Veli, Önay ORCID: 0009-0001-4748-8950
(2025).
THE REGULATION OF TNF SIGNALING IN METABOLISM-RELATED DISEASES.
PhD thesis, Universität zu Köln.
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Abstract
Tumor necrosis factor (TNF) is a proinflammatory cytokine with a well-established role in diabetes pathogenesis. In addition to driving inflammation, TNF can induce cell death; yet the contribution of TNF-induced cell death to β-cell loss in diabetes remains unclear. Receptor- interacting protein kinase 1 (RIPK1) is a key regulator of TNF-induced cell death. The kinase function of RIPK1 is essential for TNF-induced necroptosis and under certain circumstances, apoptosis. In contrast, the scaffolding function of RIPK1 is essential to limit cell death. We studied the role of TNF-induced cell death in the pathogenesis of diabetes by dissecting the kinase versus scaffold function of RIPK1. To study the role of this important checkpoint in TNF signaling in β-cells, we used two models that induce hyperglycemia by either β-cell toxicity or insulin resistance. These are the Multiple Low Dose Streptozotocin (MLDSTZ) and the high-fat diet models, respectively. Using Ripk1D138N mice, which express a kinase-inactive version of RIPK1, we demonstrated that RIPK1 kinase activity does not drive the pathogenesis of type 1 diabetes (T1D), at least in the Multiple Low Dose Streptozotocin (MLDSTZ) model. We then generated Ripk1β-KO mice, which lack RIPK1 specifically in pancreatic β-cells. Notably, these mice are viable and have normal glycemic control over their lifespan. Moreover, Ripk1β-KO mice were not sensitized to MLDSTZ or high-fat diet (HFD). Hence, although RIPK1 is vital for the survival of many cell types, including keratinocytes and immune cells, its deletion in β-cells did not result in any spontaneous phenotype. Islets isolated from Ripk1β-KO mice were resistant to TNF-induced cell death and were not sensitized to cytokine cocktail-induced cell death. While RIPK1 is crucial for the survival of many cell types, its deletion in β-cells did not sensitize them to cell death. Interestingly, we found that β-cells express high levels of cFLIP relative to Caspase-8, likely conferring resistance to TNF-induced apoptosis. Additionally, β-cells exhibited low RIPK3 expression, explaining their resistance to necroptosis. These findings suggest that β-cells fine-tune the expression of prosurvival and pro-death proteins to limit TNF-induced cell death, thereby preserving β-cell mass and maintaining normoglycemia. In a second project, we investigated the role of LUBAC, another important checkpoint of TNF signaling in glycogen metabolism. LUBAC, an E3 ubiquitin ligase, acts downstream of various immune and cytokine receptors and is crucial for limiting TNF-induced cell death. In humans, LUBAC deficiency leads to autoinflammation and immunodeficiency. Unexpectedly, many patients also develop cardiomyopathy and myopathy due to toxic polyglucosan (a less- branched form of glycogen) accumulation in cardiac and skeletal muscles. 9 To explore LUBAC's role in glycogen metabolism, we generated HoipMuscle-KO mice, lacking HOIP, the main catalytic component of LUBAC, in cardiac and skeletal muscle. These mice were viable and displayed no overt phenotypes. However, histological analysis revealed polyglucosan accumulation in the heart, reinforcing the hypothesis that LUBAC regulates glycogen metabolism through an unknown mechanism. Our preliminary results suggest that LUBAC may regulate autophagy, as ATG16L1, a key scaffold protein for LC3 lipidation, was significantly downregulated in HoipMuscle-KO hearts. The precise mechanism by which LUBAC controls glycogen metabolism warrants further investigation. In summary, my work revealed that β-cells regulate TNF-mediated cell death pathways in an alternative way, independent of classical checkpoints. While elevated cFLIP levels may play a key role in this resistance, other molecular mechanisms are likely involved and warrant further investigation. Additionally, LUBAC, a critical checkpoint in TNF signaling, appears to regulate glycogen metabolism, potentially by facilitating autophagy-mediated clearance of polyglucosan bodies.
Item Type: | Thesis (PhD thesis) | ||||||||||||
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URN: | urn:nbn:de:hbz:38-783216 | ||||||||||||
Date: | 26 May 2025 | ||||||||||||
Language: | English | ||||||||||||
Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||||||
Divisions: | CECAD - Cluster of Excellence Cellular Stress Responses in Aging-Associated Diseases | ||||||||||||
Subjects: | Natural sciences and mathematics Life sciences |
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Date of oral exam: | 4 December 2024 | ||||||||||||
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Refereed: | Yes | ||||||||||||
URI: | http://kups.ub.uni-koeln.de/id/eprint/78321 |
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