Sall2 Transcription Factor: A Novel Regulator Of G1-S Cyclins.

Spalt-like proteins are Zinc finger transcription factors present from C. elegans to vertebrates, with critical roles in development. In vertebrates, four paralogues have been identified (Sall1-4), with Sall2 being the most dissimilar member of the family. Sall2 has shown to be important for brain and eye development and–unlike its vertebrate paralogues–it has also been proposed as a tumor suppressor. Sall2 activity promotes cell cycle arrest and cell death and its expression is downregulated in several types of cancer. Despite this, very few Sall2 transcriptional targets have been identified until date. In agreement with previous reports, characterization of primary and immortalized mouse embryonic fibroblasts (MEFs) showed a proliferative advantage of isogenic Sall2-/- vs Sall2+/+ cells. In order to identify the molecular mechanisms underlying Sall2 function during cell cycle, iMEFs were synchronized at G2/M phase with nocodazole. Flow cytometry, Western blot, and qRT-PCR analyses were performed after nocodazole release. In accordance with the role of Sall2 in promoting cell cycle arrest, a more rapid progression from G2/M to G1/S was noticed in Sall2-deficient cells, which correlates with an increased expression of Cyclin D1 (Ccnd1) and Cyclin E1 (Ccne1). Similar results were obtained in HEK293 and SKOV3 human cell lines, suggesting that Sall2 controls cell cycle progression by downregulating G1-S cyclins. Sall2-mediated repression of CCND1 and CCNE1 was confirmed by luciferase reporter assays. In addition, bioinformatic analysis led to the identification of several putative Sall2 binding sites within CCND1 and CCNE1 promoters. Sall2 binding to CCND1 and CCNE1 promoters was demonstrated by chromatin immunoprecipitation. Analysis of tissues from Sall2+/+ and Sall2-/- mice confirmed the inverse correlation between expression of SALL2 and G1-S cyclins. Importantly, the same correlation was found in different types of tumor by analyzing publicly available databases, suggesting that Sall2-Cyclin D1-Cyclin E1 axis could be downregulated in cancer. Thus, we have identified two novel Sall2 transcriptional targets at G1-S transition. Altogether, our findings support the role of Sall2 as a tumor suppressor, by acting as a transcriptional repressor during cell cycle. Citation Format: Viviana E. Hermosilla, David E. Escobar, Matias I. Hepp, Elizabeth N. Riffo, Ginessa Salgado, Violeta Morin, Mario Galindo, Ariel F. Castro, Roxana Pincheira, Roxana Pincheira. Sall2 transcription factor: A novel regulator of G1-S cyclins [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr A32.