The subcellular distribution of annexin V, a calcium-dependent phospholipid- and membrane-binding protein, in a human-derived cell line, GL15, was investigated by immunocytochemistry at light and electron microscope levels. Annexin V was found diffusely in the cytoplasm and associated with plasma membranes, membranes delimiting cytoplasmic vacuoles, membranes of the endoplasmic reticulum, and filamentous structures the identity of which remains to be established. By immunocytochemistry at the light microscope level and immunochemistry, the expression of annexin V in these cells was found to depend on cellular growth state, being maximal soon after plating and progressively declining thereafter. However, re-expression of annexin V was observed whenever cell proliferation slowed down or arrested. These findings suggest that annexin V in glioma cells is mostly expressed in connection with cell differentiation. Also, the present ultrastructural data suggest that plasma membranes, membranes of the endoplasmic reticulum and the cytoskeleton are prominent sites of action of annexin V in vivo, thus lending support to the possibility that this protein might have a role in the regulation of cytoskeleton elements and/or of the structural organization of membranes.
