Introduction
The previous section outlines that insulin gene transcription is a highly controlled process. The product of this process, pre-proinsulin mRNA, is unusually constant in pancreatic β cells and it is further balanced as glucose concentrations raise [37]. As such, there is normally an abundant source of preproinsulin mRNA in the β-cell cytosol offered for translation. Actually, it is the personal regulation of preproinsulin mRNA translation that is the predominant regulate mechanism for insulin production in the β cell under typical circumstances. This permits the β cell to quickly replenish insulin stores spine to suitable levels, after they have actually been depleted by stimulated insulin secretion, and is a lot more economic energy-wise to the β cell, because translational regulate of insulin production bypasses the requirement for insulin gene transcription and preproinsulin mRNA maturation.
Structure of the insulin molecule
The primary structure A- and B-chain of insulin itself has actually been known for close to 50 years [88]. However, it was not until at the very least 10 years after this discovery that it was realized insulin is actually synthesized as a solitary polypeptide chain precursor molecule, preproinsulin (Figure 6.3) [88]. The N-terminal signal peptide (24 amino acids) is cleaved cotranslationally to generate proinsulin. The proinsulin molecule is a 12-kDa solitary chain polypeptide that encompasses the B-chain (30 amino acids) and the A-chain (21 amino acids) of insulin joined by the connecting peptide, C-peptide (Figure 6.3). Proinsulin to insulin conversion occurs by cleavage at two dibasic amino acids sequences by the B-chain/C-peptide and C-peptide/A-chain junctions to release the C-peptide moiety yielding the insulin molecule along with both independent disulphide-linked A- and B-chains correctly aligned [88].
Proinsulin biosynthesis: translation and translocation
Essentially, translation of preproinsulin mRNA to preproinsulin healthy protein occurs in a fashion normal of a lot of eukaryotic mRNAs destined to enter the cell’s secretory pathway [88–90]. Throughout the translation process, the emerging signal sequence of pre- proinsulin binds the signal recognition particle (SRP) that after that docks to the SRP-receptor, which is an integral ER membrane protein. This locates the preproinsulin mRNA/ribosomal translational complex to the ER, which is the significant site of proinsulin biosynthesis in the β cell. As SRP binds to the SRP receptor, the nascent signal peptide of the newly forming preproinsulin dissociates and is transferred to yet another ER integral membrane protein, the signal sequence receptor (SSR). SSR is section of a “translocation pore” that facilitates transport of the newly forming preproinsulin polypeptide across the ER membrane in to the ER lumen, marking the entrance of the newly synthesized preproinsulin in to the β-cell’s secretory pathway. The signal peptide is cleaved by yet another RER protein, the signal peptidase, resulting in the nascent proinsulin molecule located to the ER lumen. There, the proinsulin molecule undergoes right folding, assisted by the molecular chaperons and formation of disulfide bonds catalyzed by ER disulfide isomerase activity [88].
Proinsulin biosynthesis: effectors
and stimulus-response coupling mechanisms Proinsulin biosynthesis is translationally controlled by certain nutrients, neurotransmitters, and hormones, however glucose is the a lot of physiologically relevant [91]. This translational regulate response to glucose is rapid. Considerable glucose-induced proinsulin biosynthesis can easily be observed after a twenty – 30-moment lag period that reaches a max fee (∼twenty – 30-fold raise above basal) by 60 moment [91]. Of the peptide hormones that stimulate proinsulin biosynthesis, perhaps the incretins, GLP-1 and GIP, are the a lot of physiologically relevant. GLP-1/GIP do not raise proinsulin biosynthesis translation in their own right, however potentiate glucose-induced proinsulin biosynthesis as they do glucose-induced insulin secretion [92]. Epinepherine is additionally worthy of mention for specifically inhibiting glucose-induced proinsulin biosynthesis as it does glucose-induced insulin secretion [91].
Unlike that for glucose-induced insulin secretion, the secondary signals that cause an raise in proinsulin biosynthesis are much less well defined. Glucose metabolism is called for for glucose-induction of the two proinsulin biosynthesis and insulin secretion, however several lines of evidence indicate that the stimulus-response coupling mechanism for these β-cell functions are fairly distinct [91,92]. For example, sulfonylureas stimulate and diazoxides inhibit glucose-induced insulin secretion, however these compounds have actually no effect on glucose-induced proinsulin biosynthesis indicating the signal transduction mechanism for regulating proinsulin biosynthesis is independent of the KATP-channel and raise in cytosolic Ca2+, unlike that for glucose-stimulated insulin secretion [91,92]. Somatostatin is a potent inhibitor of insulin secretion, however has actually no effect on proinsulin biosynthesis [91,92]. Long-chain fatty acids, as soon as applied acutely, are marked potentiators of glucose-induced insulin secretion, however if anything modestly inhibit glucose-induced proinsulin biosynthesis [91,92]. It has actually been indicated that a secondary signal for glucose-induced proinsulin biosynthesis could be mitochondrial export of succinate and cytosolic accumulation of succinyl-CoA, however just what lies downstream of succinate/ succinyl-CoA to specifically upregulate preproinsulin mRNA translational machinery needs further investigation.
Proinsulin biosynthesis: translational regulate mechanism
Glucose modestly enhances general healthy protein synthesis in the β cell ∼1.5–2-fold. However, the effect of glucose on proinsulin synthesis translation is considerably greater, and can easily reach ≥10-fold stimulation above basal [91,92]. This indicates a specific effect of glucose on translational regulate of proinsulin biosynthesis. Such personal regulate of glucose-induced proinsulin biosynthesis in the β cell, resides in cis-elements in the 5′- and 3′-untranslated regions (UTRs) of preproinsulin mRNA itself [92]. In the 3′ -UTR of preproinsulin mRNA, simply downstream of the polyadenylation signal, there is a highly conserved primary sequence containing a UUGAA cis-element core, that has actually been reported to be involved in glucose-regulated preproinsulin mRNA stability along with the pyrimidine-rich sequence (insPRS) [37,92] (observe earlier). There is some degree of cooperativity in between the preproinsulin mRNA 5′ – and 3′ -UTRs for the specific glucose-induced translational regulate of proinsulin biosynthesis, however it appears that the 5′-UTR preproinsulin mRNA has actually the significant motivate [92]. There is additionally a conserved cis-element that is called for for glucose-induced translational regulate of proinsulin biosynthesis, named ppIGE (for preproinsulin glucose element) [92]. The ppIGE has actually a highly conserved ppIGE palindromic core of GUCxn CUG or GUUxn UUG (where n ≤ 4 bases). A cytosolic healthy protein trans-acting factor (ppIE-BP) binds to this translational regulate ppIGE cis-element of preproinsulin mRNA in a glucose-dependent manner, however the identity of the ppIGE-BP has actually yet to be revealed [92]. It ought to be noted that proinsulin is just among a small subset of β-cell proteins (∼50 in all) [92] whose biosynthesis is regulated by glucose at the translational level. These are mostly β-granule proteins, including the proinsulin processing endopeptidases, PC2 and PC1/3 [92]. Indeed, the ppIGE is additionally conserved in the 5′-UTR of the majority insulin secretory granule proteins’ mRNAs, and thus, the glucose-induced personal translational regulate of proinsulin biosynthesis and that for insulin secretory granules can easily be coordinated and is the principal regulate mechanism for insulin secretory biogenesis in β cells [92]. The glucose-induced translational regulate of the proinsulin processing endopeptidases, proPC2 and proPC3, supplies a means whereby proinsulin conversion is not compromised upon increased proinsulin biosynthesis [92].
