Regulation of phosphoinositide 3-kinase gamma in inflammation and metabolism

Manipulation of Phosphoinositide 3-kinase gamma (PI3Kg) attenuates inflammation, and ameliorates symptoms and progression of various inflammatory diseases. Activity of PI3Kg in leukocytes is determined by two regulatory proteins termed p84 and p101, but their detailed roles have only been rudimentarily studied yet. In the first study we investigated allergic inflammation characterized by IgE-mediated mast cell activation and the potential of Ras inhibitors to specifically attenuate p84-dependent PI3Kg activation. Pharmacologic Ras targeting using farnesyl-transferase inhibitors (FTIs), geranyl-geranyl-transferase inhibitors (GGTIs) and statins (inhibitors of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase) abolished p84-dependent mast cell outputs while p101-dependent macrophage activation remained unaffected. Therefore Ras inactivation could effectively inhibit mast cell -mediated allergic response without constraining monocyte and macrophage -dependent immunity in a cell culture-based model. However, current Ras inhibitors interfere with post-translational modification of many proteins and have pleiotropic effects on numerous critical signaling pathways, which disqualifies them as potential low side effect anti-allergic treatments.
Complementary to pharmacological inhibition, genetically modified mice devoid of p84 or p101 are powerful model systems to study the function of these signaling molecules in primary tissue, in bone marrow derived cultured cells and in live animal disease models. In the second study, a high fat diet (HFD) -dependent obesity model was used to provoke low-grade chronic inflammation leading to weight gain, hyperglycemia and hyperinsulinemia in p84-/- and p101-/- transgenic mice. p84 and p101- deficiency displayed distinct phenotypes from each other and from mice lacking p110g catalytical subunit of PI3Kg. p84 negatively regulates protein kinase A (PKA)-dependent hormone-sensitive lipase (HSL) activation but without net effect on body weight or fat mass. Despite adiposity p84-/- animals are protected from hyperinsulinemia and liver steatosis, proposing a change in insulin effects and/or in fatty acid flux from adipose compartment to liver. p101 null mice on HFD also become obese, yet they show improved fasting glucose and glycated hemoglobin (HbA1c) levels. p101 is responsible for chemokine-mediated signaling in myeloid leukocytes including macrophages. p101-dependent migration and activation of lymphocyte populations in white adipose tissue (WAT), and possibly other organs such as pancreas or liver, increase systemic glycemia, and accelerate the progression of adiposity-related type II diabetes. These findings disclose unexpected non-redundant roles of p84 and p101 in obesity and metabolic syndrome.
For the third project p84-/-/p110d D910A/D910A and p101-/-/p110d D910A/D910A double mutant mice were generated to study the consequences of disbalanced PI3Kg and PI3Kd signaling on innate and adaptive immunity. While p84-/-/p110d D910A/D910A mice do not exert any gross anatomical or hematological phenotypes, p101-/-/p110d D910A/D910A mice show thymus hypoplasia, impaired red and white zone differentiation of the spleen, neutrophilic and eosinophilic infiltrations in the gut mucosa, lymphopenia and eosinophilia, IgE class switch and Th2 cytokine elevation. The large discrepancy between p84-/-/p110d D910A/D910A and p101-/-/p110d D910A/D910A genotype implicates specific involvement of p101-dependent processes to maintain proper lymphocyte development and immune tolerance. On the other hand, blockage of the myeloid-restricted p84 subunit minimally impacts humoral immunity and preserves proper Th1/Th2 balance.