Differential sorting of lysosomal enzymes out of the regulated secretory pathway in pancreatic beta-cells.
In cells specialized for secretory granule exocytosis, lysosomal hydrolases may enter the regulated secretory pathway. Using mouse pancreatic islets and the INS-1 beta-cell line as models, we have compared the itineraries of procathepsins L and B, two closely related members of the papain superfamily known to exhibit low and high affinity for mannose-6-phosphate receptors (MPRs), respectively. Interestingly, shortly after pulse labeling INS cells, a substantial fraction of both proenzymes exhibit regulated exocytosis. After several hours, much procathepsin L remains as precursor in a compartment that persists in its ability to undergo regulated exocytosis in parallel with insulin, while procathepsin B is efficiently converted to the mature form and can no longer be secreted. However, in islets from transgenic mice devoid of cation-dependent MPRs, the modest fraction of procathepsin B normally remaining within mature secretory granules is increased approximately fourfold. In normal mouse islets, immunoelectron microscopy established that both cathepsins are present in immature beta-granules, while immunolabeling for cathepsin L, but not B, persists in mature beta-granules. By contrast, in islets from normal male Sprague-Dawley rats, much of the proenzyme sorting appears to occur earlier, significantly diminishing the stimulus-dependent release of procathepsin B. Evidently, in the context of different systems, MPR-mediated sorting of lysosomal proenzymes occurs to a variable extent within the trans-Golgi network and is continued, as needed, within immature secretory granules. Lysosomal proenzymes that fail to be sorted at both sites remain as residents of mature secretory granules.
Kuliawat
R
R
Diabetes Research Center and Division of Endocrinology, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
Klumperman
J
J
Ludwig
T
T
Arvan
P
P
D000818:Animals; D002401:Cathepsin B / Q000378:metabolism*; D056668:Cathepsin L; D002403:Cathepsins / Q000378:metabolism; D002451:Cell Compartmentation; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004792:Enzyme Precursors / Q000378:metabolism*; D005089:Exocytosis; D006031:Glycosylation / Q000187:drug effects; D006056:Golgi Apparatus / Q000378:metabolism; D007150:Immunohistochemistry; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000201:enzymology* / Q000648:ultrastructure; D008247:Lysosomes / Q000201:enzymology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D051381:Rats; D017207:Rats, Sprague-Dawley; D017527:Receptor, IGF Type 2 / Q000378:metabolism; D014415:Tunicamycin / Q000494:pharmacology
D004791:Enzyme Inhibitors; D004792:Enzyme Precursors; D007328:Insulin; D017527:Receptor, IGF Type 2; D014415:Tunicamycin; D002403:Cathepsins; C057832:procathepsin B; C063905:procathepsin L; D002401:Cathepsin B; D056668:Cathepsin L