Insulin Synthesis and Secretion − Rakyat Biologi

Insulin represents a key hormone for regulation of blood glucose levels. Pancreatic β-cells maintain these levels by their capacity to secrete insulin upon glucose stimulation. The main source of the primary translation product of the insulin gene, i.e. preproinsulin mRNA, is the pancreatic β-cell. Transcription of the preproinsulin gene is enhanced by cAMP- and Ca²⁺-mediated activation of separate sites on the insulin promoter (Inagaki et al., 1992; Lawrence et al., 2001). After excision of the noncoding introns, the mRNA is translocated to the cytoplasm where preproinsulin is translated on membrane bound ribosomes. Glucose-mediated insulin synthesis is strongly regulated at the translational level, i.e. cytoplasmic insulin mRNA is increasingly translocated to the ribosomes at glucose concentrations >3.3 mM (Lee and Gorospe, 2010; Welsh et al., 1986). Once synthesized in the endoplasmic reticulum, preproinsulin is enzymatically modified to proinsulin through removal of its signal peptide and is subsequently transported in vesicles along the microtubule network to the cis part of the Golgi apparatus (Doyle and Egan, 2003). However, it is in the trans part of the Golgi network that proinsulin is ultimately converted to the mature hormone insulin by the prohormone-converting endopeptidases PC3 and PC2, and carboxypeptidase H. Subsequently, insulin is packaged into secretory vesicles, forming a cytosolic pool ready for regulated transport to the plasma membrane and ultimate exocytosis into the bloodstream.

The rapid pulsatile release of insulin from β-cells, is of major importance in regulating blood glucose levels. Glucose mediates insulin synthesis and secretion by entering eukaryotic cells mostly via the facilitative glucose transporters (GLUTs) (Thorens and Mueckler, 2010). GLUTs refer to a family of membrane proteins found in most mammalian cells, which allow for facilitated uptake of glucose through the cellular membrane. GLUTs are integral membrane proteins which contain 12 membrane spanning helices with both the amino and carboxyl termini exposed on the cytoplasmic side of the plasma membrane. Binding of glucose to the extracellular site of GLUTs, provokes a conformational change associated with transport and releases glucose to the other side of the membrane. The individual isotypes of GLUT exhibit different substrate specificity, kinetic characteristics, and expression profiles, thereby allowing a tissue-specific adaption of glucose uptake through regulation of their gene expression. The well-established GLUT isoforms, GLUTs 1–4, are known to have distinct regulatory and/or kinetic properties that reflect their specific roles in cellular and whole body glucose homeostasis. GLUT2 in particular, has an important role in transporting glucose into the pancreatic β-cells and hepatocytes, whereas GLUT3 is the major neuronal glucose transporter, and GLUT4 is mostly expressed in adipose tissue and skeletal muscle (Thorens and Mueckler, 2010).

Once transported into the cytoplasm, glucose is rapidly phosphorylated to glucose 6-phosphate by glucokinase, which subsequently acts as the β-cell ‘sensor’ of blood glucose and modulator of pyruvate generation for entry into the tricarboxylic acid (TCA) cycle in mitochondria. Subsequent oxidative metabolism provides the link between the products of glucose metabolism and insulin secretion. The resultant generation of cytosolic ATP at the expense of ADP, causes depolarization of the plasma membrane by closure of the ATP-sensitive K⁺ channels in a concentration-dependent manner (Gopel et al., 1999; Rorsman and Renstrom, 2003). This permits opening of voltage-dependent Ca²⁺ channels and an increase in cytosolic Ca²⁺, which then triggers fusion of insulin-containing secretory vesicles to the plasma membrane, and exocytosis of insulin follows rapidly (Ammala et al., 1993; Rorsman and Renstrom, 2003). It is thought that the time-course of glucose-induced insulin secretion is biphasic. i.e. the rate of insulin secretion rapidly accelerates before slowing down (first phase), and eventually stabilizes or progressively increases again (second phase) (Henquin, 2009). First-phase insulin secretion is triggered by the rise in cytosolic Ca²⁺ that occurs synchronously in all β-cells in response to a sudden increase in the glucose concentration. Its time course and duration are shaped by those of the Ca²⁺ signal, and its amplitude is modulated by the magnitude of the cytosolic Ca²⁺ rise. During the second phase, synchronous Ca²⁺ oscillations in all β-cells of an individual islet induce pulsatile insulin secretion.