Impaired Intestinal Proglucagon Processing in Mice Lacking Prohormone Convertase 1

Abstract

The neuroendocrine prohormone convertases 1 and 2 (PC1 and PC2) are expressed in endocrine intestinal L cells and pancreatic A cells, respectively, and colocalize with proglucagon in secretory granules. Mice lacking PC2 have multiple endocrinopathies and cannot process proglucagon to mature glucagon in the pancreas. Disruption of PC1 results in dwarfism and also multiple neuroendocrine peptide processing defects. This study compares the pancreatic and intestinal processing of proglucagon in mice lacking PC1 expression with that in age-matched wild-type controls. Because proglucagon was found to precipitate in acidic extracts, the intestinal processing profile was analyzed in both acidic and neutral extracts by gel filtration, HPLC, and RIA. Supporting a central role for PC2 in glucagon biosynthesis, we found normal processing of proglucagon to glucagon in the pancreas, whereas the intestinal proglucagon processing showed marked defects. Tissue proglucagon levels in null mice were elevated, and proglucagon processing to glicentin, oxyntomodulin, and glucagon-like peptide-1 and -2 (GLP-1 and GLP-2) was markedly decreased, indicating that PC1 is essential for the processing of all the intestinal proglucagon cleavage sites. This includes the monobasic site R(77) and, thereby, production of mature, biologically active GLP-1. We also found elevated glucagon levels, suggesting that factors other than PC1 that are capable of processing to mature glucagon are present in the secretory granules of the L cell. These findings strongly suggest that PC1 is essential for intestinal proglucagon processing in vivo and, thereby, plays an important role in production of the incretin hormone GLP-1 and the intestinotrophic hormone GLP-2.

The Physiology of Glucagon-like Peptide 1

“…Interestingly, the NH 2 -terminal cleavage site of GLP-1 is not a classical pair of basic amino acids, but represents a single Arg residue; however, in vitro coexpression of PC 1/3 and proglucagon has demonstrated efficient cleavage at this site (255). In agreement with this, mutations in PC 1/3 lead to abnormalities in GLP-1 processing and secretion, associated with multiple endocrinopathies (143) (undoubtedly because PC 1/3 is important for processing of many regulatory peptides/hormones), and mice with a targeted deletion of the PC-1/3 gene are unable to process proglucagon to GLP-2 and GLP-1 (295,344). Interestingly, it was recently demonstrated that adenovirus-mediated expression of PC 1/3 in the pancreatic alpha cells increases islet GLP-1 secretion, resulting in improved glucose-stimulated insulin secretion and enhanced survival in response to cytokine treatment as well as enhanced performance after transplantation to mouse models of type 1 diabetes, a finding of considerable clinical interest (see below) (331).…”

“…The processing of proglucagon in the intestinal L-cells results from the actions of coexpressed prohormone convertase (PC) 1/3, which is both necessary and sufficient for the complete processing (295,344). Interestingly, the NH 2 -terminal cleavage site of GLP-1 is not a classical pair of basic amino acids, but represents a single Arg residue; however, in vitro coexpression of PC 1/3 and proglucagon has demonstrated efficient cleavage at this site (255).…”

Resolution of type 2 diabetes following gastric bypass surgery: involvement of gut-derived glucagon and glucagonotropic signalling?

“…Interestingly, in total PC1 deficiency in humans exaggerated postprandial glucagon responses have been observed [31] and in PC1-deficient mice the processing of proglucagon to GLP-1 and GLP-2 is significantly reduced, whereas plasma glucagon levels are increased [32]. These observations suggest that L cells are able to process proglucagon to glucagon, possibly by using PC2.…”

GLP2: an underestimated signal for improving glycaemic control and insulin sensitivity

“…GLP2 is also produced in a discrete population of neurons in the brainstem, which projects mainly to the hypothalamus, brain area that plays a key role in control of food intake (Vrang et al 2007). Prohormone convertase 1/3 (PC1/3) processes proglucagon in the gastrointestinal tract and in the brain, resulting in glucagon-like peptide-1 (GLP1), GLP2, intervening peptide-2, oxynthomodulin and glicentin (Ugleholdt et al 2004). The studies on proglucagon-derived peptides have supplied two classes of glucose-lowering agents, the dipeptidyl peptidase IV (DPP-IV) inhibitors and GLP1 receptor agonists, useful tools for the treatment of type 2 diabetes (T2D) (Drucker & Nauck 2006).…”