Overexpression of Bcl-x(L) in beta-cells prevents cell death but impairs mitochondrial signal for insulin secretion.
To study effects of Bcl-x(L) in the pancreatic beta-cell, two transgenic lines were produced using different forms of the rat insulin promoter. Bcl-x(L) expression in beta-cells was increased 2- to 3-fold in founder (Fd) 1 and over 10-fold in Fd 2 compared with littermate controls. After exposure to thapsigargin (10 microM for 48 h), losses of cell viability in islets of Fd 1 and Fd 2 Bcl-x(L) transgenic mice were significantly lower than in islets of wild-type mice. Unexpectedly, severe glucose intolerance was observed in Fd 2 but not Fd 1 Bcl-x(L) mice. Pancreatic insulin content and islet morphology were not different from control in either transgenic line. However, Fd 2 Bcl-x(L) islets had impaired insulin secretory and intracellular free Ca(2+) ([Ca(2+)](i)) responses to glucose and KCl. Furthermore, insulin and [Ca(2+)](i) responses to pyruvate methyl ester (PME) were similarly reduced as glucose in Fd 2 Bcl-x(L) islets. Consistent with a mitochondrial defect, glucose oxidation, but not glycolysis, was significantly lower in Fd 2 Bcl-x(L) islets than in wild-type islets. Glucose-, PME-, and alpha-ketoisocaproate-induced hyperpolarization of mitochondrial membrane potential, NAD(P)H, and ATP production were also significantly reduced in Fd 2 Bcl-x(L) islets. Thus, although Bcl-x(L) promotes beta-cell survival, high levels of expression of Bcl-x(L) result in reduced glucose-induced insulin secretion and hyperglycemia due to a defect in mitochondrial nutrient metabolism and signaling for insulin secretion.
Zhou
Y P
YP
Department of Medicine, Section of Endocrinology, University of Chicago, Chicago, Illinois 60637, USA.
Pena
J C
JC
Roe
M W
MW
Mittal
A
A
Levisetti
M
M
Baldwin
A C
AC
Pugh
W
W
Ostrega
D
D
Ahmed
N
N
Bindokas
V P
VP
Philipson
L H
LH
Hanahan
D
D
Thompson
C B
CB
Polonsky
K S
KS
D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D001786:Blood Glucose / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D053938:DNA Fragmentation / Q000187:drug effects; D015870:Gene Expression; D005947:Glucose / Q000494:pharmacology; D005951:Glucose Tolerance Test; D007150:Immunohistochemistry; D007328:Insulin / Q000032:analysis / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D008822:Mice, Transgenic; D008928:Mitochondria / Q000502:physiology*; D011189:Potassium Chloride / Q000494:pharmacology; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000032:analysis / Q000235:genetics*; D051381:Rats; D019284:Thapsigargin / Q000494:pharmacology; D051020:bcl-X Protein
C494812:Bcl2l1 protein, mouse; C494813:Bcl2l1 protein, rat; D001786:Blood Glucose; D007328:Insulin; D019253:Proto-Oncogene Proteins c-bcl-2; D051020:bcl-X Protein; D011189:Potassium Chloride; D019284:Thapsigargin; D005947:Glucose; D002118:Calcium