Dual signaling mechanisms of estrogen: Phosphorylation of P53 at Ser15 in aging HMEC in the absence of estrogen receptor

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

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

2D‐DIGE analysis revealed reduced cytokeratin signaling in placenta with preeclampsia

Preeclampsia represents a severe pregnancy disorder associated with premature delivery and fetal growth retardation which also involves certain imbalances of the placental maturation and the placental integration into the surrounding tissues. To characterize possible differences in the development of normal and preeclamptic placentae, two‐dimensional SDS‐PAGE of 10 normal placental protein homogenates was performed and compared to the protein pattern of homogenates from 10 preeclamptic placentae. Thus, acidic proteins particularly of about 56 kDa were identified in normal placentae which were nearly undetectable in protein homogenates of preeclamptic placentae. Peptide mass finger printing by MALDI identified cytokeratins, especially cytokeratin‐10 as one of the differentially expressed protein. Quantitative data were obtained by two‐dimensional difference‐gel electrophoresis (2D DIGE). Labeling of normal and preeclamptic placental proteins with the fluorophors Cy5 and Cy3, respectively, and subsequent separation of these mixed samples by 2D DIGE revealed a differentially expressed protein spot at a 12.8‐fold higher fluorescence intensity in normal placentae as compared to preeclamptic placentae. MALDI analysis of this differentially expressed protein spot identified cytokeratin. In order to verify these results in individual placenta probes, cytokeratin‐10 Western blots were performed in 3 normal and preeclamptic placental homogenates of different gestational ages, respectively. Whereas a marked 56 kDa cytokeratin‐10 expression appeared in all normal placentae, there was only little if any detectable cytokeratin‐10 present in the preeclamptic placentae, respectively. These findings suggest that preeclampsia is accompanied by a significantly reduced cytokeratin signaling provided by 2D‐DIGE‐coupled MALDI analysis which represents a suitable technique to identify e. g. disease‐related alterations in protein patterns.

Retrodifferentiation and reversibility of aging: forever young?

Maturation of stem cells or precursor cells is associated with the acquisition of certain properties finally resulting in specifically functional cell types within the diverse tissues. This maturation process requires distinct steps of differentiation and is accompanied by a constantly increasing process of aging paralleled by a progressively reduced proliferative capacity. The eventually growth arrested and terminally differentiated cells perform their appropriate specific functions associated with developing senescence by STASIS (stress or aberrant signaling‐inducing senescence) and/or by replicative senescence. Finally, elimination via apoptosis concludes their life span. However, nature also provides a surprise within this concept of life: Sometimes, differentiation and aging steps are reversible. A biological phenomenon of completely reversible differentiation events has been characterized as retrodifferentiation rather than dedifferentiation. Thus, all morphological and functional properties of retrodifferentiated and previously more undifferentiated cells are indistinguishable. Consequently, reversible differentiation may simultaneously be associated with a reversibility of the aging process and therefore, contributes to longevity and rejuvenation. Tissue renewals or regenerative potential for tissue‐specific requirements, if not sufficiently compensated by the appropriate stem cells, may necessitate the generation of undifferentiated precursors by retrodifferentiation followed by a subsequent transdifferentiation process with the consequence of cell type conversion which also includes the risk for tumor development. This interference with the normal biological clock mediated by threshold effects in certain individual cells, raises important questions: What signals trigger retrodifferentiation and what would be the finite life span of cells with a retrodifferentiation capacity?