Expression of annexins as a function of cellular growth state.

Abstract

Annexins are a structurally related family of Ca2+ binding proteins of undetermined biological function. Annexin I (also called lipocortin 1) is a substrate for the EGF-stimulated tyrosine kinase and is postulated to be involved in mitogenic signal transduction. To investigate further the involvement of lipocortin 1 in cell proliferation, we measured lipocortin 1 levels in normal diploid human foreskin fibroblasts (HFF) to determine whether its expression changes as a function of growth status. For comparison, the expression of annexin V (also called endonexin II) was measured in HFF cells. Endonexin II is a protein with similar Ca2+ and phospholipid binding properties as lipocortin 1, but it is not a substrate for tyrosine kinases. Quiescent HFF cultures were induced to proliferate by either subculture to lower cell density, EGF stimulation, or serum stimulation. In all three protocols, proliferating HFF cells contained three- to fourfold higher levels of lipocortin 1 and three- to fourfold higher levels of lipocortin 1 and three- to fourfold lower levels of endonexin II than quiescent HFF cells. In contrast, the expression of annexin II (also called calpactin I) and annexin IV (also called endonexin I) remained relatively unchanged in growing and quiescent HFF cells. Lipocortin 1 synthesis rate was eightfold higher and its turnover rate was 1.5-fold slower in proliferating compared to quiescent HFF cells. Endonexin II synthesis rate remained constant but its turnover rate was 2.2-fold faster in proliferating compared to quiescent HFF cells. In a separate set of experiments, annexin expression levels were measured in cultures of rat PC-12 cells, a pheochromocytoma that ceases proliferation and undergoes reversible differentiation into nondividing neuronlike cells in response to nerve growth factors (NGF). After NGF treatment, PC-12 cells expressed fivefold higher levels of endonexin II and 32-fold higher levels of calpactin 1. Lipocortin 1 and endonexin I were not expressed in PC-12 cells. In summary, lipocortin 1 expression exhibited a positive correlation with cell proliferation in HFF cells. The increased expression of endonexin I in quiescent HFF cells and differentiating PC-12 cells implies that this protein may play a more prominent role in nondividing cells.