Novel Roles for the Tumor Suppressor APC Through Regulation of GSK-3

Adenomatous Polyposis Coli (APC) is a tumor suppressor and essential negative regulator of the Wnt signaling pathway. Wnt signaling is crucial for proper patterning and cell fate specification during development and regulates stem cell homeostasis throughout adulthood. Mutations in Apc are strongly linked to human colorectal cancers and these mutations aberrantly activate Wnt signaling. How APC regulates the Wnt pathway and how oncogenic Apc mutations activate Wnt signaling and promote tumorigenesis are not fully understood. To address these questions, we utilized in vitro reconstitution assays, as well as Apc knockdown or mutation in human cells, zebrafish, and mice. We find APC directly enhances activity of Glycogen Synthase Kinase-3 (GSK-3), an essential regulator of diverse signaling pathways. Furthermore, APC loss of function mimics GSK-3 inhibition by reducing phosphorylation of Glycogen Synthase and by activating mechanistic Target of Rapamycin in Complex 1 (mTORC1). Thus we identify novel roles for APC as a regulator of GSK-3 activity and GSK-3 dependent signaling. Wnts reduce GSK-3 activity through an unknown mechanism to activate downstream signaling. We find Wnts induce rapid APC dissociation from the GSK-3/Axin complex, suggesting a new mechanism for how Wnts reduce GSK-3 activity. GSK-3 also regulates mTORC1, and we find oncogenic Apc mutations activate mTORC1. Furthermore, many Apc mutant phenotypes are partially rescued by mTORC1 inhibition, while others are rescued by combined mTORC1 and Wnt inhibition. These observations demonstrate roles for mTORC1 activation downstream of Apc mutation. Taken together, our findings provide new insight into APC function and the effects of oncogenic Apc mutation

The Msi Family of RNA-Binding Proteins Function Redundantly as Intestinal Oncoproteins

Members of the Msi family of RNA-binding proteins have recently emerged as potent oncoproteins in a range of malignancies. MSI2 is highly expressed in hematopoietic cancers, where it is required for disease maintenance. In contrast to the hematopoietic system, colorectal cancers can express both Msi family members, MSI1 and MSI2. Here, we demonstrate that, in the intestinal epithelium, Msi1 and Msi2 have analogous oncogenic effects. Further, comparison of Msi1/2-induced gene expression programs and transcriptome-wide analyses of Msi1/2-RNA-binding targets reveal significant functional overlap, including induction of the PDK-Akt-mTORC1 axis. Ultimately, we demonstrate that concomitant loss of function of both MSI family members is sufficient to abrogate the growth of human colorectal cancer cells, and Msi gene deletion inhibits tumorigenesis in several mouse models of intestinal cancer. Our findings demonstrate that MSI1 and MSI2 act as functionally redundant oncoproteins required for the ontogeny of intestinal cancers

The mTORC1-mediated activation of ATF4 promotes protein and glutathione synthesis downstream of growth signals

The mechanistic target of rapamycin complex 1 (mTORC1) stimulates a coordinated anabolic program in response to growth-promoting signals. Paradoxically, recent studies indicate that mTORC1 can activate the transcription factor ATF4 through mechanisms distinct from its canonical induction by the integrated stress response (ISR). However, its broader roles as a downstream target of mTORC1 are unknown. Therefore, we directly compared ATF4-dependent transcriptional changes induced upon insulin-stimulated mTORC1 signaling to those activated by the ISR. In multiple mouse embryo fibroblast and human cancer cell lines, the mTORC1-ATF4 pathway stimulated expression of only a subset of the ATF4 target genes induced by the ISR, including genes involved in amino acid uptake, synthesis, and tRNA charging. We demonstrate that ATF4 is a metabolic effector of mTORC1 involved in both its established role in promoting protein synthesis and in a previously unappreciated function for mTORC1 in stimulating cellular cystine uptake and glutathione synthesis.ISSN:2050-084