Investigating the Hippo signaling pathway in pancreatic beta-cells

Abstract

Apoptosis and loss of function are hallmarks of pancreatic beta-cell failure in both type 1 and type 2 diabetes. Targeting beta-cell apoptosis and dysfunction therefore represents an attractive therapeutic approach to the treatment of both T1D and T2D. The initial triggers and the mechanisms of beta-cell death are complex and not fully understood. The Hippo pathway plays a key role in organ size and development through the regulation of proliferation, apoptosis and differentiation. In the present thesis, I investigated the role of Hippo signaling pathway components including NF2, YAP and LATS2 in pancreatic beta-cells in normal physiological as well as in diabetic state. NF2 is an upstream regulator of the Hippo signaling pathway. I showed that NF2 was expressed in both INS-1E cells and primary human islets. Loss of NF2 in pancreatic beta-cells could rescue beta-cell apoptosis through inhibition of LATS2 activity without compromising beta-cell function as well as beta-cell functional identity genes. Transcriptional co-activator YAP is a terminal effector of the Hippo signaling pathway. YAP is not expressed in primary adult beta-cells. This could be the reason for the almost non-existing proliferation capacity of human beta-cells. Re-expression of the constitutively active form of YAP promoted human beta-cell proliferation by regulating transcription factor forkhead box M1 (FOXM1) without altering beta-cell function and functional identity genes. Also, YAP re-expression protected beta-cells and isolated human islets from apoptosis under diabetogenic conditions. My data showed that YAP overexpression induced small redox proteins thioredoxin-1 and thioredoxin-2 (Trx1/2) at both mRNA and protein levels in both INS-1E cells and human islets and Trx1/2 was required for the anti-apoptotic function of YAP. Together, exogenously introduced YAP functions as pro-proliferative and anti-apoptotic molecule in pancreatic beta-cells. Mechanistic target of rapamycin complex 1 (mTORC1) is a master regulator of nutritional status at the cellular and organismic level. While mTORC1 mediates beta cell growth and expansion, its hyper-activation has been observed in pancreatic islets from animal models of type 2 diabetes and leads to beta cell loss. My data showed that mTORC1 activity was highly increased in type 2 diabetic islets and in human islets exposed to increased glucose concentration, while mTORC2 signaling was diminished. Inhibition of mTORC1 by S6K1 selective inhibitor improved glucose-induced insulin secretion and restored mTORC2 activity in type 2 diabetic islets as well as in isolated diabetic islets from high-fat diet treated mice. This suggests elevated mTORC1 activation as striking pathogenic hallmark of type 2 diabetic islets contributing to impaired beta-cell function and survival in the presence of metabolic stress. Large-tumor suppressor 2 (LATS2) is a core component of the Hippo signaling pathway and an endogenous upstream regulator of YAP. My data showed that overexpression of LATS2 itself was sufficient to induce pancreatic beta-cell apoptosis and impair beta-cell function. Notably, LATS2 induced beta-cell apoptosis through activated mechanistic target of rapamycin complex 1 (mTORC1) by suppression of AMP-activated protein kinase (AMPK) signaling. In addition, while LATS2 overexpression was able to further potentiate chemically-induced defective autophagy and subsequent beta-cell apoptosis, its silencing rescued beta-cell apoptosis. Loss of LATS2 in isolated human islets and beta-cells resulted in resistance to apoptosis induced by diabetogenic conditions in vitro and improved glycemia and insulin secretion in the multiple-low dose streptozotocin (MLD-STZ) mouse model in vivo. My data suggest that LATS2 acts as a pro-apoptotic molecule in pancreatic beta-cells and its inhibition could be an important strategy to improve beta-cell survival in diabetes. Taken together, my data highlight the importance of expression and activation of Hippo signaling elements in proliferation, survival and insulin secretion of pancreatic beta-cells. My results suggest that understanding the Hippo signaling pathway in pancreatic beta-cell physiology and pathology would offer a new sight to prevent beta-cell failure in diabetes.