In the present investigation, it was addressed plasma lipid levels and HDL composition and functionality in newly diagnosed women with BC in comparison to CTR women, and with respect to the molecular classification of the tumor and the clinical stage of the disease. In comparison to CRT, it was demonstrated that BC cases had 1) similar concentrations of plasma lipids adjusted per age, and 2) HDL particles less enriched in TC, PL, and TG. Divergent results have been published concerning the association of plasma lipids and lipoproteins in the development and prognosis of BC. Total cholesterol is considered a contributor factor for many types of cancer including BC, although the presence of confounding factors such as heterogeneity of BC, disease duration and staging, disparity of ethnic population, age, menopausal state, lifestyle, and treatments may bias the interpretation and comparison among studies. Moreover, diabetes mellitus, insulin resistance, and obesity that change lipoprotein profiles are also recognized as potential contributors to BC.
Particularly, the majority of studies show increased levels of TC, LDL, and TG and reduced HDLc as strong predictors for BC [41–45]. In our case-control study, HDL plasma levels did not differ between CTR and BC cases. Body mass index was similar between groups and results adjusted per age showed similar levels of plasma TG. Conceivably, triglycerides metabolism by the lipoprotein lipase dictates HDL formation in circulation and ultimately HDLc plasma levels. Noteworthy to mention that diabetes mellitus and smoking which are conditions that reduce HDL were not included in the study and there was no interference of therapeutics since only newly diagnosed subjects were included.
Although the ability of HDL in removing cell cholesterol was preserved in all women with BC included in our casuistic as compared to CTR, the diminished content of TC, PL, and 27HC in the HDL particle from BC cases may be a consequence of alterations in cell tumors that mitigate sterol exportation to HDL. In addition to the intrinsic ability of HDL to receive cholesterol, it is important to consider that lipid efflux is modulated by cellular components, such as the bioavailability of free or unesterified cholesterol and the content and functionality of ABCA-1, ABCG-1, and SR-BI . Characteristically, ABCA-1 is expressed in the mammary gland and its content is indicated as reduced in BC and associated with positive lymph nodes . Its expression negatively affects the therapeutic efficiency of chemotherapy  and is referred to by some authors as a marker of TN tumors . On the other hand, its deficiency contributes to an increase in cellular and mitochondrial cholesterol content, mitigating cell death processes mediated by this organelle, which favors tumor cell survival .
Solid tumors accumulate large amounts of cholesterol [50, 51] through increased synthesis and uptake of lipoproteins [51, 52]. SR-B1 mediates the efflux of free cholesterol down a concentration gradient for HDL. However, it can also promote the uptake of modified lipoproteins, favoring, on the contrary, the supply of cholesterol to cells and tumor progression . Higher expression of SR-B1 is linked to greater aggressiveness and worse tumor prognosis [23, 54, 55], while mutations in the Scarb1 are related to the inhibition of tumor proliferation .
Furthermore, the lower content of lipids in HDL could be linked to the lower detachment of surface components of TG-rich lipoproteins during lipolysis mediated by lipoprotein lipase, in a process recently referred to as reverse transport of remnant cholesterol . However, it is not possible to say which mechanisms act on the composition of HDL in view of the present results but these events cannot be evidenced by the simplistic determination of HDLc in plasma, explaining some of the controversies regarding the association of HDLc with BC.
As expected, most tumors in our casuistic were of the luminal type (68%), with a predominance of stage I and II disease (70.8%). The frequency of advanced disease (stages III and IV) was highest in women with TN tumors (66.5%) and lowest in those with LA tumors (16%). Taking into account the molecular classification of the tumor it was observed that: 1) TN cases had higher levels of TC, apo B, nonHDLc, and TG in comparison to LA, LB, and HER-2, but 2) HDLc and composition and functionality of the HDL particle were similar among all types. Changes observed were irrespective of BMI and may represent a distinct feature of TN tumors, channeling lipids to tumor development in a hystotype with a more aggressive clinical history, with a relatively low-survival rate and a high rate of metastasis.
Findings agree with previous studies that evidenced increased levels of VLDL-TG in TN BC, although changes in CT/apoB and TG/HDLc did not reach statistical significance. In this regard, it was demonstrated in a retrospective cohort that a high pretreatment TG/HDLc ratio was an independent predictor of the overall survival rate in TN tumors . The enhanced expression of the LDL receptor and LDLR-related proteins 5 and 6 were found in TN tumors and associated with a higher ability of tumor growth and invasion; while the knockdown of LDLR-5 and 6 decreased tumorigenesis [51, 59–61].
A model of syngeneic tumor graft evidenced that tumor growth is accompanied by changes in the host lipid metabolism by stimulating and inhibiting, respectively, the synthesis and metabolism of very-low-density lipoproteins (VLDL). This ultimately seems to provide more energy to the tumor . In addition, by using a targeted plasma liquid chromatography-tandem mass spectrometry (LC-MS/MS), potential lipid biomarkers were selected in TN cases of BC (including ceramides, phosphatidylcholine, lysophosphatidylcholine, and diacylglycerol), reaffirming the modification of the lipid profile in this molecular type .
Unlike other studies, we did not find changes in plasma lipids in HER2 positive BC cases [44, 64, 65]. In subjects with HER2-positive breast cancer, the analysis of lipoprotein profile by nuclear magnetic resonance demonstrated an enhanced level of specific VLDL subfractions as a marker of plasma lipid alteration in comparison to control women, even in the presence of similar BMI between groups. Reductions in HDL subfractions were surrogate markers for the response to neoadjuvant chemotherapy follow-up .
According to the clinical stage of the disease, a reduced intrinsic ability of the HDL particle in removing cell cholesterol was observed in stages III and IV as compared to stages I and II. This was independent of changes in HDL composition and plasma lipids reinforcing the dissociation between plasma levels of HDLc and HDL functionality as a whole particle. Increasing levels of intracellular cholesterol favor the formation of oxysterols allowing us to infer an increase in the concentration of 27HC in the tumor microenvironment that negatively drives BC evolution. The pathophysiological basis of reduced HDL functionality in advanced stages of breast cancer was not investigated in the present study. The inflammatory, oxidative, and immunogenic insults prevalent in advanced disease may favor the modification of HDL in the tumor microenvironment, compromising its function. In this sense, HDL may even facilitate tumor propagation as previously reported in experimental studies where modified HDL was oncogenic [41, 67]. Again, this supports the idea that the tumor energy demand adapts systemic metabolism in its favor.
The concept of HDL modulation by the tumor by reverse causation may unlink HDL, per se, as a predictor of tumor risk. In this sense, HDL would be applied more as a marker of tumor evolution than as a protector or inducer of its genesis.
To the best of our knowledge, this is the first demonstration of loss of HDL function according to disease burden, independently of plasma lipids. Study limitations include the absence of more detailed clinical data, including components of metabolic syndrome and visceral adiposity, and lifestyle information. However, BMI was similar between the groups, and comorbidities frankly associated with changes in lipid metabolism were excluded. It was not possible to determine which components of the HDL structure, independently of lipids and apo A-I, were responsible for the impairment of cholesterol efflux. A more detailed analysis of HDL proteomics, lipidomics, and microRNAs may add new information.