Insulin, not C-peptide (proinsulin), is present in crinophagic bodies of the pancreatic B-cell.

Abstract

We have obtained evidence by autoradiography and immunocytochemistry that mature secretory granules of the pancreatic B-cell gain access to a lysosomal compartment (multigranular or crinophagic bodies) where the secretory granule content is degraded . Whereas the mature secretory granule content shows both insulin and C-peptide (proinsulin) immunoreactivities, in crinophagic bodies only insulin, but not C-peptide, immunoreactivity was detectable . The absence of C-peptide (proinsulin) immunoreactivity in multigranular bodies, i.e., in early morphological stages of lysosomal digestion, was compatible with the ready access and breakdown of C-peptide and/or proinsulin by lysosomal degrading enzymes, while the insulin crystallized in secretory granule cores remained relatively protected . However, in the final stage of lysosomal digestion, i .e., in residual bodies where the secretory granule core material is no longer present, insulin immunoreactivity became undetectable . Lysosomal digestion thus appears to be a normal pathway for insulin degradation in the pancreatic B-cell.The pancreatic B-cell shares with other polypeptide-secreting cells the ability to store its secretory products within cytoplasmic granules available for release by exocytosis (1, 2). However, the totality of insulin manufactured is not destined to be secreted, since it has been shown that the B-cell can degrade a significant portion of its own secretory product (3, 4). One commonly assumed mechanism of intracellular degradation ofinsulin by the B-cell is the fusion ofsecretory granules with primary lysosomes (5, 6), a mechanism called granulolysis or crinophagy and initially described in the mammotroph cells ofthe anterior pituitary (7) and pancreatic A-cells (8) . In the B-cell, morphological evidence for crinophagy consists of cytoplasmic organelles (secondary lysosomes) that present a very variable morphology but usually contain dense masses resembling a-secretory granule cores. The lysosomal nature of these structures (also called crinophagic or multigranular bodies) is evidenced by their content in marker lysosomal enzymes such as acid phosphatase or arylsulfatase (9), but whether the content ofthe granule core-like material is related to insulin polypeptides has not been ascertained. For this purpose, we have followed, by electron microscopic autoradiography, the labeling of the multigranular bodies in pulsechase experiment with tritiated leucine, and determined their 222 immunoreactive content using anti-insulin and anti-C-peptide antisera revealed by the protein A-gold method. MATERIALS AND METHODSRat islets of Langerhans in sections of pancreatic tissue or isolated by collagenase digestion (10) were studied throughout . The material examined originated from the tissue store of the laboratory . All blocks were from five untreated normal rats, the pancreas (or isolated islets) of which were fixed in 2 .5% glutaraldehyde alone or with a mixture of 1 .25% glutaraldehyde, I % paraformaldehyde, and 0 .02% trinitrocresol (11), then...

Beta cells transfer vesicles containing insulin to phagocytes for presentation to T cells

“…The importance of this finding is that the antibody exclusively reacts with the B chain peptide and not with native insulin, indicating that some intracellular vesicles contain degradative products of insulin. Extensive studies have unequivocally shown that beta cells contain vesicles bearing insulin catabolites, in a pathway referred to as crinophagy (32)(33)(34)(35)(36). It indicates an intracellular degradation of excess insulin granules, as a mode of controlling the homeostasis of insulin granule content (32,37).…”

Autophagy in adipose tissue and the beta cell: implications for obesity and diabetes

“…Physiological roles of crinophagy and autophagy Pioneering studies by Orci et al showed that insulin granules are regulated by lysosomal degradation through crinophagy and autophagy [61]. In crinophagy, the secretory granule membrane fuses with the membrane of a large vacuolar, lysosomal compartment to generate a crinophagic body, within which the insulin granule content is degraded.…”

“…Insulin granules may also reach lysosomes via autophagosomes that engulf cytosolic components containing secretory granules (macroautophagy), or via lysosomal engulfment and swallowing of a single granule (microautophagy). Insulin is relatively resistant to degradation in the acidic environment of the lysosome; its degradation is much slower than that of C-peptide or proinsulin [61,62]. This underlies the assumption that insulin degradation plays a minor role in the regulation of insulin homeostasis and beta cell function [62].…”

The insulin factory: a tour of the plant surroundings and a visit to the assembly line

“…In the B cell, morphological evidence for crinophagy consists of cytoplasmic organelles of variable morphology containing dense masses resembling t-secretory granule cores. The lysosomal nature of these structures (also called crinophagic or multigranular bodies) is evidenced by their content in lysosomal enzymes such as acid phosphatase, arylsulfatase [35] or cathepsin B (Fig. 11 C).…”

“…11 C). Whether or not the granule core-like material of multigranular bodies is related to insulin polypeptides has recently been explored by immunocytochemistry at the ultrastructural level using anti-insulin and anti-C-peptide antisera [35]. A timetable of the postulated transfer of secretory granule cores into lysosomal structures was also assessed by pulse-chase experiments with 3H-leucine on isolated islets.…”