LKB1 deletion with the RIP2.Cre transgene modifies pancreatic β-cell morphology and enhances insulin secretion in vivo

Abstract

The tumor suppressor liver kinase B1 (LKB1), also called STK11, is a protein kinase mutated in Peutz-Jeghers syndrome. LKB1 phosphorylates AMP-activated protein kinase (AMPK) and several related protein kinases. Whereas deletion of both catalytic isoforms of AMPK from the pancreatic β-cell and hypothalamic neurons using the rat insulin promoter (RIP2).Cre transgene (βAMPKdKO) diminishes insulin secretion in vivo, deletion of LKB1 in the β-cell with an inducible Pdx-1.CreER transgene enhances insulin secretion in mice. To determine whether the differences between these models reflect genuinely distinct roles for the two kinases in the β-cell or simply differences in the timing and site(s) of deletion, we have therefore created mice deleted for LKB1 with the RIP2.Cre transgene. In marked contrast to βAMPKdKO mice, βLKB1KO mice showed diminished food intake and weight gain, enhanced insulin secretion, unchanged insulin sensitivity, and improved glucose tolerance. In line with the phenotype of Pdx1-CreER mice, total β-cell mass and the size of individual islets and β-cells were increased and islet architecture was markedly altered in βLKB1KO islets. Signaling by mammalian target of rapamycin (mTOR) to eIF4-binding protein-1 and ribosomal S6 kinase was also enhanced. In contrast to Pdx1-CreER-mediated deletion, the expression of Glut2, glucose-induced changes in membrane potential and intracellular Ca2+ were sharply reduced in βLKB1KO mouse islets and the stimulation of insulin secretion was modestly inhibited. We conclude that LKB1 and AMPK play distinct roles in the control of insulin secretion and that the timing of LKB1 deletion, and/or its loss from extrapancreatic sites, influences the final impact on β-cell function.

Metabolism Regulates Exposure of Pancreatic Islets to Circulating Molecules In Vivo

“…To determine b-cell size, pancreas slices were stained for insulin, E-cadherin (rat, 1:500; Takara), and DAPI. The crosssectional area of E-cadherin insulin 1 cells in which a cut nuclei was present was measured using ImageJ, as previously described (28).…”

LKB1 couples glucose metabolism to insulin secretion in mice

“…However, targeting liver kinase B1 (LKB1) in the beta cell for the treatment of diabetes would be undesirable as knockout of Lkb1 in adult mice has a deleterious effect on haematopoietic stem cell maturation [20][21][22]. We and others have identified non-overlapping functions in beta cells for LKB1 targets, including MAP/microtubule affinityregulating kinase 2 (MARK2), salt-inducible kinase 2 (SIK2), brain-specific kinase 2 (BRSK2) as well as AMPactivated protein kinase (AMPK) itself [19,[23][24][25][26][27][28], underscoring the potential for identifying the minimal target(s) of LKB1 responsible for enhancing insulin secretion. Here, we systematically interrogated insulin secretion pathway components to identify the signals responsible for enhanced insulin secretion in Lkb1 knockout islets.…”

Protein Kinases and Pancreatic Islet Function

“…cell proliferation was enhanced in islets from mice in which the expression of both AMPK catalytic subunits was ablated selectively in pancreatic cells (26). In contrast, islets of Langerhans from mice in which the expression of an AMPK upstream kinase liver kinase B1 (LKB1) was ablated, had increased islet and cell size (181) and altered islet architecture (181)(182)(183)(184). Thus, the number of large islets, which may account for as much as 50 % of the total pancreatic cell mass in normal pancreata (185), was increased in pancreata from mice in which LKB1 expression was selectively ablated in the cell (181).…”

“…In contrast, islets of Langerhans from mice in which the expression of an AMPK upstream kinase liver kinase B1 (LKB1) was ablated, had increased islet and cell size (181) and altered islet architecture (181)(182)(183)(184). Thus, the number of large islets, which may account for as much as 50 % of the total pancreatic cell mass in normal pancreata (185), was increased in pancreata from mice in which LKB1 expression was selectively ablated in the cell (181). These data indicate that AMPK and LKB1 play distinct roles in the control of islet development and cell proliferation and may impact on the development of pharmacological reagents targetting these pathways for the treatment of diabetes.…”

“…mTOR is an important nutrient sensor that plays a central role in the regulation of cellular metabolism, growth, proliferation and apoptosis (187)(188)(189)(190). Signalling by (mTOR) to eukaryotic initiation factor 4-binding protein-1 (4E-BP1) and ribosomal S6 kinase (S6K) was enhanced in islets of Langerhans from mice in which LKB1 expression was specifically ablated in the cell (181). Likewise, there is evidence for crosstalk between mTOR and AMPK as a regulatory pathway that couples cellular fuel availability to cell apoptosis (58;59;191).…”