The RASSF1A Tumor Suppressor Binds the RasGAP DAB2IP and Modulates RAS Activation in Lung Cancer

Abstract

Lung cancer is the leading cause of cancer-related death worldwide. Lung cancer is commonly driven by mutations in the RAS oncogenes, the most frequently activated oncogene family in human disease. RAS-induced tumorigenesis is inhibited by the tumor suppressor RASSF1A, which induces apoptosis in response to hyperactivation of RAS. RASSF1A expression is suppressed in cancer at high rates, primarily owing to promoter hypermethylation. Recent reports have shown that loss of RASSF1A expression uncouples RAS from apoptotic signaling in vivo, thereby enhancing tumor aggressiveness. Moreover, a concomitant upregulation of RAS mitogenic signaling upon RASSF1A loss has been observed, suggesting RASSF1A may directly regulate RAS activation. Here, we present the first mechanistic evidence for control of RAS activation by RASSF1A. We present a novel interaction between RASSF1A and the Ras GTPase Activating Protein (RasGAP) DAB2IP, an important negative regulator of RAS. Using shRNA-mediated knockdown and stable overexpression approaches, we demonstrate that RASSF1A upregulates DAB2IP protein levels in NSCLC cells. Suppression of RASSF1A and subsequent downregulation of DAB2IP enhances GTP loading onto RAS, thus increasing RAS mitogenic signaling in both mutant- and wildtype-RAS cells. Moreover, co-suppression of RASSF1A and DAB2IP significantly enhances in vitro and in vivo growth of wildtype-RAS cells. Tumors expressing wildtype RAS, therefore, may still suffer from hyperactive RAS signaling when RASSF1A is downregulated. This may render them susceptible to the targeted RAS inhibitors currently in development.

Epigenetic regulation in lung cancer

“…88 The tumor suppressor RASSF1A or RASSF1A (as a tumor suppressor) inhibits Ras-induced tumorigenesis by inducing apoptosis after hyperactivation of RAS. 87 RASSF1A encodes a tumor suppressor that inhibits the RAS→RAF→MEK→ERK pathway and inactivates genes in human cancers. 90 RASSF1A forces a cell cycle arrest by inhibiting the activity of cyclins A and D1, and it interacts with CNK1, MST1, Salvador, and MOAP1, which may modulate apoptosis.…”

“…91 Thus, RASSF1A is considered a TSG that has been widely explored in lung cancer and other malignant tumors. 75,87 2.2.2 O 6 -methylguanine-DNA methyltransferase MGMT is a protein that has been shown to interfere in DNA repair, and it is believed that MGMT gene silencing or a lack of synthesis is what causes MGMT deficiency. 92 By eliminating adducts from the O 6 position of guanine, it can shield cells against the effects of alkylating chemicals.…”

“…The six core RASSF family proteins, which contain conserved Ras association domains, can function as Ras effectors 88 . All RASSF proteins lack enzymatic activity and appear to function as scaffolding and localization molecules that allow multiple pathways to communicate and sometimes compete with each other 87 . The RASSF family of RAS effectors, notably RASSF1 , is known to mediate many of these effects 87 .…”

“…87 The RASSF family of RAS effectors, notably RASSF1, is known to mediate many of these effects. 87 The RASSF plays significant roles in cell apoptosis, genomic and microtubule stability, and cell cycle regulation, and it is coded for two main transcripts, RASSF1A and RASSF1C, by alternative splicing. 89 RASSF1A is located within a 120-kb region of chromosome 3p21.3 and is known as a frequent target for aberrant methylation in lung cancer.…”

“…RAS (a small GTPase) oncogene families are the most commonly activated oncogenes in human diseases, and mutations in these oncogenes cause a variety of cancers 86 . Multiple mitogenic pathways are activated by RAS upon activation by its well‐documented effectors, RAF , PI3K , and RalGDS 87 . Furthermore, excessive RAS stimulation can also promote apoptosis or senescence, but it is much less well understood how these pathways lead to cell death.…”