Heme oxygenase-1 protects against Alzheimer's amyloid-β1-42-induced toxicity via carbon monoxide production

Heme oxygenase-1 (HO-1), an inducible enzyme up-regulated in Alzheimer's disease, catabolises heme to biliverdin, Fe2+ and carbon monoxide (CO). CO can protect neurones from oxidative stress-induced apoptosis by inhibiting Kv2.1 channels, which mediates cellular K+ efflux as an early step in the apoptotic cascade. Since apoptosis contributes to the neuronal loss associated with amyloid beta peptide (A beta) toxicity in AD, we investigated the protective effects of HO-1 and CO against A beta(1-42) toxicity in SH-SY5Y cells, employing cells stably transfected with empty vector or expressing the cellular prion protein, PrPc, and rat primary hippocampal neurons. A beta(1-42) (containing protofibrils) caused a concentration-dependent decrease in cell viability, attributable at least in part to induction of apoptosis, with the PrPc-expressing cells showing greater susceptibility to A beta(1-42) toxicity. Pharmacological induction or genetic over-expression of HO-1 significantly ameliorated the effects of A beta(1-42). The CO-donor CORM-2 protected cells against A beta(1-42) toxicity in a concentration-dependent manner. Electrophysiological studies revealed no differences in the outward current pre- and post-A beta(1-42) treatment suggesting that K+ channel activity is unaffected in these cells. Instead, A beta toxicity was reduced by the L-type Ca2+ channel blocker nifedipine, and by the CaMKKII inhibitor, STO-609. A beta also activated the downstream kinase, AMP-dependent protein kinase (AMPK). CO prevented this activation of AMPK. Our findings indicate that HO-1 protects against A beta toxicity via production of CO. Protection does not arise from inhibition of apoptosis-associated K+ efflux, but rather by inhibition of AMPK activation, which has been recently implicated in the toxic effects of A beta. These data provide a novel, beneficial effect of CO which adds to its growing potential as a therapeutic agent.