E-Peptides Control Bioavailability of IGF-1

Abstract

Insulin-like growth factor 1 (IGF-1) is a potent cytoprotective growth factor that has attracted considerable attention as a promising therapeutic agent. Transgenic over-expression of IGF-1 propeptides facilitates protection and repair in a broad range of tissues, although transgenic mice over-expressing IGF-1 propeptides display little or no increase in IGF-1 serum levels, even with high levels of transgene expression. IGF-1 propeptides are encoded by multiple alternatively spliced transcripts including C-terminal extension (E) peptides, which are highly positively charged. In the present study, we use decellularized mouse tissue to show that the E-peptides facilitate in vitro binding of murine IGF-1 to the extracellular matrix (ECM) with varying affinities. This property is independent of IGF-1, since proteins consisting of the E-peptides fused to relaxin a related member of the insulin superfamily, bound equally avidly to decellularized ECM. Thus, the E-peptides control IGF-1 bioavailability by preventing systemic circulation, offering a potentially powerful way to tether IGF-1 and other therapeutic proteins to the site of synthesis and/or administration.

The Complexity of the IGF1 Gene Splicing, Posttranslational Modification and Bioactivity

“…Although the deglycosylation process is expected to occur intracellularly, whether the deglycosylation of pro-IGF-IEa or the Ea-peptide also could be clipped extracellularly is not known. Considering the unique role of glycosylation in the protein biosynthesis process (100), it is possible that the Ea-peptide glycosylation might play a role in interactions with chaperones in the endoplasmic reticulum (78), or in regulation of the bioavailability of the different species of this IGF-I isoform (that is, pro-IGF-I Ea, mature IGF-I or Ea-peptide) (78,97,103). Thus, the existence of an N-linked glycosylation site in the Ea-peptide, which is absent in the Ec-and Eb-peptide, might reflect a differential and specific biological action of the IGF-IEa isoform mediated by this posttranslational modification of its Ea-peptide (25,43,97).…”

“…Moreover, it would be essential to verify whether the IGF-I isoforms could be released in the circulation as different proforms or E-peptides (3,79), or the final peptide that enters the circulation after extracellular endoproteolysis of the IGF-I prohormone is only the mature peptide (103,106).…”

“…Further evidence has been provided recently, suggesting that IGF-I splice variants may exert their actions through mature IGF-I and not the E-peptides (149), or by supporting the bioactivity of pro-IGF-I forms (97). Furthermore, there is controversial evidence regarding the role of proforms (103,136,149) or E-peptides (38) of the murine IGF-I isoforms, particularly in cell differentiation. Whether it reflects a differential mode of action of the free E-peptides compared with the pro-IGF-I isoforms, or potential differences between their exogenous administration and overexpression (or natural endogenous production), possibly should be addressed.…”

Estrogen-IGF-1 interactions in neuroprotection: Ischemic stroke as a case study

“…This C-terminal peptide, also called E-peptide, is coded on exon 4, 5 and 6. The E-peptide containing propeptide binds extracellular matrix with greater affinity, and this affinity is independent of the mature IGF-1 region (Hede et al, 2012). A muscle specific transgene of IGF-1E propetide significantly enhances muscle regeneration after injury, while muscle specific mature IGF-1 does not enhance muscle regeneration but increases serum levels of IGF-1 (Rabinovsky et al, 2003).…”

Mechano-growth factor peptide, the COOH terminus of unprocessed insulin-like growth factor 1, has no apparent effect on myoblasts or primary muscle stem cells

“…The mature protein will be referred to here as "IGF-1." In skeletal muscle, full-length, uncleaved IGF-1 has been observed by Western blot (12), suggesting the possibility that full-length, uncleaved splice forms may have distinct functions. Two members of this family of splice variants contain a sixth COOH-terminal exon, designated IGF1Ea and IGF-1Eb in rodents.…”