Cell cycle and post-transcriptional regulation of annexin expression in IMR-90 human fibroblasts.

Based on the finding that the expression of some annexins varies dramatically 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 regulation of cell growth. To further test this hypothesis, we have studied the expression of various annexins in normal human IMR-90 fibroblasts synchronized by serum deprivation. Using immunoblotting, the cellular content 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% during 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 cycle, suggesting that down-regulation of both proteins in G2/M occurred through a post-transcriptional process. In addition, cells transfected with Anx VII cDNA were shown to contain an amount of Anx VII similar to wild-type cells, despite the elevation of Anx VII mRNA content in transfected cells by approx. 2 orders of magnitude. Vector misconstruction or possible secretion of the overexpressed protein were ruled out using appropriate controls. Therefore, as with cell-cycle regulation, Anx VII expression in transfected cells is also controlled by post-transcriptional mechanisms. Furthermore, using pulse-chase analysis, we have determined that annexin VII, and other Anxs, have a slow turnover rate, 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 support the concept that post-transcriptional mechanisms may control levels of Anxs I and VII.