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

Receptor Ck-dependent signaling regulates hTERT gene transcription

BACKGROUND

Available evidence suggests that the regulation of telomerase activity primarily depends on the transcriptional control of the human telomerase reverse transcriptase (hTERT) gene. Although several activators and repressors of hTERT gene transcription have been identified, the exact mechanism by which hTERT transcription is repressed in normal cells and activated in cancer cells remains largely unknown. In an attempt to identify possible novel mechanisms involved in the regulation of hTERT transcription, the present study examined the role of Receptor Ck, a cell surface receptor specific for cholesterol, in the transcription of hTERT gene in normal human peripheral blood mononuclear cells.

RESULTS

Activated Receptor Ck was found to down-regulate hTERT mRNA expression by repressing the transcription of c-myc gene. Receptor Ck-dependent signaling was also found to down-regulate the mRNA expression of the gene coding for the ligand inducible transcription factor, peroxisome proliferator-activated receptor gamma (PPARgamma). The ligand activation of PPARgamma resulted in the down-regulation of c-myc and hTERT mRNA expression. By using specific activator and inhibitor of protein kinase C (PKC), it was demonstrated that Receptor Ck dependent down-regulation of hTERT gene transcription involved inhibition of PKC. In addition, 25-hydroxycholesterol was found to contribute to the transcriptional regulation of hTERT gene.

CONCLUSION

Taken together, the findings of this study present evidence for a molecular link between cholesterol-activated Receptor Ck and hTERT transcription, and provide new insights into the regulation of hTERT expression in normal human peripheral blood mononuclear cells.

Receptor-Ck regulates the expression of cyclin-dependent kinase inhibitors (p16; p27)

The present study was addressed to understand the interrelationship between Receptor-Ck induction-activation coupling; isoprenoids (derived from mevalonate pathway) and cyclin-dependent kinase inhibitors. The results reported here unambiguously reveal that isoprenoids regulate the expression of genes coding for CDK inhibited p16 and p27. Further, Receptor-Ck dependent signalling, known to control the mevalonate pathway, had a direct effect upon the p27 gene expression. Based upon these studies coupled with our earlier findings we propose that Receptor-Ck has an important role in the regulation of cell cycle machinery.

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.

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.