The findings of this study suggest that case and tumor characteristics were associated with selected cholesterol precursors and cholesterol metabolites reflecting different pathways of cholesterol synthesis. The most consistent associations were observed for measures of obesity including BMI and WHR and circulating levels 7-DC, 7a-HC, 7b-HC, and 7-KC. Furthermore, metabolic diseases including CVD and diabetes were associated with cholesterol precursors 7-DC and desmosterol, even after mutually adjusting these variables for each other. Breast cancer tumor characteristics such as hormone-receptor status, tumor stage, and endocrine therapy were strongly associated with circulating levels of the cholesterol precursors lanosterol and 24-DHLan, and the ROS-produced oxysterols THC and 7b-HC. Weak associations with all patient and tumor characteristics were observed for the ER-modulating oxysterols 27-HC and 25-HC. Overall, the case-characteristics evaluated in this study accounted for a low proportion of the observed variance in oxysterol concentrations.
Cholesterol precursors
The strongest association across all categories was observed for obesity, which was strongly associated with higher levels of 7-DC and 7-KC. 7-DC is a direct cholesterol precursor, while 7-KC is a secondary cholesterol metabolite that can be formed either from other cholesterol metabolites including 7a-HC and 7b-HC (strong correlations, r ≥ 0.75) or from the cholesterol precursor 7-DC (weak correlations, r < 0.1). Cholesterol precursors are considered markers for cholesterol biosynthesis15, and since cholesterol levels are commonly elevated in obesity, this may explain the observed associations between high BMI and circulating 7-DC. Furthermore, obesity is associated with oxidative stress25,26, a condition in which cholesterol oxidation and oxysterol formation was reported to be increased27,28. It is possible that the 7-DC/7-KC pathway may play a role in obesity through increased cholesterol synthesis or oxidative stress.
A previous study reported positive associations between BMI and cholesterol precursors (desmosterol, r = 0.418; lanosterol, r = 0.454) in healthy participants29, while weaker associations between BMI and cholesterol precursors were observed in patients with familial combined hyperlipidemia (desmosterol, r = 0.219; lanosterol, r = 0.203)30, in healthy participants (r ≤ 0.1)31, and in one of our previous studies (r < 0.2) evaluating continuous BMI and cholesterol precursors in women with a breast cancer diagnosis10. These results suggest that altered 7-DC and 7-KC levels are found only in women with obesity (BMI ≥ 30 kg/m2) or with potential other factors related to obesity.
Besides obesity, cholesterol precursors were strongly associated with reported disease at baseline including CVD, hypertension, and diabetes. While desmosterol, an intermediate of the Bloch-pathway, was inversely associated with diabetes and CVD, 7-DC, an intermediate of the Kandutsch-Russel-pathway, was positively associated with diabetes and CVD. A potential differential regulation of the Kandutsch-Russel (7-DC) and the Bloch (desmosterol) pathway has been suggested previously15, and could explain the opposite direction of effect for these cholesterol precursors. Furthermore, the use of statins, which function by blocking HMG-CoA reductase, the first enzyme of the cholesterol synthesis pathway, was associated with lower levels of lanosterol, desmosterol, and 24-DHLan but not with 7-DC, supporting the indication of a differential regulation of cholesterol biosynthesis pathways.
The role of cholesterol precursors as mediators in metabolic processes is of increasing interest. Previous human studies reported altered concentrations of intermediates of cholesterol synthesis in various metabolic and inflammatory diseases such as diabetes, atherosclerosis, and non-alcoholic fatty liver disease4,15. To our knowledge, we are the first to report associations between cholesterol precursors and metabolic disorders in women diagnosed with breast cancer.
A dual role of lanosterol/24-DHLan and 7-DC was also observed with respect to breast cancer characteristics. Adverse breast cancer tumor characteristics such as higher tumor stage were associated with higher levels of lanosterol and 24-DHLan and slightly lower levels of 7-DC; and endocrine therapy use was associated with lower levels of 24-DHLan and higher levels of 7-DC. Sensitivity analyses including only women whose blood was drawn three months after their diagnosis indicated that lower levels of lanosterol and 24-DHLan were indeed associated with endocrine therapy use.
To date, little is known about the role of lanosterol and 24-DHLan in breast cancer. While it has been reported that 24-DHLan may play a role in cholesterol metabolism32, the effect of lanosterol and 24-DHLan in breast cancer development and progression has not been evaluated. Tamoxifen has been reported to block enzymes of the cholesterol biosynthesis pathway leading to the accumulation of cholesterol precursors such as desmosterol and 7-DC33. In this study, we observed slightly higher levels of desmosterol and 7-DC in women reporting tamoxifen use, while significantly lower concentrations of the early cholesterol precursor lanosterol and 24-DHLan were observed, warranting further studies to investigate the underlying biological mechanisms between cholesterol precursors and breast cancer tumor and treatment characteristics.
Cholesterol metabolites: enzymatic
We observed weak associations between 27-HC, 25-HC, 24S-HC and any of the patient and tumor characteristics evaluated. In line with the weak associations observed in this study for 27-HC, a previous study of our working group in healthy women within the EPIC-Heidelberg cohort reported significant but small changes in concentration by lifestyle, diet, reproductive or anthropometric factors (≤ │10│% difference)14.
Consistent with our findings, no association between cholesterol-lowering drugs and 27-HC levels was found in two larger studies of breast cancer patients and healthy postmenopausal women14,34, whereas in another study including 42 breast cancer patients, a decrease in 27-HC levels was observed after two weeks of statin treatment13.
While 27-HC, 25-HC, and 24S-HC were weakly associated with tamoxifen or AI use in this study, a clinical study including 29 breast cancer patients receiving tamoxifen (n = 15) or AI (n = 14) over a 28-day period, reported that tamoxifen treatment significantly decreased circulating levels of 24-HC and 25-HC, whereas AI treatment significantly increased 27-HC levels35. While Dalenc et al. investigated hormone-receptor positive invasive and metastatic breast cancer patient, we included invasive, non-metastatic women with hormone-receptor positive and negative tumors in this study. Kimbung et al. reported that patients with high tumor expression of CYP27A1, the enzyme converting cholesterol to 27-HC, were less likely to receive endocrine treatment (OR = 0.62 (0.45–0.87))12, however, comparison between circulating 27-HC and tissue content are difficult as associations were reported to be weak (r = 0.029)36.
While Kloudova et al. observed differences in 27-levels by tumor size or tumor stage, we observed weak association for 27-HC in this study, potentially due to differences in hormone-receptor status and endocrine therapy between the study populations. Consistent with the results of this cross-sectional study, a previous study of 287 breast cancer cases from the EPIC-Heidelberg cohort found no association between hormone receptor status, tumor stage, and serum 27-HC levels37.
Cholesterol metabolites: non-enzymatic
Age at diagnosis was significantly associated with 7-KC but not with the other oxysterols, which is in line with a previous study on 58 women diagnosed with breast cancer9 reporting strong correlations between 7-KC and age (r = 0.94), but weaker correlations for other oxysterols (r ≤ │0.41).
Furthermore, obesity has been associated with higher levels of 7-KC, 7a-HC and 7b-HC, which can be interconverted. A previous study on serum levels of several oxysterols in men and women found no difference between obese (defined as BMI > 30 without dyslipidemia, diabetes or hypertension) and metabolically healthy participants, whereas participants with metabolic syndrome (defined as at least 3 of the following factors: abdominal obesity, elevated triglycerides, reduced HDL, elevated blood pressure, or elevated plasma glucose) had higher levels of 7a-HC, 7b-HC, Triol (here: THC), and 25-HC as compared to metabolically healthy controls38. As participants with obesity (BMI ≥ 30 kg/m2 or WHR ≥ 0.85) in the current study were more similar to participants with the metabolic syndrome in Tremblay-Franco’s study due to reported comorbidities such as diabetes and hypertension, our results are consistent with those previously reported for patients with metabolic syndrome.
It is possible that the elevated levels of 7-KC, 7a-HC, and 7b-HC observed in women with obesity may be due to oxidative stress associated with obesity and concomitant diseases25,26, as all of these oxysterols can be produced via ROS. Alternatively, 7-KC can also be formed enzymatically from the cholesterol precursor 7-DC, which was strongly associated with BMI; therefore, it is possible that higher levels of 7-KC, 7a-HC, and 7b-C may result from elevated cholesterol synthesis. The findings of this study warrant further investigation into the role of 7-KC, 7a-HC, and 7b-HC in obesity and metabolic syndrome.
Higher levels of THC were observed in hormone receptor-negative as compared to positive tumors, and correspondingly, higher levels of THC were found in women not treated with endocrine therapy and in women treated with chemotherapy, the indicated treatment for hormone receptor-negative tumors. Furthermore, THC levels were associated with unfavorable tumor characteristics such as higher tumor stage, suggesting that THC may play a role in breast cancer characteristics and the corresponding treatment. In line with our findings, a previous study reported significantly lower levels of cholestane-3β,5α,6βtriol (THC) in early breast cancer stages compared with more advanced stages (Ia vs. other)9.
Furthermore, participants who reported regular aspirin use had lower THC levels as compared to non-aspirin users. Aspirin, a non-steroidal anti-inflammatory drug (NSAID), reduces inflammation by binding and inhibiting pro-inflammatory cyclooxygenase (COX1 and COX2), which regulate prostaglandin production. Recent studies have shown beneficial effects of aspirin on cardiovascular health39 and on breast cancer development and progression40, however, associations between aspirin use and THC have not yet been reported.
Higher levels of 7b-HC were observed in hormone receptor-positive as compared to negative tumors and in women who reported tamoxifen therapy use, indicated for hormone-receptor positive tumors. While 7b-HC has not yet been investigated in relation to breast cancer, Kloudova et al. reported lower 7a-HC levels in participants with smaller tumors as compared to those with larger tumors9; due to strong correlations between 7a-HC and 7b-HC (r = 0.77), the findings of Kloudova’s study may be comparable to those of the current study.
While we observed weak associations with breast cancer tumor and treatment characteristics for the other oxysterols, Kloudova et al. found differences in 27-HC and 5b6b-EC levels by tumor size and tumor stage9. As 5b6b-EC is a precursor of THC, it is also possible that higher metabolism of these oxysterols may have caused altered THC levels, resulting in the strong associations for THC and weaker associations for 5a6a-EC and 5b6b-EC in this study.
Strength and limitations
The strength of this study lies in the evaluation of a panel of oxysterols and a number of case-related characteristics including lifestyle, reproduction, comorbidities, and clinical characteristics. With 2282 participants, this is the largest study to date characterizing circulating oxysterols in a breast cancer population.
A limitation of this study is that blood cholesterol levels were not measured in this cohort, so adjustment for total cholesterol levels was not possible, and we present absolute oxysterol concentrations rather than oxysterol/cholesterol ratios. However, it should be noted that circulating total cholesterol has been shown to be weakly to moderately correlated with oxysterols and cholesterol precursors in healthy participants (0.19 ≤ r ≤ 0.53)7,31, in patients with familial combined hyperlipidemia (r ≤ │0.2│)30, and in breast cancer patients (27-HC, r = 0.384)13.
Changes of biomarker concentration over time could not be evaluated since blood samples were collected at only one time point. In addition, we had incomplete information on the start and end date of endocrine therapy and incomplete information on the use of medications such as statins, limiting the assessment of medication use at the time of blood collection. Blood in this cohort study was not collected fasting, however, previous studies reported weak associations between 27-HC, 25-HC, 24S-HC, and 7a-HC and fasting status (8–11 h vs. ≥ 17 h fasting: ≤ 4% difference; 12–16 h vs. > 17h fasting, ≤ 8% difference)41, and between 27-HC and fasting status (≥ 3 h vs. < 3 h fasting, − 2.79%)14.
While relatively good intra-person reproducibility of 0.62 to 0.91 over a 1-year period was demonstrated for most oxysterols, intra-person reproducibility was weaker for 5a6a-EC and 5b6b-EC (r = 0.2 and r = 0.1, respectively)42, so the results for these oxysterols need to be interpreted in this context. We observed that the study region had a strong impact on concentrations of some non-enzymatically produced oxysterols, including 5a6a-EC, 5b6b-EC, and 7-KC, most likely due to differences in sample matrix (plasma vs. serum). Thus, we controlled for study region in all of the analyses. Despite careful sample handing and the use of validated protocols, there is some risk of measurement error, and we cannot rule out the possibility that oxysterol concentrations in blood samples may have been affected by long-term storage43,44,45. The CV of some oxysterols was relatively high, potentially leading to exposure misclassification. Given the number of factors investigated, it is possible that some of the results are due to multiple testing. This study is exploratory and associations need to be evaluated in future studies.
