Evidence for anti-apoptotic roles of proteasome activator 28γ via inhibiting caspase activity.

Proteasome activator PA28γ (REGγ, Ki antigen) has recently been demonstrated to display anti-apoptotic properties via enhancing Mdm2-p53 interaction, thereby facilitating ubiquitination and down-regulation of the tumor suppressor p53. In this study we demonstrate a correlation between cellular PA28γ levels and the sensitivity of cells towards apoptosis in different cellular contexts thereby confirming a role of proteasome activator PA28γ as an anti-apoptotic regulator. We investigated the anti-apoptotic role of PA28γ upon UV-C stimulation in B8 mouse fibroblasts stably overexpressing the PA28γ-encoding PSME3 gene and upon butyrate-induced apoptosis in human HT29 adenocarcinoma cells with silenced PSME3 gene. Interestingly, our results demonstrate that PA28γ has a strong influence on different apoptotic hallmarks, especially p53 phosphorylation and caspase activation. In detail, PA28γ and effector caspases mutually restrict each other. PA28γ is a caspase substrate, if PA28γ levels are low. In contrast, PA28γ overexpression reduces caspase activities, including the caspase-dependent processing of PA28γ. Furthermore, overexpression of PA28γ resulted in a nuclear accumulation of transcriptional active p53. In summary, our findings indicate that even in a p53-dominated cellular context, pro-apoptotic signaling might be overcome by PA28γ-mediated caspase inhibition.

Dosage effect of zero to three functional LBR-genes in vivo and in vitro.

The Lamin B receptor (LBR) is a pivotal architectural protein in the nuclear envelope. Mutations in the Lamin B receptor lead to nuclear hyposegmentation (Pelger-Huët anomaly). We have exactly quantified the nuclear lobulation in neutrophils from individuals with 0, 1, 2 and 3 functional copies of the lamin B receptor gene and analyzed the effect of different mutation types. Our data demonstrate that there is a highly significant gene-dosage effect between the gene copy number and the nuclear segmentation index of neutrophils. This finding is paralleled by a dose-dependent increase in LBR protein and staining intensity of the nuclear membrane in corresponding lymphoblastoid cell lines, which demonstrates a significant correlation on the protein level as well. We further show that LBR expression continually increases during granulopoiesis in vitro from human precursor cells with ovoid nuclei to multi-segmented neutrophil nuclei 11 days later, indicating relevance for regular human granulopoiesis. Altogether, LBR is a unique model that will allow the systematic study of gene-dosage effects and of modifying endogeneous and exogeneous factors on granulopoiesis.