The differentiation/retrodifferentiation program of human U937 leukemia cells is accompanied by changes of VCP/p97

Retrodifferentiation and regained proliferative capacity of growth-arrested human leukemic cells after monocyte-like differentiation requires proteolytic activities together with distinct regulatory factors. The AAA ATPase valosin-containing protein (VCP/p97) contributes to protein degradation and cell cycle regulation, respectively, and it was of interest to study a possible role of VCP/p97 during this myelomonocytic differentiation and retrodifferentiation.

Separation of autonomously proliferating human U937 myeloid leukemia cells by centrifugal elutriation demonstrated unaltered VCP/p97 expression levels throughout distinct phases of the cell cycle. However, phorbol ester-induced G0/G1 cell cycle arrest in differentiating human U937 leukemia cells was associated with a significantly increased protein and mRNA amount of this AAA ATPase. These elevated VCP/p97 levels progressively decreased again when growth-arrested U937 cells entered a retrodifferentiation program and returned to the tumorigenic phenotype. Whereas VCP/p97 was observed predominantly in the cytosol of U937 tumor and retrodifferentiated cells, a significant nuclear accumulation appeared during differentiation and G0/G1 growth arrest. Analysis of subcellular compartments by immunoprecipitations and 2D Western blots substantiated these findings and revealed furthermore a tyrosine-specific phosphorylation of VCP/p97 in the cytosolic but not in the nuclear fractions. These altered tyrosine phosphorylation levels, according to distinct subcellular distributions, indicated a possible functional involvement of VCP/p97 in the leukemic differentiation process. Indeed, a down-modulation of VCP/p97 protein by siRNA revealed a reduced expression of differentiation-associated genes in subsequent DNA microarray analysis. Moreover, DNA-binding and proliferation-associated genes, which are down-regulated during differentiation of the leukemic cells, demonstrated elevated levels in the VCP/p97 siRNA transfectants.

The findings demonstrated that monocytic differentiation and G0/G1 growth arrest in human U937 leukemia cells was accompanied by an increase in VCP/p97 expression and a distinct subcellular distribution to be reverted during retrodifferentiation. Together with a down-modulation of VCP/p97 by siRNA, these results suggested an association of this AAA ATPase in the differentiation/retrodifferentiation program.