Temporal control of bidirectional lipid-droplet motion in Drosophila depends on the ratio of kinesin-1 and its co-factor Halo.

During bidirectional transport, individual cargoes move continuously back and forth along microtubule tracks, yet the cargo population overall displays directed net transport. How such transport is controlled temporally is not well understood. We analyzed this issue for bidirectionally moving lipid droplets in Drosophila embryos, a system in which net transport direction is developmentally controlled. By quantifying how the droplet distribution changes as embryos develop, we characterize temporal transitions in net droplet transport and identify the crucial contribution of the previously identified, but poorly characterized, transacting regulator Halo. In particular, we find that Halo is transiently expressed; rising and falling Halo levels control the switches in global distribution. Rising Halo levels have to pass a threshold before net plus-end transport is initiated. This threshold level depends on the amount of the motor kinesin-1: the more kinesin-1 is present, the more Halo is needed before net plus-end transport commences. Because Halo and kinesin-1 are present in common protein complexes, we propose that Halo acts as a rate-limiting co-factor of kinesin-1.

Hippo and PI5P4K signaling intersect to control the transcriptional activation of YAP.

Phosphoinositides are essential signaling molecules. The PI5P4K family of phosphoinositide kinases and their substrates and products, PI5P and PI4,5P2, respectively, are emerging as intracellular metabolic and stress sensors. We performed an unbiased screen to investigate the signals that these kinases relay and the specific upstream regulators controlling this signaling node. We found that the core Hippo pathway kinases MST1/2 phosphorylated PI5P4Ks and inhibited their signaling in vitro and in cells. We further showed that PI5P4K activity regulated several Hippo- and YAP-related phenotypes, specifically decreasing the interaction between the key Hippo proteins MOB1 and LATS and stimulating the YAP-mediated genetic program governing epithelial-to-mesenchymal transition. Mechanistically, we showed that PI5P interacted with MOB1 and enhanced its interaction with LATS, thereby providing a signaling connection between the Hippo pathway and PI5P4Ks. These findings reveal how these two important evolutionarily conserved signaling pathways are integrated to regulate metazoan development and human disease.

PI5P4Kα supports prostate cancer metabolism and exposes a survival vulnerability during androgen receptor inhibition.

Phosphatidylinositol (PI)regulating enzymes are frequently altered in cancer and have become a focus for drug development. Here, we explore the phosphatidylinositol-5-phosphate 4-kinases (PI5P4K), a family of lipid kinases that regulate pools of intracellular PI, and demonstrate that the PI5P4Kα isoform influences androgen receptor (AR) signaling, which supports prostate cancer (PCa) cell survival. The regulation of PI becomes increasingly important in the setting of metabolic stress adaptation of PCa during androgen deprivation (AD), as we show that AD influences PI abundance and enhances intracellular pools of PI-4,5-P2. We suggest that this PI5P4Kα-AR relationship is mitigated through mTORC1 dysregulation and show that PI5P4Kα colocalizes to the lysosome, the intracellular site of mTORC1 complex activation. Notably, this relationship becomes prominent in mouse prostate tissue following surgical castration. Finally, multiple PCa cell models demonstrate marked survival vulnerability following stable PI5P4Kα inhibition. These results nominate PI5P4Kα as a target to disrupt PCa metabolic adaptation to castrate resistance.

Enhanced SREBP2-driven cholesterol biosynthesis by PKCλ/ι deficiency in intestinal epithelial cells promotes aggressive serrated tumorigenesis.

The metabolic and signaling pathways regulating aggressive mesenchymal colorectal cancer (CRC) initiation and progression through the serrated route are largely unknown. Although relatively well characterized as BRAF mutant cancers, their poor response to current targeted therapy, difficult preneoplastic detection, and challenging endoscopic resection make the identification of their metabolic requirements a priority. Here, we demonstrate that the phosphorylation of SCAP by the atypical PKC (aPKC), PKCλ/ι promotes its degradation and inhibits the processing and activation of SREBP2, the master regulator of cholesterol biosynthesis. We show that the upregulation of SREBP2 and cholesterol by reduced aPKC levels is essential for controlling metaplasia and generating the most aggressive cell subpopulation in serrated tumors in mice and humans. Since these alterations are also detected prior to neoplastic transformation, together with the sensitivity of these tumors to cholesterol metabolism inhibitors, our data indicate that targeting cholesterol biosynthesis is a potential mechanism for serrated chemoprevention.

Expanding role of PI5P4Ks in cancer: A promising druggable target.

Cancer cells are challenged by a myriad of microenvironmental stresses, and it is their ability to efficiently adapt to the constantly changing nutrient, energy, oxidative, and/or immune landscape that allows them to survive and proliferate. Such adaptations, however, result in distinct vulnerabilities that are attractive therapeutic targets. Phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks) are a family of druggable stress-regulated phosphoinositide kinases that become conditionally essential as a metabolic adaptation, paving the way to targeting cancer cell dependencies. Further, PI5P4Ks have a synthetic lethal interaction with the tumor suppressor p53, the loss of which is one of the most prevalent genetic drivers of malignant transformation. PI5P4K's emergence as a crucial axis in the expanding landscape of phosphoinositide signaling in cancer has already stimulated the development of specific inhibitors. Thus, a better understanding of the biology of the PI5P4Ks will allow for targeted and effective therapeutic interventions. Here, we attempt to summarize the mounting roles of the PI5P4Ks in cancer, including evidence that targeting them is a therapeutic vulnerability and promising next-in-line treatment for multiple cancer subtypes.

PI5P4Ks drive metabolic homeostasis through peroxisome-mitochondria interplay.

PI5P4Ks are a class of phosphoinositide kinases that phosphorylate PI-5-P to PI-4,5-P2. Distinct localization of phosphoinositides is fundamental for a multitude of cellular functions. Here, we identify a role for peroxisomal PI-4,5-P2 generated by the PI5P4Ks in maintaining energy balance. We demonstrate that PI-4,5-P2 regulates peroxisomal fatty acid oxidation by mediating trafficking of lipid droplets to peroxisomes, which is essential for sustaining mitochondrial metabolism. Using fluorescent-tagged lipids and metabolite tracing, we show that loss of the PI5P4Ks significantly impairs lipid uptake and ß-oxidation in the mitochondria. Further, loss of PI5P4Ks results in dramatic alterations in mitochondrial structural and functional integrity, which under nutrient deprivation is further exacerbated, causing cell death. Notably, inhibition of the PI5P4Ks in cancer cells and mouse tumor models leads to decreased cell viability and tumor growth, respectively. Together, these studies reveal an unexplored role for PI5P4Ks in preserving metabolic homeostasis, which is necessary for tumorigenesis.

Cachexia-associated adipose loss induced by tumor-secreted leukemia inhibitory factor is counterbalanced by decreased leptin.

Cachexia syndrome consists of adipose and muscle loss, often despite normal food intake. We hypothesized that cachexia-associated adipose wasting is driven in part by tumor humoral factors that induce adipocyte lipolysis. We developed an assay to purify secreted factors from a cachexia-inducing colon cancer line that increases lipolysis in adipocytes and identified leukemia inhibitory factor (LIF) by mass spectrometry. Recombinant LIF induced lipolysis in vitro. Peripheral LIF administered to mice caused >50% loss of adipose tissue and >10% reduction in body weight despite only transient hypophagia due to decreasing leptin. LIF-injected mice lacking leptin (ob/ob) resulted in persistent hypophagia and loss of adipose tissue and body weight. LIF's peripheral role of initiating lipolysis in adipose loss was confirmed in pair-fed ob/ob mouse studies. Our studies demonstrate that (a) LIF is a tumor-secreted factor that promotes cachexia-like adipose loss when administered peripherally, (b) LIF directly induces adipocyte lipolysis, (c) LIF has the ability to sustain adipose and body weight loss through an equal combination of peripheral and central contributions, and (d) LIF's central effect is counterbalanced by decreased leptin signaling, providing insight into cachexia's wasting, despite normophagia.