A cytoprotective role for the heme oxygenase-1/CO pathway during neural differentiation of human mesenchymal stem cells.

The inducible protein heme oxygenase-1 (HO-1) catalyzes the oxidation of heme to carbon monoxide (CO) and biliverdin, which play a concerted action in cytoprotection against oxidative stress and in the modulation of cell proliferation and differentiation. Here we report that both HO-1 expression and activity can be highly increased in undifferentiated human mesenchymal stem cells (MSCs) treated with hemin, a known HO-1 inducer. However, HO-1 mRNA and protein expression gradually decrease when MSCs undergo neural differentiation in vitro, making them extremely susceptible to glutamate-mediated cytotoxicity. A time course for HO-1 revealed that this protein is markedly down-regulated after 2 days and returns to control levels 6 days after differentiation. Treatment with glutamate (250 microM) after 2 days of neural differentiation resulted in a more pronounced lactate dehydrogenase release, a marker of cell injury, compared with undifferentiated cells. Notably, cells pretreated with hemin (50 microM) or compounds that release small amounts of CO (10 microM CORM-3 and CORM-A1) rendered cells more resistant to glutamate-induced toxicity; this effect was evident in both undifferentiated and differentiated MSCs. Our findings indicate that MSCs become more vulnerable to oxidative injury during the early stages of differentiation via mechanisms that involve a temporary inhibition of HO-1 expression. Thus, overexpression of HO-1 and CO-releasing molecules could provide a possible therapeutic strategy to improve cell viability during neural differentiation in applications that use stem cell technology. 2008 Wiley-Liss, Inc.