Activation of Raf-1/MEK-1/2/p42/44(MAPK) cascade alone is sufficient to uncouple LDL receptor expression from cell growth.
Our previous observation that induction of low density lipoprotein (LDL) receptor expression by a variety of extracellular signals is blocked by PD98059, a specific mitogen-activated protein kinase kinase inhibitor, led to the suggestion that the growth-responsive p42/44(MAPK) cascade plays a critical role in regulating LDL receptor transcription. To analyze the specific contribution of the p42/44(MAPK) cascade in regulating cell growth and LDL receptor induction, we established a HepG2-derived cell line that stably expresses an inducible form of oncogenic human Raf-1 kinase. Using this system, we provide direct evidence that specific activation of this cascade alone is not only required but is sufficient to fully induce LDL receptor expression. Interestingly, degree of p42/44(MAPK) activation determines the extent of LDL receptor induction. However, activation of p42/44(MAPK) in the above cells led to the inhibition of DNA synthesis, caused growth arrest, decrease in cyclin A and upregulation of p21(Cip) expression in a time-dependent manner. These results suggest that each of these two processes can be regulated independently of each other in response to p42/44(MAPK) activation. Thus, extent of p42/44(MAPK) activation may be important in transducing divergent cellular responses in human cells with implications for altered signaling resulting in hypercholesterolemia.
Department of Molecular and Cellular Biochemistry, The Ohio State University College of Medicine, Columbus 43210, USA.
Gurpreet S
GS
Kapoor
Brett A
BA
Atkins
Kamal D
KD
Mehta
D015152:Blotting, Northern; D002453:Cell Cycle; D002455:Cell Division; D003474:Curcumin / Q000494:pharmacology; D019925:Cyclin A / Q000378:metabolism; D004247:DNA / Q000096:biosynthesis; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D006801:Humans; D006937:Hypercholesterolemia / Q000378:metabolism; D015151:Immunoblotting; D008156:Luciferases / Q000378:metabolism; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism*; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D011973:Receptors, LDL / Q000378:metabolism*; D015398:Signal Transduction; D019311:Staurosporine / Q000494:pharmacology; D013347:Subcellular Fractions / Q000378:metabolism; D013997:Time Factors; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
D019925:Cyclin A; D004791:Enzyme Inhibitors; D005419:Flavonoids; D011973:Receptors, LDL; D004247:DNA; D008156:Luciferases; C482128:MAP2K2 protein, human; D011505:Protein-Tyrosine Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482125:MAP2K1 protein, human; D020929:Mitogen-Activated Protein Kinase Kinases; D019311:Staurosporine; D003474:Curcumin; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one