@phdthesis{TrujilloViera2022,
  author    = {Trujillo Viera, Jonathan},
  title     = {Protein kinase D2 drives chylomicron-mediate lipid transport in the intestine and promotes obesity},
  doi       = {10.25972/OPUS-26509},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-265095},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Obesity and associated metabolic syndrome are growing concerns in modern society due to the negative consequences for human health and well-being. Cardiovascular diseases and type 2 diabetes are only some of the pathologies associated to overweight. Among the main causes are decreased physical activity and food availability and composition. Diets with high content of fat are energy-dense and their overconsumption leads to an energy imbalance, which ultimately promotes energy storage as fat and obesity. Aberrant activation of signalling cascades and hormonal imbalances are characteristic of this disease and members of the Protein Kinase D (PKD) family have been found to be involved in several mechanisms mediating metabolic homeostasis. Therefore, we aimed to investigate the role of Protein Kinase D2 (PKD2) in the regulation of metabolism. Our investigation initiated with a mice model for global PKD2 inactivation, which allowed us to prove a direct involvement of this kinase in lipids homeostasis and obesity. Inactivation of PKD2 protected the mice from high-fat diet-induced obesity and improved their response to glucose, insulin and lipids. Furthermore, the results indicated that, even though there were no changes in energy intake or expenditure, inactivation of PKD2 limited the absorption of fat from the intestine and promoted energy excretion in feces. These results were verified in a mice model for specific deletion of intestinal PKD2. These mice not only displayed an improved metabolic fitness but also a healthier gut microbiome profile. In addition, we made use of a small-molecule inhibitor of PKD in order to prove that local inhibition of PKD2 in the intestine was sufficient to inhibit lipid absorption. The usage of the inhibitor not only protected the mice from obesity but also was efficient in avoiding additional body-weight gain after obesity was pre-established in mice. Mechanistically, we determined that PKD2 regulates lipids uptake in enterocytes by phosphorylation of Apolipoprotein A4 (APOA4) and regulation of chylomicron-mediated triglyceride absorption. PKD2 deletion or inactivation increased abundance of APOA4 and decreased the size of chylomicrons and therefore lipids absorption from the diet. Moreover, intestinal activation of PKD2 in human obese patients correlated with higher levels of triglycerides in circulation and a detrimental blood profile. In conclusion, we demonstrated that PKD2 is a key regulator of dietary fat absorption in murine and human context, and its inhibition might contribute to the treatment of obesity.},
  subject      = {Chylomicrons},
  language  = {en}
}
@phdthesis{Mayer2021,
  author    = {Mayer, Alexander E.},
  title     = {Protein kinase D3 signaling in the regulation of liver metabolism},
  doi       = {10.25972/OPUS-20797},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-207978},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2021},
  abstract  = {The liver plays a pivotal role in maintaining energy homeostasis. Hepatic carbohydrate and lipid metabolism are tightly regulated in order to adapt quickly to changes in nutrient availability. Postprandially, the liver lowers the blood glucose levels and stores nutrients in form of glycogen and triglycerides (TG). In contrast, upon fasting, the liver provides glucose, TG, and ketone bodies. However, obesity resulting from a discrepancy in food intake and energy expenditure leads to abnormal fat accumulation in the liver, which is associated with the development of hepatic insulin resistance, non-alcoholic fatty liver disease, and diabetes. In this context, hepatic insulin resistance is directly linked to the accumulation of diacylglycerol (DAG) in the liver. Besides being an intermediate product of TG synthesis, DAG serves as second messenger in response to G-protein coupled receptor signaling. Protein kinase D (PKD) family members are DAG effectors that integrate multiple metabolic inputs. However, the impact of PKD signaling on liver physiology has not been studied so far. In this thesis, PKD3 was identified as the predominantly expressed isoform in liver. Stimulation of primary hepatocytes with DAG as well as high-fat diet (HFD) feeding of mice led to an activation of PKD3, indicating its relevance during obesity. HFD-fed mice lacking PKD3 specifically in hepatocytes displayed significantly improved glucose tolerance and insulin sensitivity. However, at the same time, hepatic deletion of PKD3 in mice resulted in elevated liver weight as a consequence of increased hepatic lipid accumulation. Lack of PKD3 in hepatocytes promoted sterol regulatory element-binding protein (SREBP)-mediated de novo lipogenesis in vitro and in vivo, and thus increased hepatic triglyceride and cholesterol content. Furthermore, PKD3 suppressed the activation of SREBP by impairing the activity of the insulin effectors protein kinase B (AKT) and mechanistic target of rapamycin complexes (mTORC) 1 and 2. In contrast, liver-specific overexpression of constitutive active PKD3 promoted glucose intolerance and insulin resistance. Taken together, lack of PKD3 improves hepatic insulin sensitivity but promotes hepatic lipid accumulation. For this reason, manipulating PKD3 signaling might be a valid strategy to improve hepatic lipid content or insulin sensitivity. However, the exact molecular mechanism by which PKD3 regulates hepatocytes metabolism remains unclear. Unbiased proteomic approaches were performed in order to identify PKD3 phosphorylation targets. In this process, numerous potential targets of PKD3 were detected, which are implicated in different aspects of cellular metabolism. Among other hits, phenylalanine hydroxylase (PAH) was identified as a target of PKD3 in hepatocytes. PAH is the enzyme that is responsible for the conversion of phenylalanine to tyrosine. In fact, manipulation of PKD3 activity using genetic tools confirmed that PKD3 promotes PAH-dependent conversion of phenylalanine to tyrosine. Therefore, the data in this thesis suggests that PKD3 coordinates lipid and amino acid metabolism in the liver and contributes to the development of hepatic dysfunction.},
  subject      = {Metabolismus},
  language  = {en}
}
@phdthesis{Loeffler2019,
  author    = {L{\"o}ffler, Mona Christina},
  title     = {Protein kinase D1 deletion in adipocytes enhances energy dissipation and protects against adiposity},
  doi       = {10.25972/OPUS-18859},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-188593},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Adaptation to alterations in nutrient availability ensures the survival of organisms. In vertebrates, adipocytes play a decisive role in this process due to their ability to store large amounts of excess nutrients and release them in times of food deprivation. In todays western world, a rather unlimited excess of nutrients leads to high-caloric food consumption in humans. Nutrient overload together with a decreased energy dissipation result in obesity as well as associated diseases such as insulin resistance, diabetes, and liver steatosis. Obesity causes a hormonal imbalance, which in combination with altered nutrient levels can aberrantly activate G-protein coupled receptors utilizing diacylglycerol (DAG) as secondary messenger. Protein kinase D (PKD) 1 is a DAG effector integrating multiple hormonal and nutritional inputs. Nevertheless, its physiological role in adipocytes has not been investigated so far. In this thesis, evidence is provided that the deletion of PKD1 in adipocytes suppresses lipogenesis as well as the accumulation of triglycerides. Furthermore, PKD1 depletion results in increased mitochondrial biogenesis as well as decoupling activity. Moreover, PKD1 deletion promotes the expression of the β3-adrenergic receptor (ADRB3) in a CCAAT/enhancer-binding protein (C/EBP)-α and δ-dependent manner. This results in elevated expression levels of beige markers in adipocytes in the presence of a β-agonist. Contrarily, adipocytes expressing a constitutive active form of PKD1 present a reversed phenotype. Additionally, PKD1 regulates adipocyte metabolism in an AMP-activated protein kinase (AMPK)-dependent manner by suppressing its activity through phosphorylation of AMPK α1/α2 subunits. Thus, PKD1 deletion results in an enhanced activity of the AMPK complex. Consistent with the in vitro findings, mice lacking PKD1 in adipocytes demonstrate a resistance to high-fat diet-induced obesity due to an elevated energy expenditure caused by trans-differentiation of white into beige adipocytes. Moreover, deletion of PKD1 in murine adipocytes improves systemic insulin sensitivity and ameliorates liver steatosis. Finally, PKD1 levels positively correlate with HOMA-IR as well as insulin levels in human subjects. Furthermore, inhibition of PKD1 in human adipocytes leads to metabolic alterations, which are comparable to the alterations seen in their murine counterparts. Taken together, these data demonstrate that PKD1 suppresses energy dissipation, drives lipogenesis, and adiposity. Therefore, increased energy dissipation induced by several complementary mechanisms upon PKD1 deletion might represent an attractive strategy to treat obesity and its related complications.},
  subject      = {Proteinkinase D},
  language  = {en}
}