Increased anabolism is a common feature of tumors and several metabolic diseases. The high anabolic state is
typically accompanied by systemic suppression of catabolism (e.g., lysosome biogenesis and autophagy).
Paradoxically, the anabolic cells increase dependence on the lysosomal degradation pathways to counteract the
obligately increased stresses, such as malfunctioned organelle and reactive oxygen species. However, the
molecular mechanism of how cells activate lysosomal functions regardless of their anabolic state remains largely
unknown. Phosphatidylinositol 5-phosphate 4-kinase (PI5P4K) is a family of enzymes, consisting of PI5P4Ka, ß,
¿, and converts the lipid second messenger, phosphatidylinositol 5-phosphate (PI5P), to phosphatidylinositol
4,5-phosphate (PI(4,5)P2). The main function of PI5P4K is considered to control PI5P-dependent signaling, as
the bulk of PI(4,5)P2 is generated from another family of enzymes, PI4P5Ks. Genetic deletion studies of the three
genes in the PI5P4K family (Pip4k2a, Pip4k2b, and Pip4k2c) in mice indicate that PI5P4Kß plays distinct and
critical roles in mediating cellular responses to stress (e.g., nutrient deprivation, ROS) and ultimately affect
whole-body insulin sensitivity, growth, obesity, and cancer. Importantly, PI5P4Ks are atypical kinases that have
a unique property to use GTP as a phosphodonor. In particular, PI5P4Kß preferentially uses GTP rather than
ATP, and its kinase activity is regulated by physiological GTP concentrations, acting as a cellular GTP sensor
for metabolism and tumorigenesis by mechanisms yet to be defined. Pertaining to this proposal, our group has
developed isozyme selective PI5P4K inhibitors using newly developed NMR-based screening, and found that
treatment of the PI5P4K inhibitors suppressed lysosome acidification. Newly generated GTP-insensitive
Pip4k2bF205L/F205L mice developed severe steatosis compared to WT mice, and exhibited increased hypoglycemia
upon fasting, resembling the phenotype of autophagy deficiency. We hypothesize that GTP-dependent PI5P4Kß
activation promotes lysosomal acidification to counterbalance the anabolic stress for stress-resilient cellular
growth and hepatic functions. Capitalizing on our long-standing, productive collaborations with a number of
cutting-edge laboratories, we will define the mechanistic role of PI5P4Kß in transcriptionally-independent
lysosomal acidification and stress-resilient growth. Using the “structural reverse-genetics” framework that we
have developed recently, we will dissect and determine the role of kinase activity and scaffolding functions of
PI5P4Kß (Aim 1). We will test the hypothesis that GTP-dependent PI5P4Kß activity is required for hepatic
lysosomal function and whole-body energy homeostasis (Aim 2). Upon completing the proposed research, we
will identify the novel stress counteracting system through which GTP-mediated activation of PI5P4Kß promotes
lysosomal activation to support stress-resilient anabolic cell growth and protect mice from the pathogenesis of
metabolic diseases.
