[Show abstract][Hide abstract] ABSTRACT: Phenotypic analyses of mice null for the individual Akt isoforms suggested that they are functionally distinct and that only Akt2 plays a role in diabetes. We show here that Akt isoforms play compensatory and complementary roles in glucose homeostasis and diabetes. Insulin resistance in Akt2(-/-) mice was inhibited by haplodeficiency of Pten, suggesting that other Akt isoforms can compensate for Akt2 function. Haplodeficiency of Akt1 in Akt2(-/-) mice, however, converts prediabetes to overt type 2 diabetes, which is also reversed by haplodeficiency of Pten. Akt3 does not appear to contribute significantly to diabetes. Overt type 2 diabetes in Akt1(+/-) Akt2(-/-) mice is manifested by hyperglycemia due to beta-cell dysfunction combined with impaired glucose homeostasis due to markedly decreased leptin levels. Restoring leptin levels was sufficient to restore normal blood glucose and insulin levels in Akt1(+/-) Akt2(-/-) and Akt2(-/-) mice, suggesting that leptin-deficiency is the predominant cause of diabetes in these mice. These results uncover a new mechanism linking Akt to diabetes, provide a therapeutic strategy, and show that diabetes induced as a consequence of cancer therapy, via Akt inhibition, could be reversed by leptin therapy.
[Show abstract][Hide abstract] ABSTRACT: Blood can provide a valuable source for the generation of stem cells. Herein we identified a novel cell population from adult human blood, designated peripheral blood insulin-producing cells (PB-IPC). Phenotypic analysis demonstrated that PB-IPC displayed the embryonic stem (ES) cell-associated transcription factors including Oct-4 and Nanog, along with the hematopoietic markers CD9, CD45, and CD117; but lacked expression of the hematopoietic stem cell marker CD34 as well as lymphocyte and monocyte/macrophage markers. Notably, in vitro and in vivo characterization revealed that PB-IPC demonstrated characteristics of islet beta cell progenitors including the expression of beta cell-specific insulin gene transcription factors and prohormone convertases, production of insulin, formation of insulin granules, and the ability to reduce hyperglycemia and migrate into pancreatic islets after transplantation into the diabetic mice. These findings may open up new avenues for autologous blood stem cell-based therapies for diabetes.
Preview · Article · Sep 2007 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: The increasing incidence of diabetes and the need to further understand its cellular basis has resulted in the development of new diagnostic and therapeutic techniques. Nonetheless, the quest to noninvasively ascertain beta-cell mass and function has not been achieved. Manganese (Mn)-enhanced MRI is presented here as a tool to image beta-cell functionality in cell culture and isolated islets. Similar to calcium, extracellular Mn was taken up by glucose-activated beta-cells resulting in 200% increase in MRI contrast enhancement, versus nonactivated cells. Similarly, glucose-activated islets showed an increase in MRI contrast up to 45%. Although glucose-stimulated Ca influx was depressed in the presence of 100 microM Mn, no significant effect was seen at lower Mn concentrations. Moreover, islets exposed to Mn showed normal glucose sensitivity and insulin secretion. These results demonstrate a link between image contrast enhancement and beta-cell activation in vitro, and provide the basis for future noninvasive in vivo imaging of islet functionality and beta-cell mass.
No preview · Article · Feb 2006 · Cell Transplantation
[Show abstract][Hide abstract] ABSTRACT: AMP-activated protein kinase (AMPK) is an important signaling effector that couples cellular metabolism and function. The effects of AMPK activation on pancreatic beta-cell function remain unresolved. We used 5-amino-imidazole carboxamide riboside (AICAR), an activator of AMPK, to define the signaling mechanisms linking the activation of AMPK with insulin secretion. Application of 300 microM AICAR to mouse islets incubated in 5-14 mM glucose significantly increased AMPK activity and potentiated insulin secretion. AICAR inhibited ATP-sensitive K(+) (K(ATP)) channels and increased the frequency of glucose-induced calcium oscillations in islets incubated in 8-14 mM glucose. At lower glucose concentration (5mM) AICAR did not affect K(ATP) activity or intracellular ([Ca(2+)](i)). AICAR also did not inhibit (86)Rb(+) efflux from islets isolated from Sur1(-/-) mice that lack K(ATP) channels yet significantly potentiated glucose stimulated insulin secretion. Our data suggest that AICAR stimulates insulin secretion by both K(ATP) channel-dependent and -independent pathways.
No preview · Article · Jun 2005 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: Glucose-stimulated insulin secretion is associated with transients of intracellular Ca(2+) concentration [Ca(2+)](i) in the pancreatic beta-cell. We identified the expression and function of specific small-conductance Ca(2+)-activated K(+) (SK) channel genes in insulin-secreting cells. The presence of mRNA for SK1, -2, -3, and -4 (intermediate-conductance Ca(2+)-activated K(+) 1 [IK1]) channels was demonstrated by RT-PCR in rodent islets and insulinoma cells. SK2 and -3 proteins in mouse islets were detected by immunoblot and immunocytochemistry. In the tTA-SK3 tet-off mouse, a normal amount of SK3 protein was present in islets, but it became undetectable after exposure to doxycycline (DOX), which inhibits the transcription of the tTA-SK3 gene. The SK/IK channel-blockers apamin, dequalinium, and charybdotoxin caused increases in average [Ca(2+)](i) levels and in frequency of [Ca(2+)](i) oscillations in wild-type mouse islets. In SK3-tTA tet-off mice, the addition of apamin with glucose and tetraethylammonium (TEA) caused a similar elevation in [Ca(2+)](i), which was greatly diminished after DOX suppression of SK3 expression. We conclude that SK1, -2, -3, and IK1 (SK4) are expressed in islet cells and insulin-secreting cells and are able to influence glucose-induced calcium responses, thereby regulating insulin secretion.
[Show abstract][Hide abstract] ABSTRACT: Insulin secretory dysfunction of the pancreatic β-cell in type-2 diabetes is thought to be due to defective nutrient sensing
and/or deficiencies in the mechanism of insulin exocytosis. Previous studies have indicated that the GTP-binding protein,
Rab3A, plays a mechanistic role in insulin exocytosis. Here, we report that Rab3A−/− mice develop fasting hyperglycemia and upon a glucose challenge show significant glucose intolerance coupled to ablated first-phase
insulin release and consequential insufficient insulin secretion in vivo, without insulin resistance. Thein vivo insulin secretory response to arginine was similar in Rab3A−/− mice as Rab3A+/+ control animals, indicating a phenotype reminiscent of insulin secretory dysfunction found in type-2 diabetes. However, when
a second arginine dose was given 10 min after, there was a negligible insulin secretory response in Rab3A−/− mice, compared with that in Rab3A+/+ animals, that was markedly increased above that to the first arginine stimulus. There was no difference in β-cell mass or
insulin production between Rab3A−/− and Rab3A+/+ mice. However, in isolated islets, secretagogue-induced insulin release (by glucose, GLP-1, glyburide, or fatty acid) was
∼60–70% lower in Rab3A−/− islets compared with Rab3A+/+ controls. Nonetheless, there was a similar rate of glucose oxidation and glucose-induced rise in cytosolic [Ca2+]i flux between Rab3A−/− and Rab3A+/+ islet β-cells, indicating the mechanistic role of Rab3A lies downstream of generating secondary signals that trigger insulin
release, at the level of secretory granule transport and/or exocytosis. Thus, Rab3A plays an important in vivo role facilitating the efficiency of insulin exocytosis, most likely at the level of replenishing the ready releasable pool
of β-granules. Also, this study indicates, for the first time, that the in vivoinsulin secretory dysfunction found in type-2 diabetes can lie solely at the level of defective insulin exocytosis.