A rise in nuclear calcium translocates annexins IV and V to the nuclear envelope

Following incubation of human fibrobIasts with Ca²⁺ ionophore A23187, we found strong immunofluorescence labelling of the nuclear envelope by annexin IV antibody. Using confocal imaging of cells loaded with Fluo-3, we showed that A23187 generates an intense and sustained rise of Ca²⁺ in the nucleus. By contrast, stimulation without extracellular Ca²⁺ produces only a brief rise in nuclear Ca²⁺ that does not promote annexin IV translocation to the nuclear envelope, and compounds that induce only a transient increase of nuclear Ca²⁺ do not support translocation of annexin IV. In addition, annexin V was also translocated to the nuclear envelope by A23187, but distribution of annexins I, II, VI and VII is unaffected. In in vitro assays with isolated nuclei, annexin V was also found to bind to the nuclear envelope in a Ca²⁺-dependent manner. These results demonstrate that the translocation to the nuclear envelope of different types of Ca²⁺-reguIated proteins is directly triggered by a major rise of Ca²⁺ in tile nucleus.