Invited review
Molecular evolution of proglucagon

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Abstract

The vertebrate proglucagon gene encodes glucagon, and the two glucagon-like peptides GLP-1 and GLP-2. To better understand the origin and diversification of the distinct hormonal roles of the three glucagon-like sequences encoded by the proglucagon gene, we have examined the evolution of this gene. The structure of proglucagon has been largely maintained within vertebrates. Duplication of the proglucagon gene or duplications of sequences within the proglucagon gene are rare. All proglucagon gene duplications are likely to be the result of genome duplication events. Examination of the rates of amino acid sequence evolution of each hormone reveals that they have not evolved in a uniform manner. Each hormone has evolved in an episodic fashion, suggesting that the selective constraints acting upon the sequence vary between, and within, vertebrate classes. Changes in selection on a sequence often reflect changes in the function of the sequence, such as the change in function of GLP-1 from a glucagon-like hormone in fish to an incretin in mammals. We found that the GLP-2 sequence underwent rapid sequence evolution in the early mammal lineage, therefore we have concluded that mammalian GLP-2 has acquired a new biological function that is not found in other vertebrates. Comparisons of the hormone sequences show that many amino acid residues that are functionally important in mammalian hormones are not conserved through vertebrate evolution. This observation suggests that the sequences involved in hormone action change through evolution.

Introduction

The proglucagon gene encodes several peptide hormones with essential roles in regulating carbohydrate, lipid and amino acid metabolism [1], [2], [3]. Glucagon is a 29-amino acid peptide hormone that is produced and secreted by pancreatic islet A-cells and plays a direct role in regulating metabolism [1]. Proglucagon encodes two additional glucagon-like peptides, glucagon-like peptides 1 and 2 (GLP-1 and GLP-2) [4], [5], [6]. In mammals, GLP-1 is secreted by intestinal L-cells and indirectly regulates glucose metabolism by acting as an incretin hormone potentiating the release of insulin by pancreatic islet B-cells [7], [8]. In contrast, fish GLP-1 is secreted by both the intestine and pancreas and has a biological activity similar to glucagon [9], [10], [11]. To date, the biological activity of GLP-2 has only been identified in mammals, where it acts as an intestinal epithelial growth factor when administered to rodents [12].

Tissue-specific proteolytic processing in mammals [13], [14] regulates the production of glucagon, GLP-1, and GLP-2. Glucagon is secreted by pancreatic islet A cells, whereas glucagon-like peptides-1 and -2 are produced in the L-cells of the intestine and within selected neurons of the brain [2], [3]. In some non-mammalian vertebrates, alternative mRNA splicing may have a role in determining tissue-specific expression of GLP-2. For example, pancreatic proglucagon mRNAs from anglerfish (Lophus americanus), rainbow trout (Oncorhynchus mykiss), chicken (Gallus gallus), and gila monster (Heloderma suspectum) do not encode GLP-2, while intestinal proglucagon mRNAs encode GLP-2 [4], [15], [16], [17]. As a result, post-translational processing of preproglucagon in the pancreas of these species cannot yield GLP-2. In the frog Xenopus laevis, all proglucagon mRNAs in the intestine encode GLP-2, while only 50% of mRNA in the pancreas encodes GLP-2 [18]. Interestingly, proglucagon mRNA isolated from the Xenopus laevis stomach did not encode GLP-2. Frog proglucagons also differ from those of other vertebrates due to the presence of multiple GLP-1-like sequences encoded by the proglucagon gene [18], [19], [20]. The multiple GLP-1-like sequences are the result of duplication events that occurred on the frog lineage [20]. Characterization of mammalian proglucagon genes has shown that glucagon, GLP-1, and GLP-2 are each encoded by separate exons [21], [22]. This structure of the proglucagon gene suggests that these three peptides originated by triplication of an exon that encoded a glucagon-like sequence.

Previous evolutionary analyses of proglucagon sequences have indicated that the glucagon, GLP-1, and GLP-2 sequences within the proglucagon gene originated prior to the diversification of vertebrates more than 500 million years ago [23], [24]. These studies also demonstrated that the proglucagon gene has independently duplicated with the jawless fish (Agantha) and bony fish (Osteichthyes) lineages, resulting in the presence of two glucagon and/or GLP-1 sequences in many species. We have chosen to explore the evolution of all known proglucagon-derived sequences to better understand the origin and divergence of the unique biological function of glucagon, GLP-1 and GLP-2.

Section snippets

Sequences

Sequences for proglucagon, glucagon, GLP-1, and GLP-2, either deduced or predicted, are available in the literature or in GenBank from more than 50 species representing all classes of vertebrates. The species and numbers of peptide sequences used in this study are listed in Table 1. Incomplete sequences, sequences that contain internal deletions, or sequences that may not be functional are indicated in Table 1, and were not used in our analysis. All sequences used were readily alignable

Structure of proglucagon

The amino acid sequences of proglucagon-derived peptides have been deduced or predicted from 52 different vertebrate species (Table 1). In most of these species, the sequences of the individual peptide hormones, rather than the full-length proglucagon sequence is known. In 15 of the 52 species, one or two cDNA sequences (for a total of 19 cDNA sequences, see Table 1) have been determined, thereby allowing the prediction (in all but one species, see below) of full-length proglucagon sequences.

Conclusions

Our analysis of glucagon, GLP-1, and GLP-2 sequences has shown that each has evolved in a unique and independent fashion. Glucagon has evolved at an accelerated rate within bony fish. GLP-1 has either evolved at an accelerated rate in frogs, bony fish, and cartilaginous fish, or has slowed down within mammals, birds and reptiles (and few other species). In contrast, GLP-2 has experienced an episode of accelerated evolution on the lineage that leads to mammals. Some of these changes in

Acknowledgements

I thank Drs. R. Boushey, P. Brubaker, D. Drucker, H. Elshotz, T. Mommsen, M. Wheeler, and J. Youson for discussions and suggestions. This work was supported by a grant from the Natural Sciences and Engineering Research Council.

References (48)

  • S. Aparicio

    Vertebrate evolution, recent perspectives from fish

    Trends Genet

    (2000)
  • C.G. Unson et al.

    Multiple-site replacement analogs of glucagon

    J Biol Chem

    (1994)
  • E.S.W. Ngan et al.

    Functional studies of a glucagon receptor isolated from frog Rana tigrina rugulosa: implications on the molecular evolution of glucagon receptors in vertebrates

    FEBS Lett

    (1999)
  • J.C. Wriston

    Comparative biochemistry of the guinea-pig: a partial checklist

    Comp Biochem Physiol

    (1984)
  • D.F. Steiner et al.

    The new enzymology of precursor processing endoproteases

    J Biol Chem

    (1992)
  • J.M. Conlon et al.

    Multiple molecular forms of insulin and glucagon-like peptide isolated from the pacific ratfish (Hydrolagus colliei)

    Gen Comp Endocrinol

    (1989)
  • K. Adelhorst et al.

    Structure–activity studies of glucagon-like peptide-1

    J Biol Chem

    (1994)
  • S. Seino et al.

    Mutations in the guinea pig preproglucagon gene are restricted to a specific portion of the prohormone sequence

    FEBS Lett

    (1986)
  • P.J. Lefèbvre

    Glucagon and its family revisited

    Diabetes Care

    (1995)
  • T.J. Kieffer et al.

    The glucagon-like peptides

    Endocrine Rev

    (1999)
  • P.K. Lund et al.

    Pancreatic preproglucagon cDNA contains two glucagon-related coding sequences arranged in tandem

    Proc Natl Acad Sci USA

    (1982)
  • G.I. Bell et al.

    Hamster preproglucagon contains the sequence for preproglucagon and two related peptides

    Nature

    (1983)
  • L.C. Lopez et al.

    Mammalian pancreatic preproglucagon contains three glucagon-related peptides

    Proc Natl Acad Sci USA

    (1983)
  • D.J. Drucker

    The glucagon-like peptides

    Diabetes

    (1998)
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