κB-Ras proteins are fast-exchanging GTPases and function via nucleotide-independent binding of Ral GTPase-activating protein complexes

κB-Ras (NF-κB inhibitor-interacting Ras-like protein) GTPases are small Ras-like GTPases but harbor interesting differences in important sequence motifs. They act in a tumor-suppressive manner as negative regulators of Ral (Ras-like) GTPase and NF-κB signaling, but little is known about their mode of function. Here, we demonstrate that, in contrast to predictions based on primary structure, κB-Ras GTPases possess hydrolytic activity. Combined with low nucleotide affinity, this renders them fast-cycling GTPases that are predominantly GTP-bound in cells. We characterize the impact of κB-Ras mutations occurring in tumors and demonstrate that nucleotide binding affects κB-Ras stability but is not strictly required for RalGAP (Ral GTPase-activating protein) binding. This demonstrates that κB-Ras control of RalGAP/Ral signaling occurs in a nucleotide-binding- and switch-independent fashion.

Ral GTPase promotes metastasis of pancreatic ductal adenocarcinoma via elevation of TGF-β1 production

Pancreatic ductal adenocarcinoma (PDAC), caused by activating mutations in K-Ras, is an aggressive malignancy due to its early invasion and metastasis. Ral GTPases are activated downstream of Ras and play a crucial role in the development and progression of PDAC. However, the underlying mechanisms remain unclear. In this study, we investigated the mechanism of Ral-induced invasion and metastasis of PDAC cells using RalGAPβ-deficient PDAC cells with highly activated Ral GTPases. Array analysis and enzyme-linked immunosorbent assays revealed increased expression and secretion of TGF-β1 in RalGAPβ-deficient PDAC cells compared to control cells. Blockade of TGF-β1 signaling suppressed RalGAPβ deficiency-enhanced migration and invasion in vitro and metastasis in vivo to levels similar to controls. Phosphorylation of c-Jun N-terminal kinase (JNK), a repressor of TGF-β1 expression, was decreased by RalGAPβ deficiency. These results indicate that Ral contributes to invasion and metastasis of PDAC cells by elevating autocrine TGF-β1 signaling at least in part by decreasing JNK activity.

Ral GTPase-activating protein regulates the malignancy of pancreatic ductal adenocarcinoma

The small GTPases RalA and RalB are members of the Ras family and activated downstream of Ras. Ral proteins are found in GTP-bound active and GDP-bound inactive forms. The activation process is executed by guanine nucleotide exchange factors, while inactivation is mediated by GTPase-activating proteins (GAPs). RalGAPs are complexes that consist of a catalytic α1 or α2 subunit together with a common β subunit. Several reports implicate the importance of Ral in pancreatic ductal adenocarcinoma (PDAC). However, there are few reports on the relationship between levels of RalGAP expression and malignancy in PDAC. We generated RalGAPβ-deficient PDAC cells by CRISPR-Cas9 genome editing to investigate how increased Ral activity affects malignant phenotypes of PDAC cells. RalGAPβ-deficient PDAC cells exhibited several-fold higher Ral activity relative to control cells. They had a high migratory and invasive capacity. The RalGAPβ-deficient cells grew more rapidly than control cells when injected subcutaneously into nude mice. When injected into the spleen, the RalGAPβ-deficient cells formed larger splenic tumors with more liver metastases, and unlike controls, they disseminated into the abdominal cavity. These results indicate that RalGAPβ deficiency in PDAC cells contributes to high activities of RalA and RalB, leading to enhanced cell migration and invasion in vitro, and tumor growth and metastasis in vivo.

The Rheb-TORC1 signaling axis functions as a developmental checkpoint

In many eukaryotes, the small GTPase Rheb functions as a switch to toggle activity of TOR complex 1 (TORC1) between anabolism and catabolism, thus controlling lifespan, development and autophagy. Our CRISPR-generated, fluorescently tagged endogenous Caenorhabditis elegans RHEB-1 and DAF-15/Raptor are expressed ubiquitously and localize to lysosomes. LET-363/TOR and DAF-15/Raptor are required for development beyond the third larval stage (L3). We observed that deletion of RHEB-1 similarly conferred L3 arrest. Unexpectedly, robust RNAi-mediated depletion of TORC1 components caused arrest at stages prior to L3. Accordingly, conditional depletion of endogenous DAF-15/Raptor in the soma revealed that TORC1 is required at each stage of the life cycle to progress to the next stage. Reversal of DAF-15 depletion permits arrested animals to recover to continue development. Our results are consistent with TORC1 functioning as a developmental checkpoint that governs the decision of the animal to progress through development.

RalA controls glucose homeostasis by regulating glucose uptake in brown fat

Insulin increases glucose uptake into adipose tissue and muscle by increasing trafficking of the glucose transporter Glut4. In cultured adipocytes, the exocytosis of Glut4 relies on activation of the small G protein RalA by insulin, via inhibition of its GTPase activating complex RalGAP. Here, we evaluate the role of RalA in glucose uptake in vivo with specific chemical inhibitors and by generation of mice with adipocyte-specific knockout of RalGAPB. RalA was profoundly activated in brown adipose tissue after feeding, and its inhibition prevented Glut4 exocytosis. RalGAPB knockout mice with diet-induced obesity were protected from the development of metabolic disease due to increased glucose uptake into brown fat. Thus, RalA plays a crucial role in glucose transport in adipose tissue in vivo.

A role for Ral GTPase-activating protein subunit β in mitotic regulation

Ral proteins are small GTPases that play critical roles in normal physiology and in oncogenesis. There is little information on the GTPase-activating proteins (GAPs) that downregulate their activity. Here, we provide evidence that the noncatalytic β subunit of RalGAPα1/2 β complexes is involved in mitotic control. RalGAPβ localizes to the Golgi and nucleus during interphase, and relocalizes to the mitotic spindle and cytokinetic intercellular bridge during mitosis. Depletion of RalGAPβ causes chromosome misalignment and decreases the amount of mitotic cyclin B1, disturbing the metaphase-to-anaphase transition. Overexpression of RalGAPβ interferes with cell division, leading to binucleation and multinucleation, and cell death. We propose that RalGAPβ plays an essential role in the sequential progression of mitosis by controlling the spatial and temporal activation of Ral GTPases in the spindle assembly checkpoint (SAC) and cytokinesis. Deregulation of RalGAPβ might cause genomic instability, leading to human carcinogenesis.