The main function of the tumor suppressor Retinoblastoma (RB) is the regulation of the G1/S phase transition. This event is critical for the proliferation of normal cells in tissues, and its inhibition is one of the most important hallmarks leading to cancer. In resting cells RB is unphosphorylated and represses E2F-mediated gene expression. Upon entry into cell cycle, Rb becomes phosphorylated at the C-term through the action of Cyclin-Cdk complexes thus leading to its inactivation and the transcription of essential genes for S phase entry. Upon stress, cells deploy different mechanisms in order to adapt and increase their chances of survival. Among these, the control of cell division is essential. It is well established that stress-activated protein kinases (SAPKs) such as p38 play a key role in cell cycle regulation. During the G1-S transition, p38 regulates essential components of the cell cycle machinery such as the cyclin-dependent kinase (CDK) inhibitors p21Cip1, p27Kip1and p57Kip2. The CDK inhibitor p57Kip2 is phosphorylated by p38 which enhances its association to CDK2-Cyclin complexes and leads to reduced CDK2 activity. As a consequence, cells transiently arrest at the G1 cell phase [1]. However, stressed p57Kip2 knockout cells are still able, albeit to a lesser extent, to delay G1, which is an indication that other mechanisms involved in cell cycle control exist. Remarkably, upon stress, p38 also inhibits the transcription of E2F-dependent genes, which are necessary for cell cycle progression. These results prompted us to question whether p38 might be controlling Rb activity. Our results show that, upon stress p38 phosphorylates the N-term of Retinoblastoma (RB), making RB insensitive to cyclin-dependent kinase (CDK)-Cyclin inactivation and increases its affinity toward the E2F transcription factor, thus repressing E2F-mediated gene expression and delaying cell-cycle progression [2,3].This novel mechanism of RB regulation represents an opportunity for developing new cancer drug treatments for tumors by developing compounds capable of promoting the association of E2F transcription factors to the N-term of RB.

1. Joaquin M, Gubern A, Posas F. A novel G1 checkpoint mediated by the p57 CDK inhibitor and p38 SAPK promotes cell survival upon stress. Cell Cycle 2012b; 11:3339-40.
2. Gubern A, Joaquin M, Marquès M, Maseres P, Garcia-Garcia J, Amat R, González-Nuñez D, Oliva B, Real FX, de Nadal E, Posas F. The N-Terminal Phosphorylation of RB by p38 Bypasses Its Inactivation by CDKs and Prevents Proliferation in Cancer Cells. Mol Cell. 2016 Oct 6;64(1):25-36.
3. Joaquin M, de Nadal E, Posas F. An RB insensitive to CDK regulation. Mol Cell Oncol. 2016 Dec 14;4(1):e1268242.

In tumors, the recruitment of immune cells with suppressive capacity and the main characteristics of the tumor microenvironment (increased levels of adenosine, low oxygen and acidic pH) favor that T cells become anergic and the activation of the TCR does not occur or occurs in a deficient manner. Targeting and manipulation of the immune system constitutes thus a major priority in the treatment of cancer. The Diacylglycerol kinases (DGK) are a family of enzymes that phosphorylate diacylglycerol (DAG) transforming this lipid into phosphatidic acid (PA). In T lymphocytes DGK act as negative regulators of the immune response metabolizing the DAG generated upon T cell receptor (TCR) triggering. This lipid second messenger facilitates the activation of the Ras/ERK (extracellular signal-regulated kinase) cascade, providing a direct input for AP-1 (activator protein-1)-mediated transcription and the expression of activation markers such as CD69 or CD25. The over-activation and/or expression of specific DGK isoforms drive T lymphocytes into a hypofunctional or anergic state. Tumor infiltrating lymphocytes (TILs) show elevated expression of certain DGK isoforms. In addition, tumors express high levels of DGK, suggesting the contribution of this enzyme family to the mechanism that regulate tumor immune evasion. Targeting DGK activity could contribute to block tumor growth and enhance T cell immunity. The generation and validation of different tools to monitor the effectiveness of known and potential new DGK inhibitors is needed. Here we design and optimize robust and reproducible cell-based trials for screening processes as well as to develop reliable sensors for DGK activity. We used different strategies to validate the effects of the previously reported DGK inhibitor R59949 and test the effect over DGK activity of a structurally similar molecule previously characterized as an anti-anxiety, antidepressive and antipsychotic agent. In addition, we pursue to better understand the different DGK functions in tumors and immune system, focusing on the possible additional effects of DGK inhibitors and the mechanisms that modulate DGK expression and activity.