Diacylglycerol kinase is a keystone regulator of signaling relevant to the pathophysiology of asthma

Abstract

Signal transduction by G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immunoreceptors converge at the activation of phospholipase C (PLC) for the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). This is a point for second-messenger bifurcation where DAG via protein kinase C (PKC) and IP₃ via calcium activate distinct protein targets and regulate cellular functions. IP₃ signaling is regulated by multiple calcium influx and efflux proteins involved in calcium homeostasis. A family of lipid kinases belonging to DAG kinases (DGKs) converts DAG to phosphatidic acid (PA), negatively regulating DAG signaling and pathophysiological functions. PA, through a series of biochemical reactions, is recycled to produce new molecules of PIP₂. Therefore, DGKs act as a central switch in terminating DAG signaling and resynthesis of membrane phospholipids precursor. Interestingly, calcium and PKC regulate the activation of α and ζ isoforms of DGK that are predominantly expressed in airway and immune cells. Thus, DGK forms a feedback and feedforward control point and plays a crucial role in fine-tuning phospholipid stoichiometry, signaling, and functions. In this review, we discuss the previously underappreciated complex and intriguing DAG/DGK-driven mechanisms in regulating cellular functions associated with asthma, such as contraction and proliferation of airway smooth muscle (ASM) cells and inflammatory activation of immune cells. We highlight the benefits of manipulating DGK activity in mitigating salient features of asthma pathophysiology and shed light on DGK as a molecule of interest for heterogeneous diseases such as asthma.

Keywords: DAG; airway smooth muscle; asthma; diacylglycerol kinase; remodeling.

Graphical abstract

Figure 1.

Mechanism and structural description of diacylglycerol kinase (DGK) isotypes expressed in airway smooth muscle (ASM) and immunomodulatory cells. Receptor tyrosine kinase (RTK), T cell receptor (TCR), and G protein-coupled receptor (GPCR) signaling converge at phospholipase C (PLC) signaling generating two second messenger molecules, inositol 1,4,5-trisphophate (IP₃), and diacylglycerol (DAG). DAG kinase (DGK) phosphorylates DAG converting into phosphatidic acid (PA). There are 10 DGK isotypes expressed in mammalian cells with the α and ζ forms being well-characterized in ASM and immunomodulatory cell types. Type I (α, β, and γ) contains two EF motifs requiring Ca²⁺ as a cofactor for action. Type IV (ζ and ι) contains myristoylated alanine-rich C-kinase substrates (MARCKS) homology domains insinuating protein kinase C (PKC) required for maximal effect. [Image created with a licensed version of BioRender.com.]

Figure 2.

Diacylglycerol kinase (DGK) inhibition attenuates Gq-coupled GPCR-mediated airway smooth muscle (ASM) cell contraction. Gq-coupled G protein-coupled receptor (GPCR) signaling promotes ASM cell contraction by releasing Ca²⁺ from sarcoplasmic reticulum (SR) and resulting in myosin light chain phosphorylation. Protein kinase C (PKC) promotes an inhibitory effect on the myosin phosphatase (MYPT), sustaining actin-myosin cross-bridge interaction. Acute inhibition of diacylglycerol kinase (DGK) using inhibitor R59022 diminishes phospholipase C (PLC)-inositol 1,4,5-trisphophate (IP₃)-Ca²⁺ axis (red dotted line). In addition, DGK inhibition propels diacylglycerol (DAG)-PKC-ERK1/2-cyclooxygenase (COX)-prostaglandin E₂ (PGE₂)-protein kinase A (PKA) de-coupling of contractile mechanism of ASM cells (green dotted line). KO, knockout; MLCK, myosin light chain kinase; PA, phosphatidic acid; PIP₂, phosphatidylinositol 4,5-bisphosphate. [Image created with a licensed version of BioRender.com.]

Figure 3.

Targeting diacylglycerol kinase (DGK) modulates airway smooth muscle (ASM) cell proliferation via distinct pathways. Diacylglycerol (DAG) signaling is negatively mediated by its conversion into second messenger phosphatidic acid (PA) by DAG kinase (DGK). PA binds mammalian target of rapamycin (mTORC1) to promote ASM cell proliferation, however, depleting PA levels by DGK inhibition using inhibitor R59022 mitigates receptor tyrosine kinase (RTK)-mediated growth (red dotted line). Lack of DGK activity promotes antimitogenic effect via the DAG-protein kinase C (PKC)-ERK-cyclooxygenase (COX)-prostaglandin E₂ (PGE₂)-protein kinase A (PKA) signaling axis (green dotted line). KO, knockout; PI3K, phosphatidylinositol 3-kinase. [Image created with a licensed version of BioRender.com.]