Sterol dependent LDL-receptor gene transcription in lymphocytes from normal and CML patients

Cholesterol-receptor-mediated genomics in health and disease

Cross-talk between cell-surface receptor C(k) and intracellular receptors (liver X receptor-alpha and peroxisome-proliferator-activated receptors) controls a set of crucial genes that maintain a finely orchestrated balance between various cellular processes involved in cell growth, differentiation, apoptosis, cholesterol homeostasis and inflammation. Abnormal cross-talk of these receptors can lead to several human diseases, particularly atherosclerosis, cancer and autoimmune diseases. As our understanding of the complex signaling events that link these receptors to human health improves, we are beginning to appreciate the enormous potential of the proposed cross-talk model of cholesterol receptors in the prevention and/or treatment of diseases.

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 controls the expression of Bcl-2 and cyclin d genes

By making use of receptor-Ck positive lymphocytes (from normal human subjects) as well as receptor-Ck negative lymphocytes (from untreated chronic myeloid leukemic (CML) patients) as cellular models, we were able to show that receptor-Ck-dependent signalling is involved in the regulation of genes coding for Bcl-2 and cyclin D. Further, experiments directed to resolve the mechanism by which this receptor regulates these genes revealed that receptor-Ck, upon activation by cholesterol, initiates the cleavage of a 125 kDa cytoplasmic protein leading to the generation of a 47 kDa factor having specific affinity for genomic sterol regulatory element (SRE)/SRE-like sequence present in the promoter region of genes coding for Bcl-2 and cyclin D. Based upon these observations, we propose that the inability of leukemic cells to express receptor-Ck is responsible for the deregulated over-expression of genes coding for Bcl-2 and cyclin D and this phenomenon may be of importance in understanding leukemic haematopoiesis.

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.

Human platelets contain a novel 47 kDa thiol-oxidase having affinity for genomic sterol-regulatory sequence

The present study provides evidence to support that human platelets possess a 47 kDa dual functional molecule having thiol-oxidase activity as well as high affinity for the SRE sequence in the human genome. On the basis of these as well as earlier results, we propose that Receptor 'Ck' dependent regulation of this dual functional 47 kDa molecule may provide a mechanism for the maintenance of cellular cholesterol homeostasis. Further, this mechanism may also explain the molecular basis of cholesterol-feedback lesion observed under premalignant conditions.

LDL-dependent regulation of Bcl-2 and cyclin 'D' gene expression in lymphocytes from normal and CML patients

Effect of low-density lipoprotein (LDL) on the expression of Bcl-2 as well as cyclin 'D' genes was studied in Receptor 'Ck' (+ve) and Receptor 'Ck'(-ve) human lymphocytes. LDL had no effect upon the elevated levels of Bcl-2 and cyclin 'D' gene products in Receptor 'Ck' (-ve) lymphocytes (from untreated CML patients), whereas in Receptor 'Ck' (+ve) lymphocytes (from normal subjects), the exposure to LDL regulated the level of cyclin 'D' gene product without initiating the expression of bcl-2 gene product. However, blockage of Receptor 'Ck' in normal lymphocytes, through its specific antibody (Ab-RCk) in presence or absence of LDL, resulted in the induction of both cyclin 'D' (at 4 h interval) and bcl-2 (at 12 h interval) gene products. Based upon these results, we propose that Receptor 'Ck' deficiency in cells may inherit defective apoptosis and capacity proliferation leading to leukemic transformation.

Receptor-Ck-dependent regulation of genes in HL-60 cell line

The selective and conspicuous absence of a novel receptor-Ck (having affinity for cholesterol moiety in lipoproteins and intrinsic tyrosine kinase activity) in various leukemic cell lines/patients, prompted us to explore the nature of interrelationship between receptor-Ck deficiency and over-expression of genes coding for bcl-2, cyclin 'D', chimeric bcr-abl, c-myc and LDL-receptor in the human promyelocytic leukemic cell line (HL-60). This study revealed unambiguously that deregulated expression of these genes is primarily due to the inability of these cells to express the receptor-Ck gene. Based upon the observations we propose that 'receptor-Ck' deficiency may be responsible for the initiation of chronic myeloid leukemia.