Targeting beta-cell apoptosis in diabetes: The role of mammalian Sterile Kinase 1 (MST1)

Abstract

Pancreatic beta-cell death is the fundamental cause of type 1 and type 2 diabetes (T1/T2D). Loss of function and survival signals and the activation of pro-apoptotic mediators are characteristic for diabetic β-cells. Strategies to prevent beta-cell apoptosis and restore beta-cell survival are currently unavailable; thus in urgent need and critical for an effective treatment of both T1/T2D. The mechanisms of beta-cell death in a diabetic milieu are complex and not well defined; multiple triggering factors have been identified, which initiate a variety of signaling cascades in beta-cells that affect the expression of apoptotic genes and the subsequent beta-cell failure. The knowledge of the common key regulator of beta-cell apoptosis offers novel therapeutic targets for the treatment of diabetes. Mammalian sterile 20-like kinase 1 (MST1) is a serine threonine kinase, which mediates apoptosis in response to cytotoxic stress. MST1 is both, cleaved and activated by caspases, and also serves as an activator of caspases to amplify the apoptotic signaling pathways. In search for a common pro-apoptotic pathway, I investigated whether MST1 triggers beta-cell death in diabetes. In the present thesis, I explored the possible patho-physiological activation of MST1 in beta-cells under diabetic conditions and its downstream signaling, which may be a major common pathway of beta-cell death in diabetes. My data establish MST1 as a master regulator of apoptotic beta-cell death. I show that MST1 was strongly activated in beta-cells under diabetogenic conditions in vitro and in vivo. MST1 cleavage&phosphorylation was increased in human and mouse primary islets and in the beta-cell line INS-1E cells when exposed to a complex diabetic milieu. This correlated with the activation of known MST1 targets (H2B, JNK), increased beta- cell apoptosis and impaired insulin secretion. Notably, MST1 activation and beta-cell apoptosis were profoundly increased in diabetic islets from T2D patients, obese diabetic Leprdb/db mice as well as hyperglycemic high fat/ high sucrose fed mice. My data suggest a potential crosstalk between MST1 and pro-survival PI3K/AKT signaling pathways. MST1 and AKT negatively regulated each other and constitute a stress-sensitive survival pathway. Under acute stress conditions, AKT promoted cell survival by inhibiting MST1, but prolonged stress decreased AKT, which allowed pro-apoptotic MST1 signaling. Overexpression of MST1 itself increased beta-cell apoptosis and impaired function indicating 5 that MST1 alone is sufficient to promote beta-cell failure. MST1 overexpression decreased antiapoptotic PDX1, Bcl-2 and Bcl-xL and increased proapoptotic Bax, Bim and cytochrome c release and activation of caspase-9 and -3 indicating activation of the mitochondrial (intrinsic) pathway of cell death. The beta-cell transcription factor pancreatic duodenal homeobox-1 (PDX1) was identified as a novel MST1 substrate. MST1 overexpression in beta-cells strongly decreased PDX1 without changes in PDX1 mRNA levels; this demonstrates that the decrease in PDX1 expression was regulated at the post-transcriptional level and related to its reduced stability. MST1 directly phosphorylated PDX1 at Thr11, resulting in its ubiquitination and degradation and subsequent reduction in PDX1 target genes and loss of glucose-stimulated insulin secretion. Amazingly, PDX1-phosho-deficient mutant (T11-PDX1) restored PDX1 function and insulin secretion. This suggests (1) that MST1-induced PDX1 phosphorylation at T11 leads directly to PDX1 de-stabilization and impaired beta-cell function and (2) that PDX1 is a crucial target of MST1 in the regulation of beta-cell function. MST1 deficiency restored beta-cell function and survival in human and rodent cells. MST1-knock down in β-cells protected them from cell death induced by multiple diabetic stimuli. Importantly, detailed in vivo studies show that MST1 deficient mice did not develop diabetes. Together, my work shows that MST1 acts as an essential apoptotic molecule in the presence of diabetic stimuli and is a common component in the diverse signaling pathways leading to β-cell apoptosis. My results suggest that MST1 is a critical mediator of impaired beta-cell function and apoptosis. Inhibiting the MST1-pathway could be an important strategy to prevent beta-cell apoptosis.