The programmes of work in the laboratory are currently aimed at understanding the molecular mechanisms and physiological significance of intracellular signalling networks which involve a family of enzymes called phosphoinositide 3OH-kinases (PI3Ks).
PI3Ks are now accepted to be critical regulators of numerous important and complex cell responses, including cell growth, division, survival and movement.
PI3Ks catalyse the formation of one or more critical phospholipid messenger molecules, which signal information by binding to specific domains in target proteins. Currently the best understood pathway involves the activation of Class I PI3Ks by cell surface receptors.
In recent years, the Hawkins laboratory has increasingly focused on the role of PI3Ks in the signalling mechanisms which allow receptors on neutrophils (white blood cells) to control various aspects of neutrophil function.
Neutrophils are key players in the front line of our immune system, responsible primarily for the recognition and destruction of bacterial and fungal pathogens. However, they are also involved in the amplification cascades that underlie various inflammatory pathologies, e.g. Acute Respiratory Distress Syndrome (ARDS) and rheumatoid arthritis.
Insulin and other growth factors are key regulators of liver gene expression, including in metabolic diseases. Most of the phosphoinositide 3-kinase (PI3K) activity induced by insulin is considered to be dependent on PI3Kα. We used mice lacking p110α, the catalytic subunit of PI3Kα, to investigate its role in the regulation of liver gene expression in health and in metabolic dysfunction-associated steatotic liver disease (MASLD). The absence of hepatocyte PI3Kα reduced maximal insulin-induced PI3K activity and signaling, promoted glucose intolerance in lean mice and significantly regulated liver gene expression, including insulin-sensitive genes, in ad libitum feeding. Some of the defective regulation of gene expression in response to hepatocyte-restricted insulin receptor deletion was related to PI3Kα signaling. In addition, though PI3Kα deletion in hepatocytes promoted insulin resistance, it was protective against steatotic liver disease in diet-induced obesity. In the absence of hepatocyte PI3Kα, the effect of diet-induced obesity on liver gene expression was significantly altered, with changes in rhythmic gene expression in liver. Altogether, this study highlights the specific role of p110α in the control of liver gene expression in physiology and in the metabolic rewiring that occurs during MASLD.
CDS enzymes (CDS1 and 2 in mammals) convert phosphatidic acid (PA) to CDP-DG, an essential intermediate in the de novo synthesis of PI. Genetic deletion of CDS2 in primary mouse macrophages resulted in only modest changes in the steady-state levels of major phospholipid species, including PI, but substantial increases in several species of PA, CDP-DG, DG and TG. Stable isotope labelling experiments employing both 13C6- and 13C6D7-glucose revealed loss of CDS2 resulted in a minimal reduction in the rate of de novo PI synthesis but a substantial increase in the rate of de novo PA synthesis from G3P, derived from DHAP via glycolysis. This increased synthesis of PA provides a potential explanation for normal basal PI synthesis in the face of reduced CDS capacity (via increased provision of substrate to CDS1) and increased synthesis of DG and TG (via increased provision of substrate to LIPINs). However, under conditions of sustained GPCR-stimulation of PLC, CDS2-deficient macrophages were unable to maintain enhanced rates of PI synthesis via the 'PI cycle', leading to a substantial loss of PI. CDS2-deficient macrophages also exhibited significant defects in calcium homeostasis which were unrelated to the activation of PLC and thus probably an indirect effect of increased basal PA. These experiments reveal that an important homeostatic response in mammalian cells to a reduction in CDS capacity is increased de novo synthesis of PA, likely related to maintaining normal levels of PI, and provides a new interpretation of previous work describing pleiotropic effects of CDS2 deletion on lipid metabolism/signalling.
Receptor tyrosine kinases such as epidermal growth factor receptor (EGFR) stimulate phosphoinositide 3-kinases (PI3Ks) to convert phosphatidylinositol-4,5-bisphosophate [PtdIns(4,5)P] into phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P]. PtdIns(3,4,5)P then remodels actin and gene expression, and boosts cell survival and proliferation. PtdIns(3,4,5)P partly achieves these functions by triggering activation of the kinase Akt, which phosphorylates targets like Tsc2 and GSK3β. Consequently, unchecked upregulation of PtdIns(3,4,5)P-Akt signalling promotes tumour progression. Interestingly, 50-70% of PtdIns and PtdInsPs have stearate and arachidonate at -1 and -2 positions of glycerol, respectively, forming a species known as 38:4-PtdIns/PtdInsPs. LCLAT1 and MBOAT7 acyltransferases partly enrich PtdIns in this acyl format. We previously showed that disruption of LCLAT1 lowered PtdIns(4,5)P levels and perturbed endocytosis and endocytic trafficking. However, the role of LCLAT1 in receptor tyrosine kinase and PtdIns(3,4,5)P signaling was not explored. Here, we show that LCLAT1 silencing in MDA-MB-231 and ARPE-19 cells abated the levels of PtdIns(3,4,5)P in response to EGF signalling. Importantly, LCLAT1-silenced cells were also impaired for EGF-driven and insulin-driven Akt activation and downstream signalling. Thus, our work provides first evidence that the LCLAT1 acyltransferase is required for receptor tyrosine kinase signalling.