DescriptionHippo signaling controls organ size during development from Drosophila to humans. It limits tissue growth through regulation of cell proliferation and apoptosis. Deregulation of Hippo signaling leads to tissue overgrowth and tumor formation. An important characteristic of Hippo signaling is its connection to other pathways, through which Hippo signaling plays an indispensable role in diverse biological processes. We found, in addition to regulating growth during development, Hippo signaling regulates growth during regeneration. The growth suppression effect of Hippo signaling is through inhibition of a transcriptional co-activator Yorkie (Yki). Yki can promote cell proliferation and suppress apoptosis. Using the Drosophila wing imaginal disc, a mono-layered epithelial tissue, as a model, we found Yki is activated in the wing discs in response to apoptosis induction. Yki remains active over the entire wing disc regeneration. Yki activation during regeneration requires c-Jun N-terminal kinase (JNK). JNK activation of Yki is independent of cell death, and through a Hippo pathway component, Jub (a fly Ajuba LIM domain protein). Both Yki and Jub are required for wing regeneration. We also observed activation of Yki in the neoplastic tumors caused by disruption of epithelial apical-basal polarity. The growth of the neoplastic tumors is JNK-dependent. Like in regeneration, Yki activation in the neoplastic tumors also requires regulation of Jub by JNK. Reduction of Yki or Jub level suppresses tumor growth. Our work identified Jnk regulation of Hippo signaling as an important mechanism to drive the stress-responsive tissue growth, and established Jub and Yki as stress-responsive molecules. JNK regulation of Yki is also conserved in mammalian cells. Using cultured epithelial cell lines, we found manipulation of JNK activity affects the activity of YAP (the mammalian homolog of Yki), like what we observed in flies. And JNK influences the binding of the mammalian Ajuba proteins to LATS (the mammalian homolog of Wts, a core kinase that suppresses Yki activity) through a phosphorylation-dependent mechanism. Ajuba proteins may serve as a hub connecting multiple pathways to the Hippo pathway as the binding between Ajuba proteins and LATS can also be regulated by other signaling pathways. The activity of Ajuba proteins may depend on their conformation.