Metabolism of glucagon-like peptide-2 in pigs: Role of dipeptidyl peptidase IV
Introduction
The intestinotropic factor, glucagon-like peptide-2 (GLP-2) has attracted considerable attention because of its ability to stimulate intestinal growth in rodents [1], [2] and to improve both intestinal absorption and nutritional status in short-bowel patients [3]. Therefore, GLP-2 has been suggested for the treatment of intestinal insufficiency [4].
GLP-2 originates from tissue-specific posttranslational processing of the glucagon precursor, proglucagon, in the intestinal L-cells [5]. This processing also leads to the formation of one of the major incretin hormones, glucagon-like peptide-1 (GLP-1). Because of the great interest in GLP-1 as a potential therapeutic agent for type 2 diabetes [6], more is now known about GLP-1 metabolism [7], [8]. GLP-2 has approximately 40% sequence homology with GLP-1, including the two N-terminal amino acids. GLP-2 metabolism might, therefore, resemble that of GLP-1. However, little is actually known about this.
It was suggested by Mentlein et al. in 1993, that the enzyme dipeptidyl peptidase IV (DPP-IV) could be capable of cleaving GLP-2, in agreement with its N-terminal similarity to GLP-1 [9]. Subsequently, it has been shown in a number of in vivo studies [10], [11], [12], [13], [14] that intact GLP-2 (1–33), like GLP-1 [8], is a substrate for DPP-IV degradation leading to the formation of the partial agonist GLP-2 (3–33) as demonstrated in studies involving high-performance liquid chromatography and sequencing [10], [14]. Studies have shown that full length (intact) GLP-2 (1–33) is required for receptor activation [15], and that the DPP-IV cleavage product, GLP-2 (3–33), may even act as a GLP-2 receptor antagonist [16].
The few studies focusing on GLP-2 clearance suggested that the kidneys play a role [17], [18], as is the case for GLP-1 [7]. However, it was suggested by Tavares et al. [18] that other organs and mechanisms may also be involved. Again, because of the resemblance to GLP-1, it was suggested that the liver and lungs may contribute to GLP-2 clearance.
The present study was undertaken to elucidate in detail the contribution of the different organs to GLP-2 metabolism, as well as the involvement of DPP-IV, using a previously described porcine model [7], [8], [19]. A processing independent and an N-terminal-specific assay for GLP-2 were employed, allowing determination of the intact, biologically active molecule as well as the metabolite generated by DPP-IV degradation, GLP-2 (3–33) [10].
Section snippets
Materials and methods
This study conformed to the Danish legislation governing animal experimentation, and was carried out after permission was granted from the National Superintendence for Experimental Animals.
Effect of valine-pyrrolidide
The concentration profiles for intact GLP-2 (1–33), the metabolite GLP-2 (3–33) and total GLP-2 (GLP-2 (1–33) + GLP-2 (3–33)) during GLP-2 infusion without and with subtraction of basal levels and before and after valine-pyrrolidide administration are shown in Fig. 1, Fig. 2. During infusion of GLP-2 alone, plasma concentrations of intact GLP-2 and were lower (ANOVA, repeated measurement: both p < 0.001) than concentrations of total GLP-2. After termination of the infusion, basal levels were
Discussion
The main findings of the present study are, firstly, that GLP-2 is significantly degraded by DPP-IV but not nearly to the same extent as GLP-1. Secondly, that GLP-2 is significantly extracted by DPP-IV independent mechanisms in the peripheral tissues in general (here exemplified by extraction across a leg), as well as in the splanchnic bed and the kidneys. Thirdly, that the metabolite formed, GLP-2 (3–33), is eliminated much more slowly and mainly by the kidneys.
The protease DPP-IV cleaves an
Acknowledgments
The technical assistance of Letty Klarskov and Mette Olesen is gratefully acknowledged.
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