Activin receptor-like kinase 7 induces apoptosis of pancreatic beta cells and beta cell lines

Activin receptor-like kinase 7 (ALK7), a member of the type I receptor serine/threonine kinases of the TGF-β 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.

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 ³H-thymidine incorporation and apoptosis was quantified using terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling detection, DNA degradation analysis and caspase-3 assays.

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.

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.