Chloroquine diverts ACTH from a regulated to a constitutive secretory pathway in AtT-20 cells

Abstract

AtT-20 cells, a mouse pituitary line, externalize a viral membrane glycoprotein and the precursor of ACTH constitutively, that is, rapidly without storage or regulation¹. They also have a regulated pathway in which they cleave the precursor to mature hormones, ACTH and β-endorphin², store them in secretory granules³ and discharge them only in the presence of a secretagogue. An analogy exists for newly synthesized lysosomal enzymes which are either delivered to the lysosome or secreted from the cell^4,5. Targeting to the lysosomes may require a low pH step, since chloroquine causes the enzymes to be secreted from the cell^5,6. Here we show that chloroquine (200 µM) also appears to block the storage of newly synthesized ACTH in secretory granules and instead diverts it to the outside of the cell via the constitutive pathway. Chloroquine has no effect on the constitutive pathway and does not block the exocytosis of pre-packaged ACTH. Thus like lysosomal enzymes, peptide hormones are not sent to their correct destinations in the presence of chloroquine, but are diverted instead to a constitutive pathway that is chloroquine-insensitive.