PI3K/AKT, JNK, and ERK pathways are not crucial for the induction of cholesterol biosynthesis gene transcription in intestinal epithelial cells following treatment with the potato glycoalkaloid alpha-chaconine.
We previously reported that exposure of the intestinal epithelial Caco-2 cell line to noncytotoxic concentrations of potato glycoalkaloids resulted in increased expression of cholesterol biosynthesis genes. Genes involved in mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/v-akt murine thymoma viral oncogene homologue (AKT) pathways and their downstream effectors such as Jun, c-Myc, and Fos also were induced. MAPK and PI3K/AKT pathways have been described to regulate the activity of sterol regulatory element binding transcription factors (SREBPs) and consequently the expression of cholesterol biosynthesis genes. In this study, to understand the mechanism of induction of cholesterol biosynthesis upon alpha-chaconine treatment, its effect on SREBP-2 protein levels was investigated. We also examined whether MAPK and PI3K/AKT pathways are required for the observed induction of these genes following exposure of cells to alpha-chaconine. Differentiated Caco-2 cells were pretreated with LY294002 (PI3K inhibitor), PD98059 (MEK1 inhibitor), or SP600125 (JNK inhibitor) or a combination of all inhibitors for 24 h prior to coincubation with 10 microM alpha-chaconine for 6 h. Significant increases in precursor and mature protein levels of SREBP-2 were observed after alpha-chaconine exposure. We also observed that alpha-chaconine treatment resulted in significant phosphorylation of AKT, extracellular signal related protein kinase (ERK), and c-jun N terminal protein kinase (JNK) but not that of p38. In general, the kinase inhibitor experiments revealed that phosphorylation of kinases of PI3K/AKT, ERK, and JNK pathways was not crucial for the induction of expression of cholesterol biosynthesis genes, with the exception of SC5DL. The transcription of this later gene was reduced when all three pathways were inhibited. On the basis of these results, it can be postulated that other mechanisms, which may be independent of the MAPK and PI3K/AKT pathways, including possibly post-translational activation of SREBP-2, may be more pivotal for the induction of cholesterol biosynthesis genes following exposure of intestinal cells to alpha-chaconine.
Mandimika
Tafadzwa
T
RIKILTInstitute of Food Safety, Wageningen University and Research Centre, P.O. Box 230, 6700 AE Wageningen, The Netherlands.
Baykus
Hakan
H
Poortman
Jenneke
J
Garza
Cutberto
C
Kuiper
Harry
H
Peijnenburg
Ad
A
D018938:Caco-2 Cells; D002784:Cholesterol / Q000096:biosynthesis* / Q000235:genetics*; D004847:Epithelial Cells / Q000378:metabolism; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects*; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012992:Solanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D011198:Solanum tuberosum / Q000737:chemistry*; D051782:Sterol Regulatory Element Binding Protein 2 / Q000235:genetics
C497390:SREBF2 protein, human; D051782:Sterol Regulatory Element Binding Protein 2; D012992:Solanine; C011860:alpha-chaconine; D002784:Cholesterol; D019869:Phosphatidylinositol 3-Kinases; D051057:Proto-Oncogene Proteins c-akt; D048049:Extracellular Signal-Regulated MAP Kinases; D048031:JNK Mitogen-Activated Protein Kinases