Hippo Signaling: A Stress Response Pathway in Stem Cells.

The Hippo pathway, with its core components and the downstream transcriptional coactivators, controls the self-renewable capacity and stemness features of stem cells and serves as a stress response pathway by regulating proliferation, differentiation and apoptosis. The Hippo pathway interaction with other signaling pathways plays an important role in response to various stress stimuli arising from energy metabolism, hypoxia, reactive oxygen species, and mechanical forces. Depending on the energy levels, the Hippo pathway is regulated by AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR), which in turn determines stem cell proliferation (cell survival and growth) and differentiation. Oxidative stress-driven by ROS production also affects the Hippo pathway with transcriptional changes through MST/YAP/FoxO pathway and leads to the activation of pro-apoptotic genes and eventually cell death. HIF1alpha/YAP signaling is critical for the long-term maintenance of mesenchymal stem cells (MSCs) under hypoxia. In this review, we present an overview of stem cell response to stress, including mechanical, hypoxia, metabolic and oxidative stress through the modulation of the Hippo pathway. The biological effects such as autophagy, apoptosis and senescence were discussed in the context of the Hippo pathway in stem cells.

The effect of melatonin on Hippo signaling pathway in dental pulp stem cells.

Dental pulp stem cells (DPSCs) have a high capacity to differentiate into the neuronal cell lineage. Meanwhile, both Hippo signaling and melatonin are key regulators in neuronal differentiation of neuronal progenitor cells. Recently emerging evidences suggest the possible interaction between melatonin and Hippo signaling in different cell lines. But underlying mechanisms involved in the initiation or progression of neurogenic differentiation in DPSCs through this connection need to be explored. Therefore, the scope of this study is to investigate the effect of melatonin on Hippo signaling pathway through the expression of its downstream effector (YAP/p-YAPY357) after the neuronal differentiation of DPSCs. In regard with this, DPSCs were incubated with growth and dopaminergic neuronal differentiation medium with or without melatonin (10 µM) for 21 days. The morphological changes were followed by phase contrast microscopy and differentiation of DPSCs was evaluated by immunofluorescence labelling with NeuN, GFAP, and tyrosine hydroxylase. Furthermore, we evaluated the presence of neural progenitor cells by nestin immunoreactivity. Hippo signaling pathway was investigated by evaluating the immunoreactivity of YAP and p-YAPY357. Our results were also supported by western-blot analysis and SOX2, PCNA and caspase-3 were also evaluated. The positive immunoreactivity for NeuN, tyrosine hydroxylase and negative immunoreactivity for GFAP showed the successful differentiation of DPSCs to neurons, not glial cells. Melatonin addition to dopaminergic media induced tyrosine hydroxylase and decreased significantly nestin expression. The expressions of PCNA and caspase-3 were also decreased significantly with melatonin addition into growth media. Melatonin treatment induced phosphorylation of YAPY357 and reduced YAP expression. In conclusion, melatonin has potential to induce neuronal differentiation and reduce the proliferation of DPSCs by increasing phosphorylation of YAPY357 and eliminating the activity of YAP, which indicates the active state of Hippo signaling pathway.