Abstract
Annexin VIII is a calcium-dependent phospholipid-binding protein previously identified as a blood anticoagulant based on in vitro studies. However, the physiologic function of annexin VIII remains unknown. In acute promyelocytic leukemia (APL) the annexin VIII gene is highly expressed, but its expression is undetectable in the blasts of other acute leukemias. In the present investigation, we showed using the APL-derived NB4 cell line that expression of the annexin VIII gene is regulated at the transcription level during induced differentiation by all-trans retinoic acid (ATRA). The half-life of the annexin VIII mRNA is about 5 to 6 hours, as determined by using actinomycin D as a transcription inhibitor. Analysis of the expression of annexin VIII protein in NB4 cells and in APL samples showed a consistent expression of a predominant 36-kD protein and a weak 72-kD protein. After ATRA- induced differentiation of NB4 cells, the annexin VIII protein level reduced gradually, but a detectable level persisted even after 4 days of induction. Because annexin VIII mRNA becomes undetectable after 48 hours of ATRA induction, this result indicates that annexin VIII is a relatively stable protein. A multiple tissue Northern blot analysis was performed, and we found that annexin VIII is normally expressed in the placenta and the lung. Cellular localization of the annexin VIII protein was determined by immunofluorescence staining and subcellular fractionation. These results indicated that annexin VIII is predominantly localized to the plasma membrane. The annexin VIII is neither an extracellular protein nor associated with the cell surface suggesting that it does not play a role in blood coagulation in vivo. The plasma membrane localization and its property as a phospholipase inhibitor suggests that annexin VIII may have a role in the signal transduction pathway in the APL cells.
