Diacylglycerol kinases in cancer

Diacylglycerol kinases (DGK) are a family of enzymes that catalyze the transformation of diacylglycerol into phosphatidic acid. In T lymphocytes, DGKα and ζ limit the activation of the PLCγ/Ras/ERK axis, providing a critical checkpoint to inhibit T cell responses. Upregulation of these isoforms limits Ras activation, leading to hypo-responsive, anergic states similar to those caused by tumors. Recent studies have identified DGKα upregulation in tumor lymphocyte infiltrates, and cells from DGKα and ζ deficient mice show enhanced antitumor activity, suggesting that limitation of DAG based signals by DGK is used by tumors to evade immune attack. DGKα expression is low or even absent in other healthy cells like melanocytes, hepatocytes or neurons. Expression of this isoform, nevertheless is upregulated in melanoma, hepatocarcinoma and glioblastoma where DGKα contributes to the acquisition of tumor metastatic traits. A model thus emerges where tumor milieu fosters DGKα expression in tumors as well as in tumor infiltrating lymphocytes with opposite consequences. Here we review the mechanisms and targets that facilitate tumor "addiction" to DGKα, and discuss its relevance in the more advanced forms of cancer for tumor immune evasion. A better knowledge of this function offers a new perspective in the search of novel approaches to prevent inhibition of immune attack in cancer. Part of the failure in clinical progress may be attributed to the complexity of the tumor/T lymphocyte interaction. As they develop, tumors use a number of mechanisms to drive endogenous, tumor reactive T cells to a general state of hyporesponsiveness or anergy. A better knowledge of the molecular mechanisms that tumors use to trigger T cell anergic states will greatly help in the advance of immunotherapy research.

Diacylglycerol Kinases: Shaping Diacylglycerol and Phosphatidic Acid Gradients to Control Cell Polarity

Diacylglycerol kinases (DGKs) terminate diacylglycerol (DAG) signaling and promote phosphatidic acid (PA) production. Isoform specific regulation of DGKs activity and localization allows DGKs to shape the DAG and PA gradients. The capacity of DGKs to constrain the areas of DAG signaling is exemplified by their role in defining the contact interface between T cells and antigen presenting cells: the immune synapse. Upon T cell receptor engagement, both DGK α and ζ metabolize DAG at the immune synapse thus constraining DAG signaling. Interestingly, their activity and localization are not fully redundant because DGKζ activity metabolizes the bulk of DAG in the cell, whereas DGKα limits the DAG signaling area localizing specifically at the periphery of the immune synapse. When DGKs terminate DAG signaling, the local PA production defines a new signaling domain, where PA recruits and activates a second wave of effector proteins. The best-characterized example is the role of DGKs in protrusion elongation and cell migration. Indeed, upon growth factor stimulation, several DGK isoforms, such as α, ζ, and γ, are recruited and activated at the plasma membrane. Here, local PA production controls cell migration by finely modulating cytoskeletal remodeling and integrin recycling. Interestingly, DGK-produced PA also controls the localization and activity of key players in cell polarity such as aPKC, Par3, and integrin β1. Thus, T cell polarization and directional migration may be just two instances of the general contribution of DGKs to the definition of cell polarity by local specification of membrane identity signaling.

Diacylglycerol Kinases in T Cell Tolerance and Effector Function

Diacylglycerol kinases (DGKs) are a family of enzymes that regulate the relative levels of diacylglycerol (DAG) and phosphatidic acid (PA) in cells by phosphorylating DAG to produce PA. Both DAG and PA are important second messengers cascading T cell receptor (TCR) signal by recruiting multiple effector molecules, such as RasGRP1, PKCθ, and mTOR. Studies have revealed important physiological functions of DGKs in the regulation of receptor signaling and the development and activation of immune cells. In this review, we will focus on recent progresses in our understanding of two DGK isoforms, α and ζ, in CD8 T effector and memory cell differentiation, regulatory T cell development and function, and invariant NKT cell development and effector lineage differentiation.

Molecular Pathways: Targeting Diacylglycerol Kinase Alpha in Cancer

Lipid kinases have largely been neglected as targets in cancer, and an increasing number of reports suggest diacylglycerol kinase alpha (DGKα) may be one with promising therapeutic potential. DGKα is one of 10 DGK family members that convert diacylglycerol (DAG) to phosphatidic acid (PA), and both DAG and PA are critical lipid second messengers in the plasma membrane. A host of important oncogenic proteins and pathways affect cancer cells in part through DGKα, including the c-Met and VEGF receptors. Others partially mediate the effects of DGKα inhibition in cancer, such as mTOR and HIF-1α. DGKα inhibition can directly impair cancer cell viability, inhibits angiogenesis, and notably may also boost T-cell activation and enhance cancer immunotherapies. Although two structurally similar inhibitors of DGKα were established decades ago, they have seen minimal in vivo usage, and it is unlikely that either of these older DGKα inhibitors will have utility for cancer. An abandoned compound that also inhibits serotonin receptors may have more translational potential as a DGKα inhibitor, but more potent and specific DGKα inhibitors are sorely needed. Other DGK family members may also provide therapeutic targets in cancer, but require further investigation.