Abstract 1965: Hippo pathway restrains cell proliferation via assembly of the DREAM repressor complex
Authors: Litovchick Larisa , Saini Siddharth 
Published: August 2015

Abstract
Hippo pathway mediates tumor suppression in various organisms by the mechanisms that are not fully understood. The canonical Hippo pathway converges on the pro-oncogenic transcription factors YAP and TAZ that are phosphorylated by LATS1 or LATS2 protein kinases, resulting in their proteasomal degradation. Hippo pathway is regulated by various extracellular signals including lysophosphatidic acid (LPA), a bioactive lipid that has a tumor-promoting activity. High levels of LPA in the malignant effusions correlate with increased epithelial ovarian cancer (EOC) aggressiveness. However, the mitogenic effect of LPA is not well understood. We hypothesized that the Hippo signaling can regulate cell proliferation though a novel mechanism involving retinoblastoma (RB) family proteins. Previous studies show that LATS2 contributes to the function of the DREAM (DP, RB-like, E2F, and MuvB core) complex that promotes the G0/G1 arrest by repressing more than 800 cell cycle-regulated genes. The DREAM complex formation depends on phosphorylation of LIN52 subunit of the MuvB core by DYRK1A protein kinase. DYRK1A activity towards LIN52 is promoted by LATS2 phosphorylation in vitro. However, it remains to be established whether the Hippo signaling regulates the function of DREAM complex in vivo.
To understand the mechanism by which inhibition of the Hippo signaling can promote cell proliferation, we determined the effect of LPA on the DREAM complex in the ovarian cancer and the precursor cell lines. Using immunoprecipitation-Western blot assays, we show that the LPA treatment of serum-starving cells prevents the recruitment of RB-like p130 into the DREAM complex. Consequently, the cells escape from G0/G1 arrest as demonstrated by FACS analysis of the DNA synthesis. Since MuvB core also contributes to the expression of mitotic genes through binding to BMYB transcription factor, we show that this interaction is not disrupted by LPA. In support of the model that LPA effects are mediated by inhibition of the Hippo signaling, the RNAi knockdown of both LATS1 and LATS2 in ovarian cell lines also resulted in the impaired DREAM assembly. To characterize the mechanism by which LPA can interfere with the DREAM complex formation, we investigated the changes in LIN52 phosphorylation upon LPA treatment or LATS depletion. Our data support the model in which the disruption of the Hippo signaling inhibits the phosphorylation of LIN52, thereby preventing the formation of the DREAM complex and entry into G0/G1 arrest. Our findings highlight a novel mechanism by which inhibition of the Hippo tumor suppressor pathway can promote cancer cell proliferation.
Citation Format: Larisa Litovchick, Siddharth Saini. Hippo pathway restrains cell proliferation via assembly of the DREAM repressor complex. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1965. doi:10.1158/1538-7445.AM2015-1965