ERK5 FORMS ARE MODULATED BY REGULATING PHOSPHORYLATION AND PROTEIN LEVEL IN LEUKAEMIA CELLS UNDERGOING HYPOXIA-INDUCED APOPTOSIS

1 (Background). Hypoxia is a master regulator of haematopoiesis whose effects in leukaemia cell populations are not well characterized yet at either the cell biology or molecular level. 2 (Aims). Some molecular aspects of the response to severe hypoxia of myeloid leukaemia cell lines cells have been investigated in this study. 3. (Methods) Friend’s murine erythroleukaemia (MEL) cells, or the human leukaemia cell lines K562 and HL60 were incubated at 0.1% O2 in an anaerobic incubator, or in normoxia, and a number of parameters, as described below, were determined at different times of incubation. 4 (Results). Hypoxia prevented the MEL cell number increase which occurred in normoxia and determined early and massive apoptosis, as well as cell cycle arrest of surviving cells. The AKT protein, an important pro-survival signal, was cleaved in hypoxia. Hypoxia decreased the intensity and duration of ERK1/2, p38 and JNK activation occurring in normoxia, without altering the expression of these proteins. On the other hand, hypoxia suppressed the p120 ERK5 constitutive activation and protein expression, unchanged in normoxia. ERK5 mRNA was not decreased in hypoxia. Phosphorylation of a p82 ERK5 form was also abrogated in hypoxia, although protein level massively increased, but not in normoxia. This down-modulation was also observed in HL60 and K562 cells undergoing hypoxiainduced apoptosis. The disappearance of p120 ERK5, never reported before, and the dephosphorylation of p82 ERK5 were prevented by the treatment with z-VAD, a pan-caspase inhibitor. 5 (Conclusions) The results of this study are consistent with a role of ERK5 as a pro-survival signal which is suppressed in leukaemia cells undergoing hypoxia-induced apoptosis.