Abstract 5522: Cancer-associated point mutant in the START domain of DLC1 encodes a protein attenuated for tumor suppressor activity and for binding both Caveolin-1 and PLCD1 but not for Rho-GAP activity
Authors: Sanchez-Solana Beatriz , Qian Xiaolan , Wang Dunrui , Durkin Marian , Tripathi Brajendra , Papageorge Alex , Lowy Douglas R. 
Published: August 2018

Abstract
The tumor suppressor gene DLC1 encodes a multi-domain protein, including a Rho-GAP domain (Rho GTPase activating protein domain) that negatively regulates the activity of RhoA, RhoB, and RhoC, and has been hypothesized to be the basis of its tumor suppressor functions. Other well recognized DLC1 domains are an N-terminal sterile alpha motif (SAM) domain, and a C-terminal steroidogenic acute regulatory protein-related lipid transfer (START) domain. DLC1 is down-regulated in several malignancies by gene deletion, epigenetic promoter methylation, or post-translational mechanisms. In addition, we have recently determined, by analyzing the TCGA database that point mutations in the DLC1 gene occur frequently, and have provided evidence that most of the mutant proteins display attenuated tumor suppressor function. Moreover, the mutations are distributed along the entire coding region, implying that regions of DLC1 in addition to its RhoGAP domain contribute to its full tumor suppressor activity.
Several DLC1 binding partners, in addition to Rho-GTP, have been identified. They include Caveolin-1, the principal structural component and marker of caveolae, which interacts with the START domain of DLC1. The distribution of caveolae is closely related to intracellular events, such as MAPK activation or the control of intracellular Ca2+ levels. Phospholipase C delta 1 (PLCD1) transforms PtdIns(4,5)P2 (PIP2) into inositol triphosphate (IP3) and diacyglycerol (DAG), which participate in intracellular Ca2+ mobilization and protein kinase C activation, respectively. p122RhoGAP, the rat homolog of DLC1, was initially cloned as a novel PLCD1-interacting protein, but the interacting region of PLCD1 with p122RhoGAP was not identified. In addition, others have concluded the function of PLCD1 is not affected by human DLC1.
In the current study, we have characterized a colon cancer-associated DLC1 mutant (R947C) whose mutation lies in the START domain. The mutant has an attenuated tumor suppressor phenotype, as determined by deficiency in reduction of growth in soft agar and cell migration. Compared with the wild type, the binding of the mutant to PLCD1 and to Caveolin-1 is reduced, but its RhoGAP activity is intact. In addition, phosphorylation of Caveolin-1 by Src at Tyr 14 reduces its interaction with wild type DLC1, and Caveolin-1 and PLCD1 interact in vivo more efficiently with each other in the absence of DLC1. Moreover, wild type DLC1, Caveolin-1 and PLCD1 possess cooperative tumor suppressor activities in cells expressing all three genes. Overall, this study provides new insight into the role of the START domain of DLC1, and reinforces the Rho-GAP-independent regulation of tumor growth by DLC1.
Citation Format: Beatriz Sanchez-Solana, Xiaolan Qian, Dunrui Wang, Marian Durkin, Brajendra Tripathi, Alex Papageorge, Douglas R. Lowy. Cancer-associated point mutant in the START domain of DLC1 encodes a protein attenuated for tumor suppressor activity and for binding both Caveolin-1 and PLCD1 but not for Rho-GAP activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5522.