c-peptide and Chromosome-Deletion

Expression of the human insulin gene was examined in transgenic mouse lines carrying the gene with various lengths of DNA sequences 5' to the transcription start site (+1). Expression of the transgene was demonstrated by 1) the presence of human C-peptide in urine, 2) the presence of specific transcripts in pancreas, but not in other tissues, 3) the specific immunofluorescence staining of pancreatic islets for human C-peptide, and 4) the synthesis and accumulation of human (pro)insulin in isolated islets. Deletions in the injected DNA fragment of sequences upstream from positions -353, -258, and -168 allowed correct initiation of the transcripts and cell specificity of expression, while quantitative expression gradually decreased. Deletion to -58 completely abolished the expression of the gene. The amount of human product that in mice harboring the longest fragment contributes up to 50% of the total insulin does not alter the normal proportion of mice insulins I and II. These results suggest that expression of the human insulin gene in vivo results from the cooperation of several cis-regulatory elements present in the various deleted fragments. With none of the deletions used, expression of the transgene was observed in cell types other than beta-islet cells.

Topics: Animals; C-Peptide; Chromosome Deletion; Female; Gene Expression; Humans; Immunohistochemistry; Insulin; Male; Mice; Mice, Transgenic; Pancreas; RNA, Messenger

The biological function of the connecting peptide (C-peptide) of proinsulin is unknown. Comparison of all known C-peptide sequences reveals the presence of a highly conserved peptide sequence, Glu/Asp-X-Glu/Asp (X being a hydrophobic amino acid), adjacent to the Arg-Arg doublet at the B chain/C-peptide junction. Furthermore, the next amino acid in the C-peptide sequence is also acidic in many animal species. To test the possible involvement of this hydrophilic domain in insulin biosynthesis, we constructed a mutant of the rat proinsulin II gene lacking the first four amino acids of the C-peptide and expressed either the normal (INS) on the mutated (INSDEL) genes in the AtT20 pituitary corticotroph cell line. In both cases immunoreactive insulin (IRI) was stored by the cells and released upon stimulation by cAMP. In the INS expressing cells, the majority of IRI, whether stored or released in response to a secretagogue, was mature insulin. By contrast, most of the stored and releasable IRI in the INSDEL expressing cells appeared to be (mutant) proinsulin or conversion intermediate with little detectable native insulin. Release of the mutant proinsulin and/or conversion intermediates was stimulated by cAMP. These results suggest that the mutant proinsulin was appropriately targeted to secretory granules and released predominantly via the regulated pathway, but that the C-peptide deletion prevented its conversion to native insulin.

Topics: Amino Acid Sequence; Animals; Base Sequence; C-Peptide; Cell Line; Chromosome Deletion; Genes; Humans; Information Systems; Insulin; Mice; Molecular Sequence Data; Oligonucleotide Probes; Pituitary Neoplasms; Proinsulin; Protein Processing, Post-Translational; Rats; Sequence Homology, Nucleic Acid; Transfection