Inhibition of low density lipoprotein receptor expression by long-term exposure to phorbol ester via p38 mitogen-activated protein kinase pathway.
The proximal region -234 to (+58 bp) of low-density lipoprotein receptor (LDLR) is responsible for its up-regulation by sterol regulatory element binding protein (SREBP). However, the mechanism of sterol-independent repression of LDLR has not been determined yet. In this study, we observed that there was an early induction and a later repression of LDLR by phorbol ester (PMA) in SK-Hep1 hepatocarcinoma cells and investigated the mechanisms through which PMA repressed LDLR transcription. SK-Hep1 cells were exposed to PMA and LDLR mRNA was evaluated by RT-PCR and Northern blot analysis. The effect of phorbol ester on LDLR transcriptional activity was studied using transient transfection of LDLR promoter-luciferase constructs. Overexpression of N-SREBP-2, a dominant positive SREBP2, did not reverse the PMA-repressed LDLR promoter activity. Serial deletion of LDLR promoter revealed that the region between -1,563 and -1,326 was responsible for the repression. The pretreatment with SB202190, an inhibitor for p38 mitogen-activated protein kinase pathway (p38-MAPK), but not other signaling inhibitors, reversed the PMA-induced repression. The 24 h-treatment with PMA efficiently arrested the SK-Hep1 cell cycle at G0/G1 as demonstrated by FACS analysis and decreased the 3H-thymidine incorporation. The PMA-induced repression of LDLR transcription may be exerted by the factor(s), not SREBP2, induced or modified by p38-MAPK-mediated signaling pathway and associated with cell cycle blockage.
Oh
Jiyoung
J
Asan Institute for Life Sciences, University of Ulsan College of Medicine, Seoul 138-736, Korea.
Choi
Yon Sik
YS
Kim
Jun-Woo
JW
Park
Joong-Yeol
JY
Kim
Seung-Whan
SW
Park
Kun-Koo
KK
Pak
Youngmi Kim
YK
D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D015536:Down-Regulation / Q000187:drug effects; D004847:Epithelial Cells / Q000473:pathology; D005434:Flow Cytometry; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D020935:MAP Kinase Signaling System / Q000187:drug effects*; D010703:Phorbol Esters / Q000494:pharmacology*; D011401:Promoter Regions, Genetic / Q000235:genetics; D011493:Protein Kinase C / Q000378:metabolism; D011725:Pyridines / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011973:Receptors, LDL / Q000235:genetics*; D017384:Sequence Deletion; D051782:Sterol Regulatory Element Binding Protein 2 / Q000378:metabolism; D013997:Time Factors; D014158:Transcription, Genetic / Q000187:drug effects; D048051:p38 Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*
D007093:Imidazoles; D010703:Phorbol Esters; D011725:Pyridines; D012333:RNA, Messenger; D011973:Receptors, LDL; C497390:SREBF2 protein, human; D051782:Sterol Regulatory Element Binding Protein 2; D011493:Protein Kinase C; D048051:p38 Mitogen-Activated Protein Kinases; C090942:4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)imidazole