Insulin Through the Ages: Phylogeny of a Growth Promoting and Metabolic Regulatory Hormone(1).

INTRODUCTION

The discovery of insulin by Banting and Best in 1922 represented a milestone in clinical medicine but it has also contributed substantially to progress in the fields of molecular and comparative endocrinology. The importance of insulin used in the treatment of diabetes elicited great interest in this hormone and consequently resources became available for studies to characterize its structure and function. As a result, insulin was the first protein whose primary amino acid sequence was determined (Ryle et al., 1955); the radioimmunoassay technique was developed using insulin as the model (Yalow and Berson, 1970); insulin was the first hormone for which the 3-dimensional crystal structure was solved (Adams et al., 1969); proinsulin was the first hormone precursor discovered (Steiner et al., 1967); and the insulin cDNA and gene were among the first to be cloned by recombinant DNA techniques (Ullrich et al., 1977). Together with these impressive "firsts" in molecular endocrinology, there has also been considerable interest to determine the phylogenetic and evolutionary origin of insulin.

Shortly after the discovery of insulin, some researchers thought that it was a unique hormone present only in mammals and birds. However, as early as 1923 Collin reported finding insulin-like activity in a bivalve mollusc, Mya arenaria. Assisted by advances in a number of biochemical techniques, including the development of an efficient method for isolating pure insulin from the pancreas, improved immunological and immunohistochemical assays for insulin, and importantly development of the Edman degradation method for amino acid sequence analysis, scientists by 1975 had isolated and sequenced insulin molecules from representative species in all classes vertebrates including hagfish (Fig. 1). The hagfish, together with lampreys, are living representatives of the jawless vertebrates (class Agnatha) and are considered to be the most evolutionarily ancient vertebrates.

[Figure 1 ILLUSTRATION OMITTED]

In contrast to the success achieved in vertebrates, early attempts to isolate and characterize an insulin-like molecule from an invertebrate species were unsuccessful. However, in 1986 Nagasawa and coworkers in a herculean effort successfully purified and sequenced a 4 kilodalton peptide from the Silkworm Bombyx mori which they originally called 4K-prothoracicotropic hormone-II and later renamed bombyxin A2 (Nagasawa et al., 1986)). The amino acid sequence of bombyxin A2 clearly showed that it was an insulin-like molecule, i.e., it consisted of two peptide chains (A and B) crosslinked by highly conserved insulin type disulfide bonds. The overall sequence identity between bombyxin A2 and human insulin was 40%.

More recently, recombinant DNA techniques have been used to clone insulin-like peptide genes in a variety of invertebrate species, including mollusc, insects and the nematode Caenorhabditis elegans (Smit et al., 1988; Lagueux et al., 1990; Duret et al., 1998; Gregoire et al., 1998). An insulin receptor-like tyrosine kinase cDNA has also been cloned from a colenterate, hydra (Steele et al., 1996) and from C. elegans (Kimura et al., 1997). Although a previous report claimed that sponges contain an insulin mRNA (Robitzki et al., 1989), this has been retracted (Duret et al., 1998). Recently, the yeast genome was completely sequenced and no insulin-like genes were found (Chervitz et al., 1998). These results suggest that insulin-like genes have co-evolved with the appearance of the metazoa (multicellular animals with differentiated tissues).

In the present review our goals are to briefly summarize the structure and function of insulin and insulin-like growth factors (IGFs) in vertebrates and compare these with the insulin-like peptides found in invertebrates. Based on the available data we propose that the ancestral insulin-like molecule functioned primarily as a mitogenic growth factor but in vertebrates the insulin gene has evolved to become a predominantly metabolic regulatory hormone.