Abstract 3065: Loss of ARL11 function promotes growth of in situ neoplasia by activating the ras pathway

Abstract

Abstract Our previously reported whole-organ genome-wide mapping studies of bladder cancer provided evidence that a new class of genes, referred to as forerunner (FR) genes (ITM2B, P2RY5, GPR38 and ARL11), located near known tumor suppressors such as RB1, may drive early clonal expansion of neoplasia. (S Lee, et al. PNAS 104: 13732-13737, 2007; T Majewski et al. Lab Inv 88: 694-712, 2008) These studies suggested three major steps for the involvement of the RB1 locus in tumor development. In the first step, one allele of FR gene and RB1 is inactivated by large deletions. In the second step, homozygous inactivation of the FR genes is accomplished by hypermethylation or less frequently by missence mutations. Homozygous inactivation of FR genes is associated with the clonal expansion of in situ lesions referred to as low-grade intraurothelial neoplasia. In the final, third step, the contiguous tumor suppressor, such as RB1, is inactivated most commonly by a mutation. This step is associated with clonal evolution into a transformed phenotype with microscopic features of carcinoma in situ progressing to invasive cancer. Recently, we concentrated our efforts on the ARL11 candidate FR gene, which encodes ADP-ribosylation factor-like 11 protein, a ras-related member of the small GTPases, that functions as GDP/GTP nucleotide interswitch for signal transduction. We have shown that it has been silenced by hypermethylation in approximately 40% of urothelial carcinomas of the bladder and the silencing was associated with in situ expansion of intraurothelial neoplasia. Moreover, loss of ARL11 expression was almost mutually exclusive with the FGFR3 mutations (<5% of bladder tumors showed a co-existence of mutant FGFR3 and methylated ARL11). Functional studies conducted on immortalized normal urothelial cells grown in vitro showed that ARL11 reduced cell proliferation, which was mediated by the inhibition of the ras pathway. This effect was associated with the down-regulation of the active ras and ERK1/2 phosphorylation as well as with the up-regulation of negative cell cycle regulator proteins such as p27kip1 and parallel down-regulation of cyclins D1 and E. Conversely, the silencing of ARL11 conferred a tumor suppressor effect promoting cell proliferation via the activation of the ras signaling pathway. In summary, our studies show that recessive events in the RB1 locus associated with the loss of ARL11 function promote clonal expansion of intraurothelial precursor lesions by activating the ras pathway in early occult phases of human bladder carcinogenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3065.