Файл: Extracellular and Intracellular Signaling (книга).pdf

individuals.127 The integrated secretory responses of islet cells exceed those of segregated islet cell lines, thereby suggesting that interactions between the

individual cells that comprise the islet are required for normal secretory func- tion.128–131 Distortions in islet architecture have been implicated in the

development of T2DM in many animal models, including the Zucker diabetic fatty rat.132 Altered islet morphology has also been reported in maturity onset diabetes of the young (MODY). There are, however, significant di erences in cell composition and islet structure between species.133 In rodent islets the di erent cell types are clearly segregated, with b cells (about 70%) clustered at the core surrounded by a mantle of a and d cells.134 Human islets contain fewer insulincontaining b cells (rodent vs. human: 77% vs. 55%) and more glucagon-containing a cells (18% vs. 38%). In addition, the insulin-containing cells are found interspersed with glucagonand somatostatin-containing cells within the islet.135

Irrespective of the anatomical arrangement, it is emerging that the individual cells that comprise the islet may also modulate their actions via responses to their own secretions or by e ects of peptides secreted from the other cell types. Autocrine e ects of insulin on the b cell may be important within the context of islet compensation for insulin resistance.

2.9Autocrine E ects of Insulin on the Pancreatic b Cell

Experimentally, increases in compensatory insulin secretion, in part due to

increased islet mass, are observed in response to partial pancreatectomy,136 continuous glucose infusion,137 late pregnancy138–143 and increased dietary

saturated fat.106,107 Hyperplasia and hypertrophy contribute to the increase in islet mass and hypertrophy occurs through both neogenesis from ductal cells and replication of di erentiated b cells.144 An autocrine e ect of insulin has been implicated, particularly through the results of studies in mice in which

insulin resistance was engineered by ablation of either the insulin receptor (IR), or insulin receptor substrate-1 (IRS-1), or both genes.145–147 In these studies a

400-fold increase in plasma insulin levels was associated with a 40-fold increase in b-cell mass.

Insulin exerts its e ects in target cells through binding to and activating its cell surface receptors. The IR consists of two a and two b subunits that are linked together by disulfide bonds to form a heterotetrameric complex. Insulin binding to the extracellular a subunits rapidly activates the tyrosine protein kinase activity of the intracellular b subunits, resulting in autophosphorylation of the b chains. This activation of the b-subunit tyrosine kinase activity results in phosphorylation of intracellular substrates including the IRS proteins on tyrosine residues.148 This initiates a series of intracellular signaling events. IRS proteins contain multiple potential phosphorylation sites and, on tyrosine phosphorylation, they serve as multi-site docking molecules for other signaling molecules. In this way, the IRS proteins act as an interface between insulin receptors and signaling molecules further downstream.149

The finding that the insulin receptor and IRS proteins are both specifically expressed in rodent b cells150,151 has led to the prospect of a potential role of the

insulin signaling cascade in the regulation of b-cell function.151 Data from studies using isolated islets and islet cell lines suggest that insulin itself governs its own gene transcription, thereby forming a functional autocrine loop. Other insulin-

regulated processes in the pancreatic b cell have been reported to include gene transcription,152 translation,153 b-cell proliferation154,155 and finally insulin secretion itself.156–159 In accordance with this, a study determining the pattern of

gene expression in single human a, d and b cells demonstrated that the receptors and downstream elements that belong to the insulin receptor signaling pathway are expressed in human b cells at the mRNA level.160 The major argument against autocrine insulin signaling is that b cells are exposed to so much insulin that the respective signal transduction pathways must be desensitized. However, this would be predicted to be the case only in situations of chronic stimulation to secrete insulin e.g. prolonged hyperglycemia. Relative hyperinsulinemia, but at low glucose levels, in the basal (i.e. non-stimulated, post-absorptive) state, as is observed in T2DM, could exert a long-term tonic e ect on the islet itself.

Disruption of insulin signaling the level of the b-cell IR (the bIRKO mouse) does not cause a change in b-cell mass prenatally,161–163 but reduces islet insulin

content in the adult state, and the bIRKO mouse shows an age-dependent

decrease in b-cell mass, with a selective loss of GSIS and development of a T2DM-like phenotype at the age of 8 weeks.155,163 The IR exists in two isoforms

as the result of alternative splicing of the 11th exon. The A type (IR-A) lacks the 12 amino acids encoded by exon 11, whereas the B type (IR-B) contains this sequence at the C terminus of the a subunit of the IR. Pancreatic b cells express both isoforms in an almost one-to-one ratio.164 It appears that each IR isoform may have specific roles in mediating particular actions of insulin in the pancreatic b cell. Thus, it has been shown that over-expression of IR-A leads to a pro-

nounced e ect of insulin on insulin gene promoter activation, whereas overexpression of IR-B does not.165,166 By contrast, selective blocking of IR-B using

a blocking antibody raised against the 12 amino acids encoded by exon 11 abolishes insulin-stimulated upregulation of the glucokinase (GK) promoter (which would be predicted to modify islet glucose sensing), but has no e ect on the insulin-stimulated upregulation of the insulin promoter, which would allow retention of normal insulin production.165 Hence, within the context of compensatory insulin secretion, increased signaling via IR-B would be predicted to enhance islet glucose sensing and, therefore, GSIS. Of interest, a recent study has shown that chronic (48 h) exposure of insulin-secreting bTC6 cells to high glucose decreases insulin-induced tyrosine phosphorylation of the IR, in the absence of any change in expression of the IR as detected by immunoblotting.167 This may explain why poor glycemic management in DM could ultimately lead to impaired b-cell adaptation to insulin resistance.

The IRS proteins are characterized by the presence of an NH2-terminal PH domain adjacent to a PTB domain, and a variable-length COOH-terminal tail that contains several tyrosine and serine phosphorylation sites.149 These proteins lack catalytic activity of their own, but they possess a number of interaction domains

and phosphorylation motifs that enable them to function as interface molecules in signaling between activated tyrosine kinase receptors and their downstream elements.168 So far six members of this family have been identified (IRS1-6).169 Specific interactions with the IR and IGF-1 receptor kinases are mediated by the PH and PTB domains on IRS proteins, while downstream signaling proteins are recruited by the phosphorylation of tyrosine sites in the COOH terminal end.170 All four IRS proteins have been detected in pancreatic b cells.154

IRS-1 deficient islets and b-cell lines have decreased insulin content and glucoseor arginine-stimulated insulin secretion, which is partially restored by re-expression of IRS-1.154 Signaling through IRS-1 in b cells appears to maintain intracellular insulin content and synthesis,166 regulate calcium homeostasis171 and promote insulin secretion.156 However, despite whole-body insulin resistance, IRS-1 null mice do not develop diabetes because IRS-2 compensates for IRS-1 ablation by mediating b-cell growth and development and allowing compensatory insulin hypersecretion to overcome insulin resis-

tance. This is demonstrated by b-cell hypoplasia and a corresponding decrease in insulin secretion in IRS-2 null mice.155,172,173 In particular, global disruption

of IRS-2 leads to lack of compensatory b-cell hyperplasia despite the development of hepatic insulin resistance,173 and is ultimately fatal in young male mice.155 Female mice are less a ected but, nevertheless, die in middle age.155 Histological analysis shows that b-cell mass is reduced compared with the wild-type – however, although hyperplasia is defective, individual b cells show normal or increased insulin secretion in response to glucose.173

Alterations in the insulin signaling pathways that might contribute to b-cell proliferation and/or apoptosis in IRS-1 knockout mice were identified using a transplantation approach to separate host insulin resistance from islet function. The results demonstrated that the IRS-1–/– mice displayed an 80% decrease in b-cell apoptosis as compared to wild-type, and this was accompanied by a substantial increase in IRS-2 expression.146 These results indicate that the islet growth seen in IRS-1 deficient b cells is due to the compensatory increase in IRS-2 expression. Over-expression of IRS-2 in b cells was found to decrease apoptosis by more than 50% and promote b-cell proliferation.146 In addition, an increased b-cell mass is seen in food-restricted pregnant rats which express increased level of IRS-2174 and IRS2 is critical for b-cell hyperplasia in response to HF diet-induced insulin resistance.175 In this study, an animal model for human T2DM (mice with b-cell-specific GK haploinsu ciency (Gck(þ/) (–)) fed an HF diet) exhibited decreased b-cell proliferation which was reversed by over-expression of IRS2 in b cells.

During insulin stimulation, tyrosine phosphorylation sites in the IRS proteins bind specifically to SH2 domains in various downstream signaling molecules including phosphatidylinositide 3-kinase (PI3k), Grb2/son of sevenless (SOS) and SHP-2, with the Grb2/SOS complex mediating the activation of p21ras, thus triggering the ras/raf/mitogen-activated protein kinase kinase (MEK)/MAPK cascade. PI3Ks are a family of enzymes subdivided into three major classes (I, II and III) based on their structural homology, regulation and substrate specificity. PI3K class Ia is a heterodimeric enzyme that consists of an

development and function comprises one connection between poor early growth and type 2 DM. Use of a maternal low-protein diet during pregnancy and lactation (MLP model) leads to low birth-weight and a predisposition to the development of insulin resistance in later life in a rat model.103 Dahri et al.189 found a simultaneous reduction of cell proliferation, islet size, islet vascularization and insulin content in rat fetuses at E21.5, where pregnant mothers were subjected to a low-protein diet, which is consistent with either elevated fetal insulin levels (which are not observed) or abnormally high insulin signaling in the fetal pancreas during early development. These results would be consistent with altered IRS-2 expression and/or signaling in the islet itself, but this has not yet been investigated.

The relative weight of the pancreas at weaning is reduced and o spring of protein-restricted mothers show increased susceptibility to the development of insulin resistance with aging and in response to challenge to the endocrine

pancreas such as the imposition of a cafeteria190 or high-saturated-fat diet.106,107 Early protein restriction also leads to a lower pancreatic insulin

content in adulthood.191 In addition, we have identified defects in compensatory GSIS in vivo when such o spring are challenged with a diabetogenic

high-saturated-fat diet and gender-specific di erences in compensatory GSIS in response to insulin resistance.106,107,192 As islet IRS-2 deficiency impacts on

compensatory insulin secretion and is gender related, it is again possible that islet signaling via IRS-2 in adulthood is modified by early life events. Evidence that maternal protein malnutrition can permanently impact on tissue IR expression and downstream components of the insulin signaling cascade in

later life in non-islet tissues has been shown from studies of adipocytes and muscles of adult o spring of protein malnourished dams.110,193,194 In parti-

cular, adipocytes isolated from early-protein-restricted rats show increased IR levels, increased basal and insulin-stimulated levels of IRS-1-associated PI3K activities and increased Akt activities.193 In adipocytes, there are two major isoforms of the PI3K catalytic subunit (p110a and p110b). There is no change in p85 expression56 or p110a protein expression194 in adipocytes from early-protein-restricted rats compared with control, but adipocytes from early- protein-restricted rats have relatively low levels of p110b.194 The association of p85 with p110a is similar in control and low-protein adipocytes; however, there is less p110b associated with p85.110 These alterations in insulin signaling have functional implications as there are associated increases in basal and

insulin-stimulated glucose uptake by adipocytes in vitro194 and in vivo during euglycemic hyperinsulinemia in the intact animal.195,196

2.11 E ects of FA on the Pancreatic b cell

The FAs oleate (18:1), palmitate (16:0) and stearate (18:0) constitute 80% of the circulating FFA.197 Oleate is the most abundant FFA in the human circulation,197 whereas stearate is most abundant in the rodent circulation.198 Exogenous FA can trigger insulin release from pancreatic b cells at basal