Characterization of the TGN exit routes in AtT20 cells using pancreatic amylase and serum albumin

The AtT20 pituitary cell is the one that was originally used to define the pathways taken by secretory proteins in mammalian cells. It possesses two secretory pathways, the constitutive for immediate secretion and the regulated for accumulation and release under hormonal stimulation. It is in the regulated pathway, most precisely in the immature granule of the regulated pathway, that proteolytic maturation takes place. A pathway that stems from the regulated one, namely the constitutive-like pathway releases proteins present in immature granules that are not destined for accumulation in mature granules. In AtT20 cells proopiomelanocortin the endogenous precursor of the accumulated adrenocorticotropic hormone, is predominantly secreted in a constitutive manner without proteolytic maturation. In order to better understand by which secretory pathway intact proopiomelanocortin is secreted by a cell line possessing a regulated secretory pathway, it was transfected with rat serum albumin (a marker of constitutive secretory proteins), and pancreatic amylase (a marker of regulated proteins). COS cells were also transfected in order to serve as control of release by the constitutive pathway. It was observed that both the basal and stimulated secretions of albumin and proopiomelanocortin from AtT20 cells are identical. In addition, secretagogue stimulation when POMC is in transit in the trans-Golgi network decreases its constitutive secretion by 50%. It was also observed using cell fractionation and 20 degrees C secretion blocks that albumin and proopiomelanocortin are present in the regulated pathway, presumably in the immature granules, and are secreted by the constitutive-like secretory pathway. These observations show that stimulation can increase sorting into the regulated pathway, and confirm the importance of the constitutive-like secretory pathway in the model AtT20 cell line.