Novel, non-crinophagic, degradation of connecting peptide in transformed pancreatic beta cells.
Proinsulin is converted to equimolar amounts of insulin and connecting peptide (C-peptide) in pancreatic beta cell secretory granules. The fate of C-peptide, which remains soluble in granules while insulin is crystallized, was studied in a rat insulinoma cell line (INS cells). A pulse-chase approach demonstrated that insulin to C-peptide ratios (I/CP) began to rise in INS cells as soon as conversion took place, reaching 2.2:1 at 6 h of chase, and 5.3:1 by 24 h. During the first 4 h of chase, this ratio was inverted in the medium under basal conditions, possibly reflecting preferential secretion of C-peptide via the post granular constitutive-like pathway. However, the amount of C-peptide secreted to the medium (1.4%/h) was trivial compared with the net loss of C-peptide from the INS cells, estimated to be approximately 33% during the 1st h. As no extracellular degradation of insulin or C-peptide was observed, the data show progressive, extensive intracellular degradation of C-peptide. Stimulation of INS granule exocytosis revealed that the I/CP ratio in secretory granules is representative of that of cell extracts, indicating that C-peptide, but not insulin, is lost from functionally competent granules with time. This particular degradative route for C-peptide is thus distinct from crinophagy (fusion of granules and lysosomes) and was found to be much more active in INS cells than in primary islet beta cells.
Neerman-Arbez
M
M
Laboratoires de Recherche Louis Jeantet, University of Geneva Medical Center, Switzerland.
Halban
P A
PA
D000818:Animals; D002096:C-Peptide / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003594:Cytoplasmic Granules / Q000378:metabolism; D006801:Humans; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism*; D007700:Kinetics; D008247:Lysosomes / Q000378:metabolism; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D014407:Tumor Cells, Cultured
D002096:C-Peptide