Crotoxin, a phospholipase A 2 neurotoxin from snake venom, interacts with epithelial mammary cells, is internalized and induces secretion

Abstract

Prolactin (PRL) induces liberation of arachidonic acid (AA) from phospholipids of lactating mammary epithelial cells and stimulates casein secretion. In order to investigate the possible involvement of phospholipase A 2 (PLA 2) activity in the hormonal control of casein secretion by PRL, we examined the effects of crotoxin, a PLA 2 neurotoxin from snake venom, on mammary epithelial cells. Crotoxin is made of two subunits: a basic PLA 2 with low toxicity (component B, CB) and an acidic, non-toxic and enzymatically inactive component A (CA) which enhances the pharmacological action of CB. While CA is inactive, the PLA 2 subunit (CB) induces an accumulation of secretory products in the lumen of mammary acini, an extensive development of the Golgi apparatus. The secretion of newly synthesized casein is increased in the presence of CB and this effect is inhibited by nordihydroguaiaretic acid (NDGA) and caffeic acid, two inhibitors of the lipoxygenase pathway which also prevent stimulation of secretion by PRL. Further, CB transiently induces the release of radiolabelled AA from mammary tissues previously labelled with [ 14C]AA, the highest release being observed between 15 s and 5 min of contact with CB and CA. Immunofluorescence labelling by anti-CB antibodies of epithelial mammary tissues previously incubated with CA, CB or a combination of CA and CB indicates that CB binds to epithelial cells and is internalized, at least in part, and that CA enhances both CB binding and its internalization. These observations emphasize the involvement of PLA 2 in the control of casein secretion and suggest that PLA 2 acts intracellularly.