Activin receptor-like kinase 7 induces apoptosis of pancreatic beta cells and beta cell lines.
OBJECTIVE
Activin receptor-like kinase 7 (ALK7), a member of the type I receptor serine/threonine kinases of the TGF-beta superfamily, was recently reported to regulate cell proliferation and apoptosis. We hypothesised that ALK7 may play a role in modulating pancreatic beta cell proliferation and/or apoptosis.


METHODS
We detected ALK7 expression in beta cells using RT-PCR, immunostaining and western blotting. Constitutively active, dominant negative or wild-type ALK7 was introduced into beta cells using adenoviral delivery. Proliferation was assessed using (3)H-thymidine incorporation and apoptosis was quantified using terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling detection, DNA degradation analysis and caspase-3 assays.


RESULTS
Induction of constitutively active ALK7 in beta cells resulted in growth inhibition and enhanced apoptosis; no effect was seen with INS-1 cells expressing wild-type or dominant negative ALK7. Elevated glucose concentrations and fatty acid (palmitate) markedly increased expression levels of ALK7 transcripts and proteins in INS-1 and rat islets and increased beta cell apoptosis. Activation of ALK7 increased Smad2 phosphorylation, reduced protein kinase B (Akt) kinase activity and was associated with increased levels of the bioactive forms of caspase-3, whereas co-expression of constitutively active ALK7 with dominant negative Smad2 or constitutively active Akt significantly diminished ALK7-induced growth inhibition and apoptosis in INS-1 cells. Although overexpression of constitutively active Akt significantly reduced ALK7-induced growth inhibition and ALK7-enhanced beta cell apoptosis, ALK7-stimulated Smad2 phosphorylation was not affected.


CONCLUSIONS
These results suggest that the pancreatic beta cell apoptosis induced by ALK7 activation occurs via the activation of two distinct downstream pathways: the suppression of Akt activation and the activation of the Smad2-caspase-3 cascade.
Zhang
N
N
Division of Endocrinology and Metabolism, St Michael's Hospital, 30 Bond Street, Room 7005, M5B 1W8 Toronto, ON, Canada.
Kumar
M
M
Xu
G
G
Ju
W
W
Yoon
T
T
Xu
E
E
Huang
X
X
Gaisano
H
H
Peng
C
C
Wang
Q
Q
D030201:Activin Receptors, Type I / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis / Q000235:genetics; D053148:Caspase 3 / Q000378:metabolism; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D005227:Fatty Acids / Q000494:pharmacology; D005786:Gene Expression Regulation; D005947:Glucose / Q000494:pharmacology; D050417:Insulin-Secreting Cells / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D051899:Smad2 Protein / Q000378:metabolism
D005227:Fatty Acids; D051899:Smad2 Protein; C497784:Smad2 protein, rat; D051057:Proto-Oncogene Proteins c-akt; D030201:Activin Receptors, Type I; C486044:Acvr1c protein, rat; D053148:Caspase 3; D005947:Glucose