Proteomic analysis of acute promyelocytic leukemia: PML-RARα leads to decreased phosphorylation of OP18 at serine 63

In the present study, we employed 2‐DE to characterize the effect of the acute promyelocytic leukemia (APL)‐specific PML‐RARα fusion protein on the proteome. Differentially expressed proteins, a number of which are related to the cell cycle function, including oncoprotein18 (OP18), heat shock protein70, glucose‐regulated protein75, and peptidyl‐prolyl isomerase, were identified by MS. Subsequent bioinformatic pathway discovery revealed an integrated network constituting SMARCB1, MYC, and TP53‐regulated pathways. The data from the DNA microarray and proteomic experiments demonstrated the correlation between the translocation and higher expression of OP18 at mRNA and protein levels. Transient cotransfection assay revealed that PML‐RARα is a potent activator of OP18 promoter and this transcriptional activation is retinoic acid sensitive. PML‐RARα induction also leads to decreased phosphorylation on Ser63 residue of OP18, which is okadaic acid sensitive suggesting the involvement of a phosphatase pathway. Overexpression of a constitutively phosphorylated Ser63 mutant of OP18 in PML‐RARα expressing APL patient, PR9, and NB4 cells led to a G2/M‐phase arrest in contrast to a phosphorylation‐deficient Ser63 mutant and untransfected control. Taken together, our results demonstrate the significance of decreased Ser63 phosphorylation of OP18 in PML‐RARα‐mediated effects on cell cycle.