Characterisation of Anti-Apoptotic Signalling Pathways in Hepatocytes activated by alpha-Lipoic Acid and Atrial Natriuretic Peptide

Both, the R-enantiomer of the antioxidant alpha-lipoic acid (R-LA)
and the hormone atrial natriuretic peptide (ANP) are known to exert
potent hepatoprotective action. The present work characterises
alpha-lipoic acid- and ANP-mediated signal transduction pathways
involved in the regulation of apoptotic cell death in two different
models: primary hepatocytes and ischemic isolated perfused rat
livers. alpha-lipoic acid was shown to protect isolated hepatocytes
from TNF-alpha-/ActinomycinD-induced apoptosis. Astonishingly, this
effect did not seem to be governed neither by its well described
antioxidative nor its Fe-chelating properties. In fact, the
LA-mediated activation of the PI3-K/Akt survival pathway seemed to
be responsible for the antiapoptotic properties of alpha-lipoic
acid. Consequently, incubation with a specific PI3-K-inhibitor
significantly reduced both, R-LA-mediated decrease in caspase
activity and R-LA-induced BAD phosphorylation. Thus, PI3-K-mediated
Akt activation and subsequent phosphorylation of the proapoptotic
protein BAD at Ser136 are causally involved in the antiapoptotic
signalling mediated by R-LA. Perfusion with ANP 20 min prior to the
ischemic period is known to reduce apoptotic cell death occurring
at the end of the ischemic period. We could previously show that
this preconditioning of rat livers leads to a marked activation of
p38 MAPK. Since ANP reduces apoptotic cell death, the potential
connection between this ANP-induced p38 MAPK activation and
apoptosis reduction was investigated. Astonishingly, liver
perfusion with an p38 MAPK inhibitor even decreased apoptotic cell
death, supporting a detrimental role of this kinase. PKA-specific
inhibitors demonstrated the involvement of PKA in this ANP-mediated
protection. Interestingly, it also turned out that PKA
phosphorylates the proapoptotic protein BAD at Ser112, an effect
known to contribute to the inhibition of apoptosis. In summary, the
present data show for the first time that phosphorylation of BAD at
either Ser136 or Ser112 turns out to be a central protective
mechanism to defend from hepatocyte apoptosis.