**8. Insulin synthesis and secretion**

Insulin production begins in the β-cells with the secretion of pre-proinsulin that is converted into proinsulin. Proinsulin is then transformed into insulin and C-peptide, which are stored in the form of secretory granules until they are triggered for release throughout the body during food ingestion. Insulin is mainly produced in response

to glucose. This has been validated *in vitro* : it was found that when human islets or stem cell-derived β-cells were stimulated with glucose, they secreted insulin [ 17 ]. Other hormones, such as melatonin, estrogen, leptin, growth hormone, glucagon-like peptide 1, etc. can modulate the level of insulin secretion.

 The primary signal that stimulates insulin exocytosis from granules is a process triggered by glucose (for instance, increased intake of dietary sugar) followed by a rise in intracellular calcium (Ca 2+ ) [ 8 , 18 ]. Calcium influx relies on many factors such as glucose transport, metabolic enzymes, and functioning potassium ion channels. Moreover, an elegant study showed that the growth and survival effects of glucose on β-cells require activation of proteins in the insulin signaling pathway via an autocrine mechanism ( **Figure 2** ) [ 19 ]. This, the fact that β-cells both secrete and *respond* to insulin via autoregulation may make they especially vulnerable to epigenetic changes induced by glucose ( **Figure 2** ), In the model proposed by Assmann *et al* . [ 19 ], we further posit that the identified targets may be exceptionally sensitive to epigenetic dysregulation in DM (in addition to transcriptional and/or translational dysregulation) ( **Figure 2** ). If these targets are epigenetically misregulated they may become difficult to normalize.

 Furthermore, the pancreatic endoplasmic reticulum kinase (ERK) plays a central role in regulating translational events. It regulates insulin translation through phosphorylation of eukaryotic initiation factor 2 alpha (eIF2a). ERK mutation is linked with permanent neonatal DM in humans [ 8 ]. One such example is Wolcott-Rallison syndrome (WRS), a rare autosomal recessive disease characterized by neonatal/earlyonset non-autoimmune insulin-requiring DM associated with skeletal dysplasia and growth retardation [ 20 ]. Because glucose is critical in activating insulin signaling, let us look at glucose in more detail.

### **Figure 2.**

 *Diagram of a link between glucose and insulin signaling in β-cells and indicating epigenetic effects (blue arrows). (A) Potential direct effects of glucose and/or its metabolites on proteins in the insulin/IGF-1 signaling pathway. (B) Potential indirect effects of glucose and direct effects of insulin following exocytosis of insulin. Akt, v-akt murine thymoma viral oncogene homolog; FoxO-1, forkhead box O1; GRB2, growth factor receptor-bound protein 2; PIP3, phosphatidylinositol (3,4,5)-trisphosphate; mTOR, mammalian target of rapamycin; 4EBP1, translation initiation factor 4e binding protein 1. We postulate that β-cells are especially sensitive to epigenetic perturbations (blue arrows) because unlike cells that do not produce insulin, β-cells also have an autocrine quality, in that they both produce insulin, and receive the insulin signal by receptor binding. This autoregulatory loop may be especially vulnerable to epigenetic dysregulation.* 
