Alpha-lipoic acid as a directly binding activator of the insulin receptor: protection from hepatocyte apoptosis.
OBJECTIVE
Alpha-lipoic acid has cytoprotective potential which has previously been explained by its antioxidant properties. The aim of this study was to assess LA-induced-specific cytoprotective signalling pathways in hepatocytes.


METHODS
Apoptosis of rat hepatocytes was induced by actinomycinD/TNF-alpha. Caspase-3-like activity was determined by a fluorometric; LDH by an enzymatic assay; and phosphorylation of the insulin receptor, Akt, and Bad by Western blot (after immunoprecipitation). Protein kinase and insulin receptor activities were measured by in vitro phosphorylation. Computer modeling studies were performed by using the program GRID.


RESULTS
Alpha-lipoic acid decreased actinomycinD/TNF-alpha-induced apoptosis, as did the antioxidants Trolox and N-acetylcysteine. The activation of PI3-kinase/Akt involving phosphorlyation of Bad markedly contributed to the cytoprotective action of alpha-lipoic acid. Alpha-lipoic acid but not other antioxidants protected against actinomycinD/TNF-alpha-induced apoptosis via phosphorylation of the insulin receptor. Computer modeling studies revealed a direct binding site for alpha-lipoic acid at the tyrosine kinase domain of the insulin receptor, suggesting a stabilizing function in loop A that is involved in ATP binding. Treatment of immunoprecipitated insulin receptor with LA induced substrate phosphorylation.


CONCLUSIONS
Alpha-lipoic acid mediates its antiapoptotic action via activation of the insulin receptor/PI3-kinase/Akt pathway. We show for the first time a direct binding site for alpha-lipoic acid at the insulin receptor tyrosine kinase domain, which might make alpha-lipoic acid a model substance for the development of insulin mimetics.
Diesel
Britta
B
Department of Pharmacy, Pharmaceutical Biology, Saarland University, Saarbrücken, Germany.
Kulhanek-Heinze
Stefanie
S
Höltje
Monika
M
Brandt
Birte
B
Höltje
Hans-Dieter
HD
Vollmar
Angelika M
AM
Kiemer
Alexandra K
AK
D000818:Animals; D000975:Antioxidants / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D002478:Cells, Cultured; D003609:Dactinomycin / Q000494:pharmacology; D022781:Hepatocytes / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D008297:Male; D008958:Models, Molecular; D051058:Oncogene Protein v-akt / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism; D011972:Receptor, Insulin / Q000378:metabolism*; D015398:Signal Transduction; D008063:Thioctic Acid / Q000378:metabolism / Q000494:pharmacology*; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
D000975:Antioxidants; D007328:Insulin; D014409:Tumor Necrosis Factor-alpha; D003609:Dactinomycin; D008063:Thioctic Acid; D019869:Phosphatidylinositol 3-Kinases; D020794:Receptor Protein-Tyrosine Kinases; D011972:Receptor, Insulin; D051058:Oncogene Protein v-akt