Visit our Expo - Redox and Inflammation signaling 2012

V. Phosphatases as key cell signaling intermediates Poster V, 5
Regulation of apoptosis signal-regulating kinase 1 (ASK1) by polyamine levels via
protein phosphatase 5
Mikhail A. Kutuzov, Alexandra V. Andreeva and Tatyana A. Voyno-Yasenetskaya.
Department of Pharmacology, College of Medicine, University of Illinois at Chicago,
Chicago, Illinois 60607, U.S.A. Email: kutuzov@uic.edu
Protein phosphatase 5 (PP5) is a member of the PPP family of protein Ser/Thr phosphatases,
conserved from protists to plants and animals. Recent years have seen increasing evidence for
PP5 involvement in a variety of signaling pathways, suggesting that PP5 is multifunctional
phosphatase, similarly to more extensively characterized PPP phosphatases PP1 and PP2A. A
number of proteins have been identified that interact with PP5 and either regulate its activity,
or are PP5 substrates, or play a role of scaffold. PP5 may also be regulated, at least in vitro,
by low molecular weight compounds, such as polyunsaturated fatty acids and fatty acyl-CoA
esters. We found that PP5 is strongly inhibited by micromolar concentrations of a natural
polyamine spermine. This inhibition was observed both with p-nitrophenyl phosphate and
protein substrates phosphocasein and apoptosis signal-regulating kinase 1 (ASK1, thought to
be a physiological substrate of PP5). Inhibition of polyamine biosynthesis in COS-7 cells by
alpha-difluoromethylornithine (DFMO) led to accelerated dephosphorylation of oxidative
stress-activated ASK1. This effect of DFMO was suppressed by okadaic acid and by siRNAmediated
PP5 depletion, indicating that the effect of polyamine levels on ASK1
dephosphorylation was mediated by PP5. Polyamine depletion in COS-7 cells abrogated
oxidative stress-induced activation of caspase-3 (which executes ASK1-induced apoptosis), as
well as caspase-3 activation induced by ASK1 overexpression, but had no effect on basal
caspase-3 activity. These results implicate polyamines, emerging intracellular signaling
molecules, as potential physiological regulators of PP5. Our findings also suggest a novel
mechanism of the antiapoptotic action of a decrease in polyamine levels via de-inhibition of
PP5 and accelerated dephosphorylation and deactivation of ASK1.
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