Intracellular and peptide interactions of C-peptide

Proinsulin C-peptide is depleted together with insulin in type I diabetic patients. The supplement of insulin to these patients is necessary for their survival, but it is also likely that the loss of C-peptide may contribute to both short- and long-term complications. In this thesis, molecular effects of C-peptide have been investigated. In particular, nuclear effects of C-peptide and C-peptide involved in oligomerization have been studied, as well as method development to facilitate further protein interaction analysis.

C-peptide is a small peptide hormone with known membrane-binding properties that is thought to stimulate G-protein coupled receptor associated pathways. In this thesis we report that C-peptide not only acts extracellularly, but that it is internalized via specific mechanims and interacts with cytoskeletal proteins. We show that C-peptide is transferred to the nucleus, and specifically to the rRNA-synthesizing organelle nucleolus where it stimulates transcription of rDNA.

Transcription of rDNA is related to a complex of proteins at the promoter region including histone 4 that gets acetylated upon interaction with C-peptide. We further link this transcriptional activity of C-peptide to proliferation in a model system relevant to the bone growth retardation observed in type I diabetic patients suffering from fractures. To understand more of C-peptide’s effects on the basis of the transcriptional activity observed, a genome analysis of proximal tubular cells isolated from type I diabetic rats was performed. It revealed that C-peptide within 2 hrs exerts tight effects on transcription with ~500 genes affected and the majority of them being repressed. This observation suggests that C-peptide treatment corrects malfunctioning pathways, especially pathways of circulatory and inflammatory diseases.We have also studied oligomerization of C-peptide, and find that C-peptide oligomers are disrupted by insulin in addition to a previous study reporting that C-peptide disrupts insulin hexamers. The C-peptide oligomers are formed via electrostatic interactions, and can further lead to aggregates with a high content of ß-sheets.

In summary, this thesis provides data on C-peptide being an intracrine hormone with intracellular effects in addition to having extracellular activity via classical endocrinological pathways. We also discuss the implications of the C-peptide oligomers we observe, which provide evidence that C-peptide may act as an insulin chaperone. It is evident that a fine-balanced homestasis of C-peptide is necessary for optimal health in both type I and II diabetic patients.

List of scientific papers

I. Lindahl E, Nyman U, Melles E, Sigmundsson K, Ståhlberg M, Wahren J, Öbrink B, Shafqat J, Joseph B, Jörnvall H. (2007) Cellular internalization of proinsulin C-peptide. Cell Mol Life Sci. 64(4):479-86.
https://doi.org/10.1007/s00018-007-6467-6

II. Lindahl E, Nyman U, Zaman F, Palmberg C, Cascante A, Shafqat J, Takigawa M, Sävendahl L, Jörnvall H, Joseph B. (2010) Proinsulin C-peptide regulates ribosomal RNA expression. J Biol Chem. 285(5):3462-9.
https://doi.org/10.1074/jbc.M109.053587

III. Lindahl E, Nordquist L, Müller P, Agha E, Friederich M, Dahlman-Wright K, Palm F, Jörnvall H. Early transcriptional regulation by C-peptide in freshly isolated rat proximal tubular cells. [Submitted]

IV. Jägerbrink T, Lindahl E, Shafqat J, Jörnvall H. (2009) Proinsulin C-peptide interaction with protein tyrosine phosphatase 1B demonstrated with a labeling reaction. Biochem Biophys Res Commun. 387(1):31-5.
https://doi.org/10.1016/j.bbrc.2009.06.074

V. Jörnvall H, Lindahl E, Astorga-Wells J, Lind J, Holmlund A, Melles E, Alvelius G, Nerelius C, Mäler L, Johansson J. (2010) Oligomerization and insulin interactions of proinsulin C-peptide: Threefold relationships to properties of insulin. Biochem Biophys Res Commun. 391(3):1561-6.
https://doi.org/10.1016/j.bbrc.2009.12.125

VI. Lind J, Lindahl E, Perálvarez-Marín A, Holmlund A, Jörnvall H, Mäler L. (2010) Structural features of proinsulin C-peptide oligomeric and amyloid states. FEBS J. 277(18):3759-3768.
https://doi.org/10.1111/j.1742-4658.2010.07777.x