Objective: To evaluate pp32 expression as a signal for the complex process of differentiation in normal and malignant cells. Background: pp32 is a highly-conserved multi-functional nuclear protein whose biologic activities include inhibition of oncogene-mediated transformation, inhibition of histone acetyl transferase activity(INHAT), a role in the caspase-independent apoptotic pathway, and message stability of specific cytokine and oncogene messages. It has been previously shown that reduction of pp32 in a neoplastic cell line induced differentiation that accompanied inhibition of proliferation. Further, we recently reported that pp32 expression is nearly absent in poorly differentiated pancreatic ductal adenocarcinoma cells (Modern Pathology 2007 (20), 1238-1244). In adult tissues, pp32 is found in self-renewing (often basal) cells as well as in neoplastic cells, but not in most terminally differentiated cells. These findings suggest that in normal or cancer cells, pp32 expression is either involved in the control of differentiation, or is regulated by mechanisms that control differentiation.
Methods: We evaluated whether pp32 expression correlated with differentiation both in in vitro models and in vivo during murine embryogenesis. Undifferentiated HL-60 promyelocytic leukemia cells as well as ML-1 and K562 myeloid leukemia cell lines were assayed for levels of pp32 mRNA and protein. To determine the pattern of pp32 expression at different gestational ages, we examined transverse sections of embryos from Days 8, 10, 12 and 15 and sagittal sections from embryos from Days 8 - 15.
Results: In all three cell line models, mRNA and protein levels decreased dramatically following phorbol ester-stimulated differentiation into different lineages. In the K562 system, reduction of pp32 expression induces morphology that is consistent with megakaryocytic differentiation. In murine embryos, pp32 mRNA levels are generally high in most tissues on Day 10, but are decreased after Day 12. As tissues differentiate, only a small fraction of cells continue to express pp32 at high levels.
Conclusions: Based upon these results and our previous findings, we conclude that pp32 is differentiation-regulated in normal tissues, neoplastic cells, and during embryogenesis and is most likely a critical signal and marker for this process. These findings provide evidence that manipulation of pp32 expression may provide a novel ‘differentiation-induced’ therapy against different types of cancers that have the plasticity to differentiate. This strategy, in theory, would be more specific and safer than recent, similar approaches using HDAC inhibitors against cancers.