A novel two-chain IGF-II-derived peptide from purified β-cell granules.
OBJECTIVE
Insulin-like growth factor II (IGF-II) is a potent mitogen that regulates prenatal growth and development in both humans and rodents. Its role in post-natal life is less clear although immunohistochemical studies have observed IGF-II-like immunoreactivity (IGF-II-LI) associated with insulin-producing pancreatic β-cells. Here we isolated secretory granules from a β-cell line, βTC6-F7, and characterized the nature of the IGF-II-LI located therein.


METHODS
Secretory granules were isolated from cultured mouse βTC6-F7 cells by ultracentrifugation. Granule protein content was separated by reversed-phase HPLC, and assayed for IGF-II (radioimmunoassay) prior to identification by gas-phase NH(2)-terminal sequencing and MALDI-TOF MS. Effects of glucose incorporation into muscle glycogen were determined by incubating with isolated rat soleus muscle strips.


RESULTS
βTC6-F7 cells contained 60 ± 8 pmol of IGF-II-LI per 10⁶ cells compared to 340 ± 44 pmol insulin-LI per 10⁶ cells. IGF-II immunoreactive fractions were found to contain an IGF-II-like molecule with a molecular mass of 6847.6 Da. The protein was found to be a two-chain insulin-like product of Igf2 that corresponds to mouse des(37-40)IGF-II, which we termed 'vesiculin'. This molecule was also detectable in βTC6-F7 cells by intact-cell mass spectrometry. Mouse vesiculin evoked concentration-dependent stimulation of muscle glycogen synthesis ex vivo with an EC(50) value of 131 nM ± 1.35.


CONCLUSIONS
Vesiculin, des(37-40)IGF-II, is a novel two-chain insulin-like hormone and the major "IGF-II-like" peptide found in purified mouse βTC6-F7 secretory granules. It stimulated ex vivo muscle glycogen synthesis with an efficacy greater than or equal to the intrinsic potency of IGF-II when compared to insulin derived from the same species.
Buchanan
Christina M
CM
Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, New Zealand. c.buchanan@auckland.ac.nz
Phillips
Anthony R J
AR
Cooper
Garth J S
GJ
D000818:Animals; D002460:Cell Line; D006003:Glycogen / Q000096:biosynthesis; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000031:analogs & derivatives* / Q000302:isolation & purification / Q000378:metabolism* / Q000494:pharmacology; D050417:Insulin-Secreting Cells / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D051379:Mice; D018482:Muscle, Skeletal / Q000378:metabolism; D010446:Peptide Fragments / Q000302:isolation & purification; D051381:Rats; D017208:Rats, Wistar; D022142:Secretory Vesicles / Q000737:chemistry / Q000378:metabolism*
D010446:Peptide Fragments; C555343:insulin-like growth factor II, des(37-40); C443100:preptin; D007335:Insulin-Like Growth Factor II; D006003:Glycogen