CELL-CYCLE AND POSTTRANSCRIPTIONAL REGULATION OF ANNEXIN EXPRESSION IN IMR-90 HUMAN FIBROBLASTS

Based on the finding that the expression of some annexins varies drama tically as a function of cellular proliferation state [Schlaepfer and Haigler (1990) J. Cell Biol. 111, 229-238], it has been proposed that the cellular level of the annexins might be critical for the regulatio n of cell growth. To further test this hypothesis, we have studied the expression of various annexins in normal human IMR-90 fibroblasts syn chronized by serum deprivation. Using immunoblotting, the cellular con tent of annexins (Anxs) II, V and VI was found to vary by less than 10 % during the cell cycle. However, Anx IV expression increased by 50% d uring S-phase and the levels of Anxs I and VII were reduced by 40% in early G2/M. However, using RNase protection assays, the mRNAs of Anxs I and VII were found to be uniformly expressed throughout the cell cyc le, suggesting that downregulation of both proteins in G2/M occurred t hrough a post-transcriptional process. In addition, cells transfected with Anx VII cDNA were shown to contain an amount of Anx VII similar t o wild-type cells, despite the elevation of Anx VII mRNA content in tr ansfected cells by approx. 2 orders of magnitude. Vector misconstructi on or possible secretion of the overexpressed protein were ruled out u sing appropriate controls. Therefore, as with cell-cycle regulation, A nx VII expression in transfected cells is also controlled by post-tran scriptional mechanisms. Furthermore, using pulse-chase analysis, we ha ve determined that annexin VII, and other Anxs, have a slow turnover r ate, consistent with the limited changes of expression throughout the cell cycle. Taken together, these results question the hypothesis that cellular expression of Anxs plays a general role in cell growth and s upport the concept that post-transcriptional mechanisms may control le vels of Anxs I and VII.