Fig. 6. The OxLDL/LDL ratios in GCF and plasma. GCF (square) and plasma samples (triangle) were collected from the 14 healthy subjects, and the concentrations of OxLDL and LDL were determined by ELISA to obtain OxLDL/LDL ratios. The mean ± standard deviation was indicated with closed circle. It is noted that the OxLDL/LDL ratio in GCF was 17 times higher than that in plasma on average. (Cited from ref. 41 in a modified form under permission from the publisher.) involved in the inflammatory responses in oral tissues. The oral epithelial cell line Ca9-22 was incubated with OxLDL and the effect on the production of inflammatory factors was examined. Ca9-22 exhibited increased amounts of interleukin-8 (IL-8), pros- taglandin E2 (PGE2) and IL-1β when stimulated with OxLDL.(51) Treatment of the cells with lipopolysaccharide from P. gingivalis did not induce any change in the production of these inflammatory mediators, suggesting that OxLDL itself, rather than bacterial infection, stimulates oral epithelial cells. Oral keratinocytes, primary culture cells originating from human oral tissues, were also responsive to OxLDL and displayed increased production of IL-8, PGE2 and IL-1β in the presence of OxLDL.(51) The cellular signaling mechanisms for the OxLDL-dependent responses have been extensively studied using macrophages and endothelial cells. There are many receptor candidates for the mediation of the cell stimulation by OxLDL. Scavenger receptors bind OxLDL and take it up into cells. Among the reported scavenger receptors, CD36 and LOX-1 are thought to mediate cellular signaling as well as OxLDL uptake.(52–54) In addition to the scavenger receptors, toll-like receptor 4 (TLR4), a receptor for bacterial endotoxin lipid A, also binds to OxLDL and is involved in OxLDL signaling.(55) In the interior of cells, there also are multiple pathways activated by OxLDL, such as the MAP-kinase pathway and peroxisome proliferator-activated receptor γ (PPARγ).(56–58) Recently, another signaling molecule TLR4 that is activated in a variety of cells by OxLDL has come to be a focus of attention.(59–61) It is evidently a very complex situation in which the responsible receptors and signaling mechanisms are not only multiple they also depend on the cell type. The effects of OxLDL on Ca9-22 cells were attenuated by fucoidan or dextransulfate, which are negatively charged polysaccharides, which suggests that there are scavenger receptors involved in the response.(62) Ca9-22 cells express the mRNA of the major scavenger receptors, including CD36 and LOX-1, but we could not detect TLR4 mRNA (unpub- lished data). More work is needed to clarify the responsible receptor(s) for the OxLDL-dependent stimulation which takes place in oral epithelial cells. We also found that the nuclear factor- kappa B (NF-κB) pathway is involved in the OxLDL-dependent production of IL-8 in Ca9-22 cells and human oral keratino- cytes.(51) The NF-κB pathway is a multi-functional gene activating system, and it is well known that oxidative stress such as ROS activate the NF-κB pathway in a great variety of cells. Conclusion These results, taken together with those reported by others, suggest that OxLDL is a pro-inflammatory stimulant, and further- more, that it is involved in not only atherosclerosis, but also other types of diseases, including periodontitis. Since OxLDL is present not only in local sites but also the systemic circulation local sites in various tissues, it has the capacity to induce inflammation in a variety of tissues, such as the vessel wall, retina, and gingiva. The systemic OxLDL level in the body may readily lend itself to the development of tools which will enable an early diagnosis and faster initiation of treatment. No potential conflicts of interest were disclosed. Conflict of Interest Acknowledgments I thank Dr. Tatsuya Takano (Teikyo University Faulty of Pharmaceutical Sciences) for the suggestion of the link to atherosclerosis research. I thank all the collaborators, especially Dr. Makiko Ueda (Osaka City University School of Medicine) for the studies on cardiovascular diseases, Dr. Masaaki Uno (Tokushima University School of Medicine) for the studies on carotid arteriosclerosis, and Dr. Matsuo Yamamoto (Showa University Dental Hospital) for the studies on collaboration in periodontal studies. I also thank Drs. Rina Kato and Takashi Obama (Showa University School of Pharmacy) for an extensive contribution in driving forward a series of our research. Pacific Edit reviewed the manuscript prior to submission. Abbreviations AMD age-related macular degeneration AMI acute myocardial infarction apoB apolipoprotein B apoE apolipoprotein E apoE-KO apolipoprotein E-knockout ELISA enzyme-linked immunosorbent assay GCF gingival crevicular fluid IL interleukin LDL low-density lipoprotein mAb monoclonal antibody NF-κB nuclear factor-kappa B OxLDL oxidatively modified low-density lipoprotein OxPC oxidized phosphatidylcholine pAb polyclonal antibody PC phosphatidylcholine PGE2 prostaglandin E2 PUFA polyunsaturated fatty acids ROS reactive oxygen species TLR4 Toll-like receptor 4 References 6 1 Stocker R. Lipoprotein oxidation: mechanistic aspects, methodological approaches and clinical relevenace. Curr Opin Lipidol 1994; 5: 422–433. 2 Steinbrecher UP, Parthasarathy S, Leake DS, Witztum JL, Steinberg D. Modification of low density lipoprotein by endothelial cells involves lipid doi: 10.3164/jcbn.1100020R1 ©2012 JCBN