TIS21(/BTG2) inhibits breast cancer growth and progression by differential regulation of mTORc1 and mTORc2-AKT1-NFAT1-PHLPP2 signaling axis

It has been reported that PI3K/AKT pathway is altered in various cancers and AKT isoforms specifically regulate cell growth and metastasis of cancer cells; AKT1, but not AKT2, reduces invasion of cancer cells but maintains cancer growth. We propose here a novel mechanism of the tumor suppresser, TIS21(/BTG2), that inhibits both growth and invasion of triple negative breast cancer cells via AKT1 activation by differential regulation of mTORc1 and mTORc2 activity. Transduction of adenovirus carrying TIS21(/BTG2) gene and transfection of short interfering RNAs were employed to regulate TIS21(/BTG2) gene expression in various cell lines. Treatment of mTOR inhibitors and mTOR kinase assays can evaluate the role of mTORc in the regulation of AKT phosphorylation at S473 residue by TIS21(/BTG2) in breast cancer cells. Open data and immunohistochemical analysis were performed to confirm the role of TIS21(/BTG2) expression in various human breast cancer tissues. We observed that TIS21(/BTG2) inhibited mTORc1 activity by reducing Raptor-mTOR interaction along with upregulation of tsc1 expression, which lead to significant reduction of p70S6K activation as opposed to AKT1(S473), but not AKT2, phosphorylation via downregulating PHLPP2 (AKT1-specific phosphatase) in breast cancers. TIS21(/BTG2)-induced pAKT(S473) required Rictor-bound mTOR kinase, indicating activation of mTORc2 by TIS21(/BTG2) gene. Additionally, the TIS21(/BTG2)-induced pAKT(S473) could reduce expression of NFAT1 (nuclear factor of activated T cells) and its target genes, which regulate cancer microenvironment. TIS21(/BTG2) significantly lost in the infiltrating ductal carcinoma, but it can inhibit cancer growth via the TIS21(/BTG2)-tsc1/2-mTORc1-p70S6K axis and downregulate cancer progression via the TIS21(/BTG2)-mTORc2-AKT1-NFAT1-PHLPP2 pathway.