Identification of a nuclear localization signal and importin beta members mediating NUAK1 nuclear import inhibited by oxidative stress

NUAK1 is a serine/threonine kinase member of the AMPK-α family. NUAK1 regulates several processes in tumorigenesis; however, its regulation and molecular targets are still poorly understood. Bioinformatics analysis predicted that the majority of NUAK1 localizes in the nucleus. However, there are no studies about the regulation of NUAK1 subcellular distribution. Here, we analyzed NUAK1 localization in several human cell lines, mouse embryo fibroblasts, and normal mouse tissues. We found that NUAK1 is located in the nucleus and also in the cytoplasm. Through bioinformatics analysis and studies comparing subcellular localization of wild type and NUAK1 mutants, we identified a conserved bipartite nuclear localization signal at the N-terminal domain of NUAK1. Based on mass spectrometry analysis, we found that NUAK1 interacts with importin-β members including importin-β1 (KPNB1), importin-7 (IPO7), and importin-9 (IPO9). We confirmed that importin-β members are responsible for NUAK1 nuclear import through the inhibition of importin-β by Importazole and the knockdown of either IPO7 or IPO9. In addition, we found that oxidative stress induces NUAK1 cytoplasmic accumulation, indicating that oxidative stress affects NUAK1 nuclear transport. Thus, our study is the first evidence of an active nuclear transport mechanism regulating NUAK1 subcellular localization. These data will lead to investigations of the molecular targets of NUAK1 according to its subcellular distribution, which could be new biomarkers or targets for cancer therapies.

Abstract A32: 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.