Mutations in the small Ras GTPases are key drivers in 30% of all human cancers, making Ras GTPases the most prominent oncogenes in human disease. Ral GTPases are critical for cellular transformation by oncogenic Ras mutants, but the underlying reasons remain poorly characterized. Our recent work demonstrates that Ral GTPase activity is a critical driver of pancreatic cancer and can promote tumor progression through direct effects on proliferation/invasion of cells. Furthermore, Ral GTPases play a role in tissue inflammation and acinar plasticity in the pancreas rendering them critical factors in tumor initiation (Beel S et al. Nat Commun, 2020). An important focus of our work therefore is to understand how Ral GTPases affect these processes and to unravel the connection to control of inflammation and cell death. In this context, we also aim to identify targetable mediators of tumorigenic Ral signaling to investigate potentially new therapeutic avenues for this dire disease.
Ral GTPases are involved in diverse cellular processes such as exocytosis, endocytosis, actin organization, cell migration and autophagy. Ral GTPases localize to the plasma membrane, endosomes or secretory vesicles, and localization has been suggested to affect activation, effector usage and consequently signaling outcome and cellular response. We there study how localization-specific Ral activity is established and translates into specific cellular responses. Such knowledge will not only be interesting from a cell biological but also from a biomedical point of view, as it would provide a basis for interference with certain functions of Ral GTPases or Ral activation under specific (pathological) conditions.
Another facet of Ral biology that is poorly understood is the control of Ral activity through Ral-GAP complexes and the molecular and signaling mechanism that govern Ral-GAP action. Like all small GTPases, Ral GTPases function as molecular switches in cells by cycling through inactive GDP- and active GTP-bound states, a process that is in the case of Ral proteins controlled by several guanine exchange factors (GEFs) and two GTPase activating protein (GAP) complexes. The Ral-GAP complexes are large heterotrimers comprising a common regulatory β subunit, which binds to one of two catalytic α subunits. Ral-GAPβ binding as well as association of the unusual small κB-Ras GTPases is required for Ral-GAPα activity via unknown mechanisms. We thus study localization, architecture and functionality of these complexes.
Consisting of the regulatory TSC1 and the catalytic TSC2 subunits, the TSC complex shows homologous architecture to the Ral-GAP complexes and functions as GAP for the GTPase Rheb. Rheb is required for full activation of mTORC1 (mechanistic target of rapamycin complex 1) at the lysosomal membrane and thus plays a critical role in cellular growth. Mutations in TSC1 or TSC2, which are spread over large parts of the proteins, cause tuberous sclerosis complex (TSC), a neurocutaneous syndrome characterized by benign tumors in multiple organs. TSC manifests with diverse phenotypes and different severity in individual patients with varying mutations. One of our ongoing research efforts is to understand the diverse molecular and signaling effects of pathogenic TSC complex mutations.
As for the Ral-GAP complexes, how the activity of the TSC complex is regulated under different cellular conditions to allow for proper control of cell growth remains incompletely understood. We have recently identified a novel role of TSC complex binding to phosphoinositol phosphates (PIPs) in the control cell growth via mTORC1 (Fitzian K et al. Mol Cell, 2021). We continue this research to investigate in detail how and when alterations in the lipid composition of membranes affect mTORC1 signaling.