Proteomic analysis of acute promyelocytic leukemia

We applied mass spectrometry based approach to explore the proteins
differentially regulated by PML-RARalpha a translocation
characteristic of acute promyelocytic leukemia (APL). Differentialy
expressed proteins, a number of which are related to cell cycle
function, including OP18, HSP70, GRP75 and Pin1 were identified by
mass spectrometry. Further analysis of the OP18 pathway indicated
that mRNA expression of OP18 was higher in APL patients and the
increased OP18 protein expression upon PML-RAR induction was
overcome by retinoic acid treatment. PML-RARalpha induced cell
cycle progression and led to mitotic exit. RNA interference
experiments revealed that siRNA against OP18 overcomes PML-RARalpha
effects on cell cycle progression. In addition to increased OP18
expression by PML-RARalpha, 2D gel electrophoresis revealed an
isomer of OP18, subsequently confirmed as Ser63 phosphomer to be
downregulated by PML-RARalpha. Based on these findings, point
mutation experiments indicated that decreased phosphorylation of
Ser63 in OP18 is important for PML-RARalpha mediated cell cycle and
mitotic index effects since constitutive phosphorylated mutant
(Ser63-asp) of OP18 overcame the PML-RARalpha effects in U937-PR
cells, NB4 and APL patients. In summary, our results demonstrate
that the effect of PML-RAR on cell cycle progression and mitotic
exit is via two mechanisms: increasing the expression of OP18 and
decreasing the phosphorylation of OP18 at Ser63.