Translocation of lipocortin (annexin) 1 to the membrane of U937 cells induced by phorbol ester, but not by dexamethasone

Abstract

1 Induction of lipocortin 1 secretion by dexamethasone has been demonstrated, although the secretory mechanism is still unknown. We have studied the effects of 12-tetradecanoyl phorbol 13-acetate (TPA) and/or dexamethasone on the expression, translocation, and secretion of lipocortin 1 in U937 cells. 2 The expression of lipocortin 1 and its mRNA increased during TPA-induced differentiation of U937 cells to a maximum of 1.9 fold and 8.2 fold, respectively, after 48 h. Both the protein and the mRNA levels decreased after 48 h. 3 TPA caused the translocation of lipocortin 1 from the cytosol to the membrane of U937 cells in a time-dependent manner, as determined by Western blot analysis. The translocation was concurrent with the differentiation of the cells. After 48 h of TPA treatment, 82.6+6.5% of lipocortin 1 was present in the membrane fraction compared to 41.6+1.7% in untreated cells. 4 The amount of lipocortin 1 that was externally bound (associated) with the membrane increased to 3.2 fold as the cytosol to membrane translocation of lipocortin 1 increased. 5 Dexamethasone decreased the externally bound lipocortin 1, but had no effect on the cytosol to membrane translocation. 6 This offers a model system with which the function and the secretion mechanism of lipocortin 1 can be studied. Our data is consistent with the hypothesis that the secretory mechanism is through an unknown pathway, involving the translocation of lipocortin 1 from the cytosol to the internal membranes, and then, its secretion to the external membrane.

Annexin I is a stress protein induced by heat, oxidative stress and a sulfhydryl‐reactive agent

“…Rabbit antiserum was raised against human annexin I produced in Escherichia coli as described previously [15]. Mouse anti-(annexin I) mAb was purchased from Zymed (USA).…”

Inhibition of Cytosolic Phospholipase A2 by Annexin I

“…Upon activation of cells, intracellular calcium concentration is elevated from 0.1 to 1 M, and cPLA 2 is phosphorylated and translocated from the cytosol to the membrane (11). ANX-I is also translocated upon activation of cells (46,47). Since the degree of inhibition due to ANX-1 is similar at calcium concentrations of 0.1 and 1 M (Figs.…”

Differential effects of annexins I, II, III, and V on cytosolic phospholipase A2 activity: specific interaction model

“…Annexins (ANXs) (also called lipocortins) are a family of intracellular proteins that bind phospholipids and membranes in a Ca 2+ ‐dependent manner [1]. Several members of the ANX family have been implicated in the regulation of cellular processes including membrane fusion, inflammation, proliferation, and differentiation [2–5]. However, despite many functional possibilities, well‐characterized biological functions or relationships between structure and function have not been determined for any of the ANXs.…”