systemic inflammatory markers including serum CRP and interleukin (IL)-8 levels and blood neutrophil count in 144 colorectal cancer patients [27], which indicate systemic inflammation may influence tumorigenesis and regulate lipid metabolism in the same period, thus, enabling some kinds of serum lipid markers to correlate with tumor characteristics and provide prognostic information.
Larger tumor size always associated with a higher risk of LNM in PTC. Although we find that lower Apo-A1 and Lp (a) levels are significantly associated with larger tumor size in male PTC patients, but they do not show a correlation with LNM. How to precisely predict LNM before surgery is always a trouble for all surgeons. We wished to excavate some information about the relationship between lipid metabolism and LNM, but the results disappointed us in this respect.
Moreover, we found that the rate of accuracy of evaluating LNM for PTC before surgery was not satisfied, even ultrasound is the best way for LNM prediction of PTC currently, the false prediction rate for TNM is about 23.5% or higher according to our analysis. More precise instruments and forecasting models for predicting LNM before surgery should be exploited for clinical use in the future.

Conclusion
In conclusion, the present study identified Apo-A1 and Lp (a) are two protective factors for male PTC patients, patients with higher level of Apo-A1 and Lp (a) are more likely to have a smaller tumor size.
Gender differences exhibited in the association between PTC and serum lipid level providing us new clues to explore the origination of this cancer and the underlying molecular mechanism of lipid metabolism in PTC patients require further investigation.