Impairment of bile salt-dependent lipase secretion in AR4-2J rat pancreatic cells induces its degradation by the proteasome.
Bile salt-dependent lipase (BSDL, EC 3.1.1.13) is a lipolytic enzyme normally secreted by the pancreatic acinar cell. Co- and post-translational modifications, such as N- and O-linked glycosylation, regulate the secretion of this enzyme; therefore it was of first importance to determine the behaviour of BSDL under conditions that impaired its secretion. Using AR4-2J pancreatic cells as model, we showed, particularly when BSDL secretion is impaired, that proteasome inhibitors increased the amount of intracellular BSDL, suggesting that the proteasome is involved in the degradation of this protein. This was strengthened by the detection of ubiquitinated BSDL and of degradation product. Our results suggested that both ubiquitination and degradation of the enzyme occurred at the level of the cytosolic side of microsome membranes. ATP hydrolysis appears essential in ubiquitinated BSDL association with membranes and degradation. Furthermore, under normal secretory conditions, we have shown that a fraction of ubiquitinated BSDL is neither O-glycosylated nor N-glycosylated, suggesting that the N-glycosylation-deficient proteasome substrate does not reach the Golgi and could be degraded by the ER-associated degradation machinery. However, another fraction of ubiquitinated BSDL that is deficient in O-glycosylation, carries out endoglycosidase H-insensitive N-linked glycans, meaning that a second system, that detects abnormal BSDL molecules, could also operate at the level of the Golgi compartment. Consequently, it appears that impairment of BSDL secretion consecutive to secretion inhibition or to a deficient glycosylation leads to the proteasome-ubiquitin-dependent degradation of the protein. Therefore, this pathway is part of the quality control involved in BSDL secretion.
INSERM Unité 260, Faculté de Médecine-Timone, 27 bld Jean Moulin, 13385 cedex 05, Marseilles, France.
J
J
Le Petit-Thevenin
A
A
Verine
A
A
Nganga
O
O
Nobili
D
D
Lombardo
N
N
Bruneau
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002850:Chromatography, Gel; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D007976:Leupeptins / Q000494:pharmacology; D008861:Microsomes / Q000378:metabolism; D008985:Monensin; D009097:Multienzyme Complexes / Q000378:metabolism*; D010179:Pancreas / Q000378:metabolism*; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D051381:Rats; D002787:Sterol Esterase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D014452:Ubiquitins / Q000737:chemistry
D007976:Leupeptins; D009097:Multienzyme Complexes; D014452:Ubiquitins; D000255:Adenosine Triphosphate; D008985:Monensin; C054784:bile salt-stimulated lipase; D002787:Sterol Esterase; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde