Activation of Hippo signaling limits cell growth and proliferation through a kinase cascade that results in the phosphorylation and inhibition of the transcriptional coactivator YAP by the kinases LATS1 and 2 (LATS1/2). In contrast, the mammalian target of rapamycin (mTOR) pathway promotes cell growth and proliferation. In the presence of growth factors that activate the lipid kinase phosphoinositide 3-kinase (PI3K), mTOR phosphorylates and activates various substrates, including the kinase Akt, the ribosomal protein S6, and p70 ribosomal S6 kinase (S6K). Tumaneng et al. (see also Csibi and Blenis) investigated the mechanisms of crosstalk between the Hippo and mTOR pathways. Knockdown of LATS1/2 or overexpression of YAP in MCF10A cells resulted in increased phosphorylation and activation of S6K and Akt, whereas knockdown of YAP decreased S6K and Akt phosphorylation. Mice that overexpressed YAP showed increased phosphorylation of S6K and Akt in the intestinal crypt compartment compared with control mice. YAP overexpression in MCF10A cells decreased the abundance of endogenous PTEN, a lipid phosphatase that opposes activation of the mTOR pathway. In YAP-overexpressing cells, the increase in the phosphorylation of S6K and Akt was attenuated by overexpression of PTEN, and, in YAP-deficient cells, phosphorylation of these mTOR substrates was restored by knockdown of PTEN. In addition, PTEN abundance was decreased in MCF10A cells by thrombin treatment, which activates YAP, and in the livers of mice lacking the Hippo pathway kinases MST1 and 2. Genome-wide deep sequencing of YAP-overexpressing MCF10A cells revealed increased abundance of microRNAs in the miR-29 family, and these PTEN-targeting microRNAs were also increased in control MCF10A cells by thrombin treatment. Transfection of a third miR-29 family member, miR-29c, into HEK293T cells reduced the activity of a luciferase reporter containing the 3′-UTR of PTEN and PTEN abundance. Chromatin immunoprecipitation assays in MCF10A cells showed that YAP bound to the promoter of miR-29c. The decrease in PTEN abundance and the increase in the phosphorylation of S6K and S6 caused by overexpression of YAP in MCF10A cells were attenuated by transfection of miR-29a, miR-29b, or miR-29c. YAP overexpression increased the size of MCF10A cells, which was attenuated by treatment with the mTOR inhibitor rapamycin or the PI3K inhibitor LY294002. Mice that overexpress YAP in the epidermis show thickening and hyperplasia of the epidermis, a phenotype that was reduced by treatment with LY294002, which also reduced phosphorylation of S6. Thus, inhibition of the Hippo pathway results in the transcriptional activation of PTEN-targeting microRNAs by YAP, thereby stimulating the mTOR pathway.
K. Tumaneng, K. Schlegelmilch, R. C. Russell, D. Yimlamai, H. Basnet, N. Mahadevan, J. Fitamant, N. Bardeesy, F. D. Camargo, K.-L. Guan, YAP mediates crosstalk between the Hippo and PI(3)K–TOR pathways by suppressing PTEN via miR-29. Nat. Cell Biol. 14, 1322–1329 (2012). [PubMed]
A. Csibi, J. Blenis, Hippo–YAP and mTOR pathways collaborate to regulate organ size. Nat. Cell Biol. 14, 1244–1245 (2012). [PubMed]