Nature of aortic smooth muscle cellular activity induced by cholesterol incorporation through an LDL-receptor-independent pathway: preventive role of trifluoperazine on such activity

Molecular link between cholesterol, cytokines and atherosclerosis

Current investigation on the origin of atherosclerosis has initiated an intense debate over whether atherosclerosis results from hypercholesterolemia or an inappropriate immune response to vascular injury. Although the role of the immune system has been questioned, the overwhelming body of evidence clearly indicates that atherogenesis is initiated by the interplay between cholesterol and cellular secretion of cytokines (especially IL-6) and apolipoprotein 'E' within the arterial wall. Recent studies have revealed that cells possess two cholesterol-sensors: (a) Receptor-Ck which senses the extracellular cholesterol and initiates signalling pathway responsible for the regulation of genes involved in the cell cycle, cell death, cellular cholesterol homeostasis and cytokines including IL-6; (b) LxR alpha which senses intracellular oxysterols and controls genes involved in cell death, cellular cholesterol homeostasis and cytokine IL-8. These cholesterol sensors define the molecular mechanism responsible for cholesterol-depended regulation of cellular synthesis and secretion of cytokines (IL-6, IL-8) within arterial wall. On the basis of this mechanism, presence of cholesterol and its oxy-derivative in the modified LDL will result in transient activation/deactivation of Receptor-Ck-dependent genes which will give rise to repeated cycles of growth coupled with apoptosis leading to a situation where apoptotic-deficient cells in the arterial wall, would be selected resulting in their accumulation and formation of oligoclonal atherosclerotic plaque.

Receptor-Ck-dependent regulation of genes involved in the cell cycle

The present study was addressed to understand the interrelationship between Receptor-Ck activation, mevalonate pathway and primary response genes such as c-fos, c-myc and cyclin 'D' involved in the cell cycle. The results reported here unambiguously revealed that the phosphatidic acid (generated through the activation of Receptor-Ck by cholesterol) regulates mevalonate pathway, DNA synthesis as well as expression of genes coding for c-fos, c-myc and cyclin 'D'. By using the specific blockers of ras farnesylation as well as phospholipase D, it became apparent that phosphatidic acid regulates two processes: (a) activation of Gap-ras pathway leading to the expression of c-fos, c-myc proto-oncogenes probably through the activation of NF1 transcription factor; (b) cleavage of 125 kDa endoplasmic reticulum protein leading to the generation of 47 kDa protein factor which not only regulates mevalonate pathway but also has an ability to heterodimerize with Receptor-Ck protein and this heterodimer may be responsible for the regulation of cyclin 'D' expression probably by binding to the SRE like sequence present in the promoter region of this gene. On the basis of these findings, we propose a pathway through which Receptor-Ck upon endocytosis regulate these primary response genes (c-fos, c-myc, cyclin 'D') involved in the cell cycle.