Editors' ChoiceCell Biology

PKA Augments Hippo Signaling

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Science Signaling  18 Jun 2013:
Vol. 6, Issue 280, pp. ec138
DOI: 10.1126/scisignal.2004422

The Hippo pathway plays an important role in limiting cell growth during development and homeostasis. Activation of the kinases MST1 and MST2 (MST1/2, homologous to invertebrate Hippo) leads to activation of the kinases LATS1 and LATS2 (LATS1/2), which phosphorylate the transcriptional coactivators YAP and TAZ to prevent their nuclear translocation and activation of growth-promoting genes. Nuclear accumulation of YAP can also be inhibited by signaling through G protein–coupled receptors (GPCRs) and by disruption of the actin cytoskeleton or loss of tension on actin stress fibers. Yu et al. and Kim et al. report that protein kinase A (PKA) mediates phosphorylation of YAP downstream of both GPCR signaling and cytoskeletal changes. Yu et al. induced cyclic adenosine monophosphate (cAMP) accumulation in cultured breast cancer cells by treating the cells with inhibitors of cAMP degradation or with epinephrine, a ligand of the β2 adrenergic receptor (a GPCR), and this increase in cAMP resulted in phosphorylation of YAP and TAZ and a decrease in the abundance of TAZ. Stimulating cAMP accumulation with forskolin also induced YAP phosphorylation in several additional cultured human and mouse cell lines. cAMP stimulates the activity of PKA, and various methods for inhibiting PKA activity resulted in the reduction or complete elimination of forskolin-induced phosphorylation of YAP in several cell types. Whereas LATS1/2 was required for YAP phosphorylation in response to forskolin treatment in HEK293A cells, MST1/2 was not required. The in vitro kinase activity of LATS1 immunoprecipitated from cells treated with forskolin was increased compared with LATS1 isolated from untreated cells. Experiments with inhibitors of the Rho small guanosine triphosphatases (GTPases) and with a constitutively active form of RhoA indicated that PKA induced phosphorylation of YAP through Rho GTPases rather than acting on LATS1/2 directly. Finally, PKA-dependent inhibition of YAP and TAZ was required for PKA-induced differentiation of mouse fibroblasts into adipocytes, and in vivo experiments in Drosophila melanogaster were consistent with PKA inhibiting the activity of the YAP homolog Yki to limit cell proliferation and promote apoptosis during development. In a related study, Kim et al. found that PKA and LATS1/2, but not MST1/2, were required for YAP phosphorylation in NIH3T3 cells in response to cell detachment, which reduces tension on actin stress fibers, or treatment with the actin-depolymerizing drug latrunculin B. In contrast to the findings of Yu et al., in vitro kinase assays and experiments in transfected cells performed by Kim et al. indicated that PKA could directly phosphorylate LATS1 and LATS2 in vitro and that neurofibromin 2 (NF2), which can act as a PKA-anchoring protein (AKAP) and had been previously implicated in Hippo signaling, was required for PKA-mediated phosphorylation of LATS1 and YAP in 293T cells. Kim et al. also demonstrated that PKA-mediated inhibition of YAP was important for promoting anoikis (cell death in response to loss of cell contacts) in NIH3T3 cells and for modulating the balance between proliferation and differentiation of neural progenitor cells in the chick embryo. Despite the conflicting results regarding whether LATS1/2 are direct targets of PKA, these studies establish the Hippo pathway as an output of PKA signaling and provide another example of an input that can influence Hippo-mediated growth control.

F.-X. Yu, Y. Zhang, H. W. Park, J. L. Jewell, Q. Chen, Y. Deng, D. Pan, S. S. Taylor, Z.-C. Lai, K.-L. Guan, Protein kinase A activates the Hippo pathway to modulate cell proliferation and differentiation. Genes Dev. 27, 1223–1232 (2013). [Abstract] [Full Text]

M. Kim, M. Kim, S. Lee, S. Kuninaka, H. Saya, H. Lee, S. Lee, D.-S. Lim, cAMP/PKA signalling reinforces the LATS-YAP pathway to fully suppress YAP in response to actin cytoskeletal changes. EMBO J. 32, 1543–1555 (2013). [PubMed]