It is well established that lipid-lipoprotein particles among density classes are metabolically processed forming a sequence of diminishing size and lipid distribution, beginning with LPL activity on triglyceride in chylomicron and continuing through HDL, resulting in increasing density and loss of lipid due to enzymic activities during the course of normal lipid transport [24
]. Tumor burden also resulted in a marked reduction in adipose tissue, together with a clear hyperlipemia. The loss of fat mass is the result of two altered processes: first, an increase in lipolytic activity, which results in a significant release of both glycerol and fatty acids [25
], and second, a marked decrease in the activity of lipoprotein lipase (LPL). This enzyme is responsible for the cleavage of both endogenous and exogenous triacylglycerols to form glycerol and fatty acids, which is reflected as hypertriglyceridemia [26
Pathophysiologic effects of oxLDL in atherogenesis have been established. Recent studies show a positive correlation between increased serum oxLDL levels and an increased risk of colon, breast and ovarian cancer [27
]. An earlier study reported that lipids might primarily affect the gonads, and subsequently higher estradiol secretion could influence the development of malignancies in these sites [30
]. Later study showed that it is possible that LDL, which is susceptible to oxidation, result in high lipid peroxidation contributing to carcinogenesis [27
In the past years, there has been a growing body of evidence that excessive lipid peroxidation may play a key role in cancer development [31
] and in animal model associated with carcinogenesis [17
]. Lipid peroxidation metabolites damage DNA and can seriously inhibit DNA repair capacity through their direct interaction with repair proteins [34
]. Circulating oxLDL is also quite reliable biomarker of lipid peroxidation [17
]. OxLDL have been reported as a potent independent mitogenic factor [35
] that induces proliferation or cell death [36
] and furthermore, could contribute to the release of cytokines and growth factors-associated with cancer [37
]. However, a meta-analysis on patients suffering from various diseases revealed that only under severe pathological conditions, e.g. HIV infection, all the indices of oxidative stress correlate with each other [38
The present investigation, for the first time, identified cytotoxicity on the proliferation of Eca-109 cell line and showed time and dose dependency. The apoptosis of Eca-109 cells treated by oxLDL also showed in time- and dose- manner. The RT-PCR expression of Bcl-2, Bax and caspase-3 genes mRNA and protein expression of them confirmed the mechanism of the inducing apoptosis by oxLDL. This fact suggests that the physiological amount of oxLDL involves in the process of cell apoptosis, clears the mutation cells or controls the proliferation of cancer cells in vivo. At the advanced stage of cancer, the activity of lipoprotein lipase in vivo decreases, and in consequence, the level of oxLDL declines, promoting an uncontrolled proliferation of tumor cell. Meanwhile, the activity of lipolysis increases, leading to occurrences of the cancer-associated cachexia.
It is confirmed that non-oxidized low-density lipoprotein (nLDL) has no effect on cell lines viability and proliferation [35
]. OxLDL with cytotoxicity or autophagy dependences cell types. Under some conditions, oxLDL can be cytotoxic. OxLDL can induce changes in cell cycle protein distribution and expression characteristic of a controlled, adaptive response to a chronic pathological condition. Autophagy is another form of programmed cell death mediated by the lectin-like oxLDL receptor-1(LOX-1)-dependent [39
]. Previous studies showed that oxLDL (more than 10 μg/ml) has cytotoxic effects on cancer cells in vitro and activate apoptosis and autophagy. OxLDL, applied at physiologic concentrations, decreased cell viability and proliferation in a dose-dependent manner in cell lines tested such as HT29 (colon), OVCAR3 (ovarian), HeLa (cervical), MCF7 (breast), A549 (lung), and PC3 (prostate) [40
]. However, when the quiescent human fibroblasts and rabbit smooth muscle cells (VSMC) were treated with oxLDL at physiological ranges (0, 10, or 50 μg/ml) at 24-48 h, the total cell number of them significantly increased [35
Now in another test, we also observed, the human umbilical vein endothelial cells (HUVEC) was treated with the dose lass than 40 μg/ml oxLDL at physiological ranges for 24-48 h. The resulted in significant increases in total HVEC cell counts at both time points. However, when the cell treated with larger than 40 μg/ml oxLDL, the inhibiting proliferation phenomenon observed at the same time points for the cells (data not shown).
More than two decades ago, epidemiological studies showed a U-shaped relationship between total cholesterol (TC) levels and risk of all-cause mortality. The relationship between the baseline serum cholesterol level to total mortality was attributed to the high number of deaths associated with serum cholesterol level at the high end of the distribution (mainly due to coronary heart disease) and at the low end (mainly due to cancer) [41
]. Recent study reported that in atherosclerosis, ox-LDL linkage with its receptor LOX-1 activates the inflammatory pathway through NF-κB, leading to cell transformation. LOX-1 is important for maintaining the transformed state in developmentally diverse cancer cell lines and for tumor growth, suggesting a molecular connection between atherogenesis and tumorigenesis [10
]. One study reported that low dose oxLDL has bilateral adjustment characteristics on the proliferation of quiescent human fibroblasts and rabbit smooth muscle cells. Western blot analysis revealed that oxLDL-stimulated cell proliferation was associated with significant increases in the expression of proteins that regulate entry into and progression through the cell cycle. Surprisingly, the expression of cell cycle inhibitors (p21 and p27) was stimulated by oxLDL as well, but this was to a lesser extent than the effects on cell cycle-activating proteins [36
]. In the present study, we also found, the Eca-109 cells were treated with more than 40 μg/ml oxLDL the cytotoxicity of it was significantly increase on a time- and dose- dependency. It is possible that the levels of oxLDL in vivo are dynamical changes based on the body's conditions. Under some condition, oxLDL can be cytotoxic.
One clinical study indicated that the levels of oxLDL were increased among both breast and ovarian cancer patients as compared to the control subjects [44
]. Another previous study showed that fasting lipid and lipoprotein studies on 38 consecutively diagnosed children with acute lymphoblastic leukemia (ALL). The results showed that the level of LDL decreased in the process of chemotherapy of the malignant tumor and recovered to normal level after the treatment. For those patients with ALL who had a long-term survival, the normal levels of LDL also were important [45
]. The effects of tamoxifen treated the patients with breast cancer showed that LDL particle diameter correlated negatively with plasma triglyceride (TG) (r = -0.62; p < 0.001). Tamoxifen-induced fatty liver in breast cancer patients may be atherogenic, via increased TG and consequent small, easily oxidized LDL particles [46
]. The smaller diameter of LDL particles, the more easily oxidized into oxLDL.
Recent review the results of cancer-related malnutrition patients with and without nutritional supplements showed the weight loss is multifactorial but can be generally separated into two components. One component included those outcomes that are the result of the metabolic abnormalities as a direct consequence of the tumor, and a second component including results of treatments, psychological issues, and others. For people with cancer the implement nutrition interventions are effective, but there is no clarity in best options for nutrition management [47
]. More clinical observation of the dynamic changes of oxLDL for other type cancer in chemotherapy need to be confirm in future.
Another major finding of the present study is a synergistic effect on inducing apoptosis of Eca-109 cells treated by the combination of 40 μg/ml oxLDL and 0.1 μg/ml ADM. The proportion rate of apoptosis ECA-109 cells treated with their combination significantly increased (19.79 ± 1.32), compared with that 40 μg/ml oxLDL (15.11 ± 0.61), 0.1 μg/ml ADM (11.27 ± 0.54). As well known, the chemical-therapy drugs of cancer, such as ADM, have toxicity, if oxLDL combined with the cytotoxic drugs could use in the clinic, may be more benefit for the patient's therapy. Further studies are required in order to elucidate whether oxLDL play a causative or merely consequential role in cancer process and to designate a novel approach in the combination therapeutic strategies.