The pro-apoptotic kinase Mst1 and its caspase cleavage products are direct inhibitors of Akt1

Protective effect of melatonin against blue light-induced cell damage via the TRPV1-YAP pathway in cultured human epidermal keratinocytes

Although blue light has been known to negatively affect skin cells, its detailed signaling mechanisms and anti-blue light agents have not been clearly elucidated. We investigated the involvement of Yes-associated protein (YAP)-mediated Hippo signaling in blue light-induced apoptosis, depending on the degree of blue light exposure. Additionally, we elucidated the effects of melatonin on blue light-irradiated keratinocytes and examined their action mechanisms. After blue light irradiation, its effects and antagonizing effects of melatonin on cell proliferation, apoptosis, DNA damage, and transient receptor potential vanilloid 1 (TRPV1)/YAP-mediated signaling were examined in HaCaT cells using western blots, image analysis, flow cytometric analysis, co-immunoprecipitation, and immunocytochemistry. We found that melatonin treatment attenuated the reduced cell viability and increased production of reactive oxygen species (ROS) in response to blue light irradiation. In the experiments to investigate the mechanism of action of blue light and melatonin, we found that YAP changed its binding protein, either p73 or TEAD, depending on the degree of blue light exposure. Melatonin treatment reduced blue light-induced phosphorylation of TRPV1 and MST1/2. Upon treatment with capsazepine, an antagonist of TRPV1, MST1/2 activation also reduced. Furthermore, we found that prolonged blue light irradiation induced DNA damage, which in turn induced YAP-p73 nuclear translocation. These effects were also notably attenuated by melatonin. These findings indicate that depending on the duration of blue light irradiation, two different YAP-mediated Hippo signaling pathways are activated. Additionally, these findings suggest that melatonin could be a potential therapeutic agent for blue light-induced skin damage.

Mechanism and therapeutic potential of hippo signaling pathway in type 2 diabetes and its complications

Loss of pancreatic islet cell mass and function is one of the most important factors in the development of type 2 diabetes mellitus, and hyperglycemia-induced lesions in other organs are also associated with apoptosis or hyperproliferation of the corresponding tissue cells. The Hippo signaling pathway is a key signal in the regulation of cell growth, proliferation and apoptosis, which has been shown to play an important role in the regulation of diabetes mellitus and its complications. Excessive activation of the Hippo signaling pathway under high glucose conditions triggered apoptosis and decreased insulin secretion in pancreatic islet cells, while dysregulation of the Hippo signaling pathway in the cells of other organ tissues led to proliferation or apoptosis and promoted tissue fibrosis, which aggravated the progression of diabetes mellitus and its complications. This article reviews the mechanisms of Hippo signaling, its individual and reciprocal regulation in diabetic pancreatic pathology, and its emerging role in the pathophysiology of diabetic complications. Potential therapeutics for diabetes mellitus that have been shown to target the Hippo signaling pathway are also summarized to provide information for the clinical management of type 2 diabetes mellitus.

MST2 Acts via AKT Activity to Promote Neurite Outgrowth and Functional Recovery after Spinal Cord Injury in Mice

Spinal cord injury (SCI) constitutes a significant clinical challenge, and there is extensive research focused on identifying molecular activities that can facilitate the repair of spinal cord injuries. Mammalian sterile 20-like kinase 2 (MST2), a core component of the Hippo signaling pathway, plays a key role in apoptosis and cell growth. However, its role in neurite outgrowth after spinal cord injury remains unknown. Through comprehensive in vitro and in vivo experiments, we demonstrated that MST2, predominantly expressed in neurons, actively participated in the natural development of the CNS. Post-SCI, MST2 expression significantly increased, indicating its activation and potential role in the early stages of neural recovery. Detailed analyses showed that MST2 knockdown impaired neurite outgrowth and motor function recovery, whereas MST2 overexpression led to the opposite effects, underscoring MST2's neuroprotective role in enhancing neural repair. Further, we elucidated the mechanism underlying MST2's action, revealing its interaction with AKT and positive regulation of AKT activity, a well-established promoter of neurite outgrowth. Notably, MST2's promotion of neurite outgrowth and motor functional recovery was diminished by AKT inhibitors, highlighting the dependency of MST2's neuroprotective effects on AKT signaling. In conclusion, our findings affirmed MST2's pivotal role in fostering neuronal neurite outgrowth and facilitating functional recovery after SCI, mediated through its positive modulation of AKT activity. In conclusion, our findings confirmed MST2's crucial role in neural protection, promoting neurite outgrowth and functional recovery after SCI through positive AKT activity modulation. These results position MST2 as a potential therapeutic target for SCI, offering new insights into strategies for enhancing neuroregeneration and functional restoration.

Hippo signaling modulation and its biological implications in urological malignancies

Although cancer diagnosis and treatment have rapidly advanced in recent decades, urological malignancies, which have high morbidity and mortality rates, are among the most difficult diseases to treat. The Hippo signaling is an evolutionarily conserved pathway in organ size control and tissue homeostasis maintenance. Its downstream effectors, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), are key modulators of numerous physiological and pathological processes. Recent work clearly indicates that Hippo signaling is frequently altered in human urological malignancies. In this review, we discuss the disparate viewpoints on the upstream regulators of YAP/TAZ and their downstream targets and systematically summarize the biological implications. More importantly, we highlight the molecular mechanisms involved in Hippo-YAP signaling to improve our understanding of its role in every stage of prostate cancer, bladder cancer and kidney cancer progression. A better understanding of the biological outcomes of YAP/TAZ modulation will contribute to the establishment of future therapeutic approaches.

Mst1-mediated phosphorylation of FoxO1 and C/EBP-β stimulates cell-protective mechanisms in cardiomyocytes

The molecular mechanisms by which FoxO transcription factors mediate diametrically opposite cellular responses, namely death and survival, remain unknown. Here we show that Mst1 phosphorylates FoxO1 Ser209/Ser215/Ser218/Thr228/Ser232/Ser243, thereby inhibiting FoxO1-mediated transcription of proapoptotic genes. On the other hand, Mst1 increases FoxO1-C/EBP-β interaction and activates C/EBP-β by phosphorylating it at Thr299, thereby promoting transcription of prosurvival genes. Myocardial ischemia/reperfusion injury is larger in cardiac-specific FoxO1 knockout mice than in control mice. However, the concurrent presence of a C/EBP-β T299E phospho-mimetic mutation reduces infarct size in cardiac-specific FoxO1 knockout mice. The C/EBP-β phospho-mimetic mutant exhibits greater binding to the promoter of prosurvival genes than wild type C/EBP-β. In conclusion, phosphorylation of FoxO1 by Mst1 inhibits binding of FoxO1 to pro-apoptotic gene promoters but enhances its binding to C/EBP-β, phosphorylation of C/EBP-β, and transcription of prosurvival genes, which stimulate protective mechanisms in the heart.

Targeting the Hippo Pathway in Cutaneous Melanoma

Melanoma is the most aggressive form of skin cancer. In the advanced stage of development, it is resistant to currently available therapeutic modalities. Increased invasiveness and metastatic potential depend on several proteins involved in various signal transduction pathways. Hippo signaling plays a vital role in malignant transformation. Dysfunctions of the Hippo pathway initiate the expression of tumor growth factors and are associated with tumor growth and metastasis formation. This review summarizes the recent achievements in studying the role of the Hippo pathway in melanoma pathogenesis and points to the potential specific targets for anti-melanoma therapy.

Inhibition of MST1 ameliorates neuronal apoptosis via GSK3β/β-TrCP/NRF2 pathway in spinal cord injury accompanied by diabetes

AIMS

Spinal cord injury (SCI) is a devastating neurological disease that often results in tremendous loss of motor function. Increasing evidence demonstrates that diabetes worsens outcomes for patients with SCI due to the higher levels of neuronal oxidative stress. Mammalian sterile 20-like kinase (MST1) is a key mediator of oxidative stress in the central nervous system; however, the mechanism of its action in SCI is still not clear. Here, we investigated the role of MST1 activation in induced neuronal oxidative stress in patients with both SCI and diabetes.

METHODS

Diabetes was established in mice by diet induction combined with intraperitoneal injection of streptozotocin (STZ). SCI was performed at T10 level through weight dropping. Advanced glycation end products (AGEs) were applied to mimic diabetic conditions in PC12 cell line in vitro. We employed HE, Nissl staining, footprint assessment and Basso mouse scale to evaluate functional recovery after SCI. Moreover, immunoblotting, qPCR, immunofluorescence and protein-protein docking analysis were used to detect the mechanism.

RESULTS

Regarding in vivo experiments, diabetes resulted in up-regulation of MST1, excessive neuronal apoptosis and weakened motor function in SCI mice. Furthermore, diabetes impeded NRF2-mediated antioxidant defense of neurons in the damaged spinal cord. Treatment with AAV-siMST1 could restore antioxidant properties of neurons to facilitate reactive oxygen species (ROS) clearance, which subsequently promoted neuronal survival to improve locomotor function recovery. In vitro model found that AGEs worsened mitochondrial dysfunction and increased cellular oxidative stress. While MST1 inhibition through the chemical inhibitor XMU-MP-1 or MST1-shRNA infection restored NRF2 nuclear accumulation and its transcription of downstream antioxidant enzymes, therefore preventing ROS generation. However, these antioxidant effects were reversed by NRF2 knockdown. Our in-depth studies showed that over-activation of MST1 in diabetes directly hindered the neuroprotective AKT1, and subsequently fostered NRF2 ubiquitination and degradation via the GSK3β/β-TrCP pathway.

CONCLUSION

MST1 inhibition significantly restores neurological function in SCI mice with preexisting diabetes, which is largely attributed to the activation of antioxidant properties via the GSK3β(Ser 9)/β-TrCP/NRF2 pathway. MST1 may be a promising pharmacological target for the effective treatment of spinal cord injury patients with diabetes.

The potential role of Hippo pathway regulates cellular metabolism via signaling crosstalk in disease-induced macrophage polarization

Macrophages polarized into distinct phenotypes play vital roles in inflammatory diseases by clearing pathogens, promoting tissue repair, and maintaining homeostasis. Metabolism serves as a fundamental driver in regulating macrophage polarization, and understanding the interplay between macrophage metabolism and polarization is crucial for unraveling the mechanisms underlying inflammatory diseases. The intricate network of cellular signaling pathway plays a pivotal role in modulating macrophage metabolism, and growing evidence indicates that the Hippo pathway emerges as a central player in network of cellular metabolism signaling. This review aims to explore the impact of macrophage metabolism on polarization and summarize the cell signaling pathways that regulate macrophage metabolism in diseases. Specifically, we highlight the pivotal role of the Hippo pathway as a key regulator of cellular metabolism and reveal its potential relationship with metabolism in macrophage polarization.

Discordant interactions between YAP1 and polycomb group protein SCML2 determine cell fate

The Polycomb group protein SCML2 and the transcriptional cofactor YAP1 regulate diverse cellular biology, including stem cell maintenance, developmental processes, and gene regulation in mammals and flies. However, their molecular and functional interactions are unknown. Here, we show that SCML2 interacts with YAP1, as revealed by immunological assays and mass spectroscopy. We have demonstrated that the steroid hormone androgen regulates the interaction of SCML2 with YAP1 in human tumor cell models. Our proximity ligation assay and GST pulldown showed that SCML2 and YAP1 physically interacted with each other. Silencing SCML2 by RNAi changed the growth behaviors of cells in response to androgen signaling. Mechanistically, this phenomenon is attributed to the interplay between distinct chromatin modifications and transcriptional programs, likely coordinated by the opposing SCML2 and YAP1 activity. These findings suggest that YAP1 and SCML2 cooperate to regulate cell growth, cell survival, and tumor biology downstream of steroid hormones.

Crosstalk between the mTOR and Hippo pathways

Cell behavior changes in response to multiple stimuli, such as growth factors, nutrients, and cell density. The mechanistic target of the rapamycin (mTOR) pathway is activated by growth factors and nutrient stimuli to regulate cell growth and autophagy, whereas the Hippo pathway has negative effects on cell proliferation and tissue growth in response to cell density, DNA damage, and hormonal signals. These two signaling pathways must be precisely regulated and integrated for proper cell behavior. This integrative mechanism is not completely understood; nevertheless, recent studies have suggested that components of the mTOR and Hippo pathways interact with each other. Herein, as per contemporary knowledge, we review the molecular mechanisms of the interaction between the mTOR and Hippo pathways in mammals and Drosophila. Moreover, we discuss the advantage of this interaction in terms of tissue growth and nutrient consumption.

The Molecular Mechanism of the TEAD1 Gene and miR-410-5p Affect Embryonic Skeletal Muscle Development: A miRNA-Mediated ceRNA Network Analysis

Muscle development is a complex biological process involving an intricate network of multiple factor interactions. Through the analysis of transcriptome data and molecular biology confirmation, this study aims to reveal the molecular mechanism underlying sheep embryonic skeletal muscle development. The RNA sequencing of embryos was conducted, and microRNA (miRNA)-mediated competitive endogenous RNA (ceRNA) networks were constructed. qRT-PCR, siRNA knockdown, CCK-8 assay, scratch assay, and dual luciferase assay were used to carry out gene function identification. Through the analysis of the ceRNA networks, three miRNAs (miR-493-3p, miR-3959-3p, and miR-410-5p) and three genes (TEAD1, ZBTB34, and POGLUT1) were identified. The qRT-PCR of the DE-miRNAs and genes in the muscle tissues of sheep showed that the expression levels of the TEAD1 gene and miR-410-5p were correlated with the growth rate. The knockdown of the TEAD1 gene by siRNA could significantly inhibit the proliferation of sheep primary embryonic myoblasts, and the expression levels of SLC1A5, FoxO3, MyoD, and Pax7 were significantly downregulated. The targeting relationship between miR-410-5p and the TEAD1 gene was validated by a dual luciferase assay, and miR-410-5p can significantly downregulate the expression of TEAD1 in sheep primary embryonic myoblasts. We proved the regulatory relationship between miR-410-5p and the TEAD1 gene, which was related to the proliferation of sheep embryonic myoblasts. The results provide a reference and molecular basis for understanding the molecular mechanism of embryonic muscle development.

Interplay of Developmental Hippo-Notch Signaling Pathways with the DNA Damage Response in Prostate Cancer

Prostate cancer belongs in the class of hormone-dependent cancers, representing a major cause of cancer incidence in men worldwide. Since upon disease onset almost all prostate cancers are androgen-dependent and require active androgen receptor (AR) signaling for their survival, the primary treatment approach has for decades relied on inhibition of the AR pathway via androgen deprivation therapy (ADT). However, following this line of treatment, cancer cell pools often become resistant to therapy, contributing to disease progression towards the significantly more aggressive castration-resistant prostate cancer (CRPC) form, characterized by poor prognosis. It is, therefore, of critical importance to elucidate the molecular mechanisms and signaling pathways underlying the progression of early-stage prostate cancer towards CRPC. In this review, we aim to shed light on the role of major signaling pathways including the DNA damage response (DDR) and the developmental Hippo and Notch pathways in prostate tumorigenesis. We recapitulate key evidence demonstrating the crosstalk of those pathways as well as with pivotal prostate cancer-related 'hubs' such as AR signaling, and evaluate the clinical impact of those interactions. Moreover, we attempt to identify molecules of the complex DDR-Hippo-Notch interplay comprising potentially novel therapeutic targets in the battle against prostate tumorigenesis.

Noncanonical HIPPO/MST Signaling via BUB3 and FOXO Drives Pulmonary Vascular Cell Growth and Survival

RATIONALE

The MSTs (mammalian Ste20-like kinases) 1/2 are members of the HIPPO pathway that act as growth suppressors in adult proliferative diseases. Pulmonary arterial hypertension (PAH) manifests by increased proliferation and survival of pulmonary vascular cells in small PAs, pulmonary vascular remodeling, and the rise of pulmonary arterial pressure. The role of MST1/2 in PAH is currently unknown.

OBJECTIVE

To investigate the roles and mechanisms of the action of MST1 and MST2 in PAH.

METHODS AND RESULTS

Using early-passage pulmonary vascular cells from PAH and nondiseased lungs and mice with smooth muscle-specific tamoxifen-inducible Mst1/2 knockdown, we found that, in contrast to canonical antiproliferative/proapoptotic roles, MST1/2 act as proproliferative/prosurvival molecules in human PAH pulmonary arterial vascular smooth muscle cells and pulmonary arterial adventitial fibroblasts and support established pulmonary vascular remodeling and pulmonary hypertension in mice with SU5416/hypoxia-induced pulmonary hypertension. By using unbiased proteomic analysis, gain- and loss-of function approaches, and pharmacological inhibition of MST1/2 kinase activity by XMU-MP-1, we next evaluated mechanisms of regulation and function of MST1/2 in PAH pulmonary vascular cells. We found that, in PAH pulmonary arterial adventitial fibroblasts, the proproliferative function of MST1/2 is caused by IL-6-dependent MST1/2 overexpression, which induces PSMC6-dependent downregulation of forkhead homeobox type O 3 and hyperproliferation. In PAH pulmonary arterial vascular smooth muscle cells, MST1/2 acted via forming a disease-specific interaction with BUB3 and supported ECM (extracellular matrix)- and USP10-dependent BUB3 accumulation, upregulation of Akt-mTORC1, cell proliferation, and survival. Supporting our in vitro observations, smooth muscle-specific Mst1/2 knockdown halted upregulation of Akt-mTORC1 in small muscular PAs of mice with SU5416/hypoxia-induced pulmonary hypertension.

CONCLUSIONS

Together, this study describes a novel proproliferative/prosurvival role of MST1/2 in PAH pulmonary vasculature, provides a novel mechanistic link from MST1/2 via BUB3 and forkhead homeobox type O to the abnormal proliferation and survival of pulmonary arterial vascular smooth muscle cells and pulmonary arterial adventitial fibroblasts, remodeling and pulmonary hypertension, and suggests new target pathways for therapeutic intervention.

Apoptosis in Type 2 Diabetes: Can It Be Prevented? Hippo Pathway Prospects

Diabetes mellitus is a heterogeneous disease of complex etiology and pathogenesis. Hyperglycemia leads to many serious complications, but also directly initiates the process of β cell apoptosis. A potential strategy for the preservation of pancreatic β cells in diabetes may be to inhibit the implementation of pro-apoptotic pathways or to enhance the action of pancreatic protective factors. The Hippo signaling pathway is proposed and selected as a target to manipulate the activity of its core proteins in therapy-basic research. MST1 and LATS2, as major upstream signaling kinases of the Hippo pathway, are considered as target candidates for pharmacologically induced tissue regeneration and inhibition of apoptosis. Manipulating the activity of components of the Hippo pathway offers a wide range of possibilities, and thus is a potential tool in the treatment of diabetes and the regeneration of β cells. Therefore, it is important to fully understand the processes involved in apoptosis in diabetic states and completely characterize the role of this pathway in diabetes. Therapy consisting of slowing down or stopping the mechanisms of apoptosis may be an important direction of diabetes treatment in the future.

Neuroprotective effect of L-deprenyl on the expression level of the Mst1 gene and inhibition of apoptosis in rat-model spinal cord injury

Objectives

After primary tissue damage as a result of spinal cord injury (SCI), there is a period of secondary damage, which includes several cellular and inflammatory biochemical cascades. As a novel pro-apoptotic kinase, Mst1 (serine/threonine kinase 4) promotes programmed cell death in an inflammatory disease model. This study aimed to evaluate Mst1 gene expression levels in rats with spinal cord injury treated with L- deprenyl.

Materials and Methods

The rats were divided into control (contusion), laminectomy, sham-operated (contused rats received 1 ml normal saline intraperitoneal), and treatment (contused rats received 5 mg/kg of L-deprenyl intraperitoneal; once a day for 7 days). The BBB (Basso, Beattie, and Bresnahan) scales were performed to assess motor function following SCI. Rats were sacrificed 28 days after SCI and the spinal cord lesion area was removed. Apoptosis and cavity formation in the spinal cord were determined by H&E staining and TUNEL assay, respectively. The mRNA levels of the Mst1, Nrf2, Bcl-2, and PGC1 α genes were analyzed using real-time quantitative PCR.

Results

The results showed significant improvement in motor function in the L- deprenyl group compared with the untreated group. Histological analysis showed a significant reduction in the number of tunnel-positive cells after injection of L-deprenyl, as well as a decrease in the volume of the cavity. In addition, L-deprenyl treatment increased the expression of the Nrf2, Bcl-2, and PGC1 α genes, while reducing the expression of the Mst1 gene in the spinal nerves.

Conclusion

These results suggest that L-deprenyl is a promising treatment for spinal cord injury.

The Hippo pathway: an emerging role in urologic cancers

The Hippo pathway controls several biological processes, including cell growth, differentiation, motility, stemness, cell contact, immune cell maturation, organ size, and tumorigenesis. The Hippo pathway core kinases MST1/2 and LATS1/2 in mammals phosphorylate and inactivate YAP1 signaling. Increasing evidence indicates that loss of MST1/2 and LATS1/2 function is linked to the biology of many cancer types with poorer outcomes, likely due to the activation of oncogenic YAP1/TEAD signaling. Therefore, there is a renewed interest in blocking the YAP1/TEAD functions to prevent cancer growth. This review introduces the Hippo pathway components and examines their role and therapeutic potentials in prostate, kidney, and bladder cancer.

Feedback, Crosstalk and Competition: Ingredients for Emergent Non-Linear Behaviour in the PI3K/mTOR Signalling Network

The PI3K/mTOR signalling pathway plays a central role in the governing of cell growth, survival and metabolism. As such, it must integrate and decode information from both external and internal sources to guide efficient decision-making by the cell. To facilitate this, the pathway has evolved an intricate web of complex regulatory mechanisms and elaborate crosstalk with neighbouring signalling pathways, making it a highly non-linear system. Here, we describe the mechanistic biological details that underpin these regulatory mechanisms, covering a multitude of negative and positive feedback loops, feed-forward loops, competing protein interactions, and crosstalk with major signalling pathways. Further, we highlight the non-linear and dynamic network behaviours that arise from these regulations, uncovered through computational and experimental studies. Given the pivotal role of the PI3K/mTOR network in cellular homeostasis and its frequent dysregulation in pathologies including cancer and diabetes, a coherent and systems-level understanding of the complex regulation and consequential dynamic signalling behaviours within this network is imperative for advancing biology and development of new therapeutic approaches.

Resistance to Targeted Therapy and RASSF1A Loss in Melanoma: What Are We Missing?

Melanoma is one of the most aggressive forms of skin cancer and is therapeutically challenging, considering its high mutation rate. Following the development of therapies to target BRAF, the most frequently found mutation in melanoma, promising therapeutic responses were observed. While mono- and combination therapies to target the MAPK cascade did induce a therapeutic response in BRAF-mutated melanomas, the development of resistance to MAPK-targeted therapies remains a challenge for a high proportion of patients. Resistance mechanisms are varied and can be categorised as intrinsic, acquired, and adaptive. RASSF1A is a tumour suppressor that plays an integral role in the maintenance of cellular homeostasis as a central signalling hub. RASSF1A tumour suppressor activity is commonly lost in melanoma, mainly by aberrant promoter hypermethylation. RASSF1A loss could be associated with several mechanisms of resistance to MAPK inhibition considering that most of the signalling pathways that RASSF1A controls are found to be altered targeted therapy resistant melanomas. Herein, we discuss resistance mechanisms in detail and the potential role for RASSF1A reactivation to re-sensitise BRAF mutant melanomas to therapy.

Targeting the Hippo Pathway in Prostate Cancer: What's New?

Simple Summary

Prostate cancer is the most commonly diagnosed cancer in men in the UK, accounting for the deaths of over 11,000 men per year. A major problem in this disease are tumours which no longer respond to available treatments. Understanding how this occurs will reveal new ways to treat these patients. In this review, the latest findings regarding a particular group of cellular factors which make up a signalling network called the Hippo pathway will be described. Accumulating evidence suggests that this network contributes to prostate cancer progression and resistance to current treatments. Identifying how this pathway can be targeted with drugs is a promising area of research to improve the treatment of prostate cancer.

Abstract

Identifying novel therapeutic targets for the treatment of prostate cancer (PC) remains a key area of research. With the emergence of resistance to androgen receptor (AR)-targeting therapies, other signalling pathways which crosstalk with AR signalling are important. Over recent years, evidence has accumulated for targeting the Hippo signalling pathway. Discovered in Drosophila melanogasta, the Hippo pathway plays a role in the regulation of organ size, proliferation, migration and invasion. In response to a variety of stimuli, including cell–cell contact, nutrients and stress, a kinase cascade is activated, which includes STK4/3 and LATS1/2 to inhibit the effector proteins YAP and its paralogue TAZ. Transcription by their partner transcription factors is inhibited by modulation of YAP/TAZ cellular localisation and protein turnover. Trnascriptional enhanced associate domain (TEAD) transcription factors are their classical transcriptional partner but other transcription factors, including the AR, have been shown to be modulated by YAP/TAZ. In PC, this pathway can be dysregulated by a number of mechanisms, making it attractive for therapeutic intervention. This review looks at each component of the pathway with a focus on findings from the last year and discusses what knowledge can be applied to the field of PC.

PHLPPing the Script: Emerging Roles of PHLPP Phosphatases in Cell Signaling

Whereas protein kinases have been successfully targeted for a variety of diseases, protein phosphatases remain an underutilized therapeutic target, in part because of incomplete characterization of their effects on signaling networks. The pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP) is a relatively new player in the cell signaling field, and new roles in controlling the balance among cell survival, proliferation, and apoptosis are being increasingly identified. Originally characterized for its tumor-suppressive function in deactivating the prosurvival kinase Akt, PHLPP may have an opposing role in promoting survival, as recent evidence suggests. Additionally, identification of the transcription factor STAT1 as a substrate unveils a role for PHLPP as a critical mediator of transcriptional programs in cancer and the inflammatory response. This review summarizes the current knowledge of PHLPP as both a tumor suppressor and an oncogene and highlights emerging functions in regulating gene expression and the immune system. Understanding the context-dependent functions of PHLPP is essential for appropriate therapeutic intervention.

YAP mediates the interaction between the Hippo and PI3K/Akt pathways in mesangial cell proliferation in diabetic nephropathy

AIMS

Glomerular mesangial cell (MC) proliferation is one of the main pathological changes in diabetic nephropathy (DN), but its mechanism needs further elaboration. The Hippo and PI3K/Akt signalling pathways are involved in the regulation of MC proliferation, but their relationship in hyperglycaemia-induced MC proliferation has not been reported.

METHODS

We used db/db mice and high-glucose-cultured mesangial cells to generate a diabetic nephropathy model. An MST1-knockdown plasmid was used to identify whether the PI3K/Akt pathway is linked to the Hippo pathway through MST1. LY294002 and SC79 were used to verify the role of the PI3K/Akt signalling pathway in MC cells. RNA silencing and overexpression were performed by using YAP and PTEN-expression/knockdown plasmids to investigate the function of YAP and PTEN, respectively, in the Hippo and PI3K/Akt signalling pathways.

RESULTS

By examining a potential feedback loop, we found decreased phosphorylation of MST1 and Lats1 and increased PI3K/Akt activation in db/db mice and high glucose-treated MCs, along with increased MC proliferation. The results of our gene silencing experiment proved PI3K/Akt-mediated intervention in the Hippo pathway and the regulatory effect of YAP on PI3K/Akt through PTEN.

CONCLUSIONS

The Hippo pathway is inhibited under diabetic conditions, leading to YAP activation and promoting MC proliferation. The PI3K/Akt pathway is activated through the inhibitory effect of YAP on its repressor, PTEN. Finally, activation of the PI3K/Akt pathway inhibits the Hippo pathway, resulting in nuclear YAP accumulation and accelerating MC proliferation and DN formation.

Downregulation of STK4 promotes colon cancer invasion/migration through blocking β-catenin degradation

Mammalian STE20-like kinase 1 (MST1/STK4/KRS2) encodes a serine/threonine kinase that is the mammalian homolog of Drosophila Hippo. STK4 plays an important role in controlling cell growth, apoptosis, and organ size. STK4 has been studied in many cancers with previous studies indicating an involvement in colon cancer lymph node metastasis and highlighting its potential as a diagnostic marker for colon cancer. However, the role of STK4 defect in promoting colon cancer progression is still understudied. Here, we found that STK4 was significantly downregulated in colon cancer and was associated with distal metastasis and poor survival. Furthermore, STK4 knockdown enhanced sphere formation and metastasis in vitro and promoted tumor development in vivo. We found that STK4 colocalized with β-catenin and directly phosphorylated β-catenin resulting in its degradation via the ubiquitin-mediated pathway. This may suggest that STK4 knockdown causes β-catenin phosphorylation failure and subsequently β-catenin accumulation, consequently leading to anchorage-independent growth and metastasis in colon cancer. Our results support that STK4 may act as a potential candidate for the assessment of β-catenin-mediated colon cancer prognosis.

Interaction of the Hippo Pathway and Phosphatases in Tumorigenesis

The Hippo pathway is an emerging tumor suppressor signaling pathway involved in a wide range of cellular processes. Dysregulation of different components of the Hippo signaling pathway is associated with a number of diseases including cancer. Therefore, identification of the Hippo pathway regulators and the underlying mechanism of its regulation may be useful to uncover new therapeutics for cancer therapy. The Hippo signaling pathway includes a set of kinases that phosphorylate different proteins in order to phosphorylate and inactivate its main downstream effectors, YAP and TAZ. Thus, modulating phosphorylation and dephosphorylation of the Hippo components by kinases and phosphatases play critical roles in the regulation of the signaling pathway. While information regarding kinase regulation of the Hippo pathway is abundant, the role of phosphatases in regulating this pathway is just beginning to be understood. In this review, we summarize the most recent reports on the interaction of phosphatases and the Hippo pathway in tumorigenesis. We have also introduced challenges in clarifying the role of phosphatases in the Hippo pathway and future direction of crosstalk between phosphatases and the Hippo pathway.

New insights into the core Hippo signaling and biological macromolecules interactions in the biology of solid tumors

As an evolutionarily conserved pathway, Hippo signaling pathway impacts different pathology and physiology processes such as wound healing, tissue repair/size and regeneration. When some components of Hippo signaling dysregulated, it affects cancer cells proliferation. Moreover, the relation Hippo pathway with other signaling including Wnt, TGFβ, Notch, and EGFR signaling leaves effect on the proliferation of cancer cells. Utilizing a number of therapeutic approaches, such as siRNAs and long noncoding RNA (lncRNA) to prevent cancer cells through the targeting of Hippo pathways, can provide new insights into cancer target therapy. The purpose of present review, first of all, is to demonstrate the importance of Hippo signaling and its relation with other signaling pathways in cancer. It also tries to demonstrate targeting Hippo signaling progress in cancer therapy.

RASSF1A Tumour Suppressor: Target the Network for Effective Cancer Therapy

The RASSF1A tumour suppressor is a scaffold protein that is involved in cell signalling. Increasing evidence shows that this protein sits at the crossroad of a complex signalling network, which includes key regulators of cellular homeostasis, such as Ras, MST2/Hippo, p53, and death receptor pathways. The loss of expression of RASSF1A is one of the most common events in solid tumours and is usually caused by gene silencing through DNA methylation. Thus, re-expression of RASSF1A or therapeutic targeting of effector modules of its complex signalling network, is a promising avenue for treating several tumour types. Here, we review the main modules of the RASSF1A signalling network and the evidence for the effects of network deregulation in different cancer types. In particular, we summarise the epigenetic mechanism that mediates RASSF1A promoter methylation and the Hippo and RAF1 signalling modules. Finally, we discuss different strategies that are described for re-establishing RASSF1A function and how a multitargeting pathway approach selecting druggable nodes in this network could lead to new cancer treatments.

Mst1/2 kinases restrain transformation in a novel transgenic model of Ras driven non-small cell lung cancer

Non-small cell lung cancer remains a highly lethal malignancy. Using the tamoxifen inducible Hnf1b:CreERT2 (H) transgenic mouse crossed to the LsL-KrasG12D (K) transgenic mouse, we recently discovered that an Hnf1b positive cell type in the lung is sensitive to adenoma formation when expressing a mutant KrasG12D allele. In these mice, we observe adenoma formation over a time frame of three to six months. To study specificity of the inducible Hnf1b:CreERT2 in the lung, we employed lineage tracing using an mTmG (G) reporter allele. This technique revealed recombined, GFP+ cells were predominantly SPC+. We further employed this technique in HKG mice to determine Hnf1b+ cells give rise to adenomas that express SPC and TTF1. Review of murine lung tissue confirmed a diagnosis of adenoma and early adenocarcinoma, a pathologic subtype of non-small cell lung cancer. Our expanded mouse model revealed loss of Mst1/2 promotes aggressive lung adenocarcinoma and large-scale proteomic analysis revealed upregulation of PKM2 in the lungs of mice with genetic deletion of Mst1/2. PKM2 is a known metabolic regulator in proliferating cells and cancer. Using a human lung adenocarcinoma cell line, we show pharmacologic inhibition of Mst1/2 increases the abundance of PKM2, indicating genetic loss or pharmacologic inhibition of Mst1/2 directly modulates the abundance of PKM2. In conclusion, here we report a novel model of non-small cell lung cancer driven by a mutation in Kras and deletion of Mst1/2 kinases. Tumor development is restricted to a subset of alveolar type II cells expressing Hnf1b. Our data show loss of Mst1/2 regulates levels of a potent metabolic regulator, PKM2.

The role of Hippo signaling pathway in physiological cardiac hypertrophy

Introduction: The role of Hippo signaling pathway, which was identified by genetic studies as a key regulator for tissue growth and organ size, in promoting physiological cardiac hypertrophy has not been investigated. Methods: Fourteen male Wistar rats were randomly assigned to the exercise and control groups. The exercise group ran 1 hour per day, 5 days/week, at about 65%-75% VO_2max on the motor-driven treadmill with 15º slope, and the control group ran 15 min/d, 2 days/week at 9 m/min (0º inclination), throughout the eight-week experimental period. Forty-eight hours after the last session, hearts were dissected and left ventricles were weighed and stored for subsequent RT-PCR analysis. Results: Despite a significant increase in the MAP4k1 expression levels in the exercise group (P = 0.001), the Mst1 expression was inhibited compared to the control group (P < 0.001) which was followed by suppression of Lats1 expression (P = 0.001). Compared with the control group, significant increases were observed in heart weight/body weight (P = 0.024) and left ventricular weight/body weight (P = 0.034) ratios in the exercise group. The H&amp;E staining confirmed the cardiac hypertrophy that may be partly due to a significant increase in Yap1 expression level compared with the control group (P <0.001), which was confirmed by Western blot analysis. Conclusion: Increased MAP4K1 expression did not influence Lats1 activation. The exercise training protocol suppressed Mst1 and Lats1 (Hippo pathway) and caused an increase in Yap1 expression level, which led to physiological cardiac hypertrophy in healthy rats. Further studies are suggested to apply this exercise protocol for the prevention and/or rehabilitation of cardiovascular disease and health promotion.

Mst1-Deficiency Induces Hyperactivation of Monocyte-Derived Dendritic Cells via Akt1/c-myc Pathway

Mst1 is a multifunctional serine/threonine kinase that is highly expressed in several immune organs. The role of Mst1 in the activation of dendritic cells (DCs), a key player of adaptive immunity, is poorly understood. In this study, we investigated the role of Mst1 in GM-CSF-induced bone marrow-derived DCs and the underlying mechanisms. Mst1^−/− DCs in response to GM-CSF expressed higher levels of activation/maturation-related cell surface molecules, such as B7 and MHC class II than Mst1^+/+ DCs. Furthermore, the expression of proinflammatory cytokines, such as IL-23, TNF-α, and IL-12p40, was increased in Mst1^−/− DCs, indicating that Mst1-deficiency may induce the hyperactivation of DCs. Additionally, Mst1^−/− DCs exhibited a stronger capacity to activate allogeneic T cells than Mst1^+/+ DCs. Silencing of Mst1 in DCs promoted their hyperactivation, similar to the phenotypes of Mst1^−/− DCs. Mst1^−/− DCs exhibited an increase in Akt1 phosphorylation and c-myc protein levels. In addition, treatment with an Akt1 inhibitor downregulated the protein level of c-myc increased in Mst1-deficient DCs, indicating that Akt1 acts as an upstream inducer of the de novo synthesis of c-myc. Finally, Akt1 and c-myc inhibitors downregulated the increased expression of IL-23p19 observed in Mst1-knockdown DCs. Taken together, these data demonstrate that Mst1 negatively regulates the hyperactivation of DCs through downregulation of the Akt1/c-myc axis in response to GM-CSF, and suggest that Mst1 is one of the endogenous factors that determine the activation status of GM-CSF-stimulated inflammatory DCs.

Suppressor of hepatocellular carcinoma RASSF1A activates autophagy initiation and maturation

RASSF1A (Ras association domain family 1 isoform A) is a tumor suppressor and frequently inactivated by promoter hypermethylation in hepatocellular carcinoma (HCC). Autophagy is to degrade misfolded or aggregated proteins and dysfunctional organelles. Autophagy defects enhance oxidative stress and genome instability to promote tumorigenesis. Activating autophagy flux by increasing levels of the RASSF1A-interacting microtubule-associated protein 1 S (MAP1S) leads to suppression of HCC in addition to extending lifespans. Here we tested whether RASSF1A itself functions as a HCC suppressor and activates autophagy similarly as MAP1S does. We show that RASSF1A deletion leads to an acceleration of diethylnitrosamine-induced HCC and a 31% reduction of median survival times in mice. RASSF1A enhances autophagy initiation by suppressing PI3K-AKT-mTOR through the Hippo pathway-regulatory component MST1 and promotes autophagy maturation by recruiting autophagosomes on RASSF1A-stabilized acetylated microtubules through MAP1S. RASSF1A deletion causes a blockade of autophagy flux. Therefore, RASSF1A may suppress HCC and improve survival by activating autophagy flux.

The hypoxia conditioned mesenchymal stem cells promote hepatocellular carcinoma progression through YAP mediated lipogenesis reprogramming

BACKGROUND

Tumor microenvironment (TME) plays a very important role in cancer progression. The mesenchymal stem cells (MSC), a major compartment of TME, have been shown to promote hepatocellular carcinoma (HCC) progression and metastasis. As hypoxia is a common feature of TME, it is essential to investigate the effects of hypoxia on MSC during HCC progression.

METHODS

The effects of hypoxia on MSC mediated cell proliferation and HCC progression were measured by cell counting kit-8 (CCK-8) assay, Edu incorporation assay and xenograft model. The role of cyclooxygenase 2 (COX2) during this process was evaluated via lentivirus mediated COX2 knockdown in MSC. We also assessed the levels and localization of yes-associated protein (YAP) in HCC cells by immunofluorescence, western blot and real-time PCR, in order to detect the alterations of Hippo pathway. The changes in lipogenesis was examined by triacylglycerol (TG) levels, BODIPY staining of neutral lipid, and lipogenic enzyme levels. The alterations in AKT/mTOR/SREBP1 pathway were measured by western blot. In addition, to evaluate the role of prostaglandin E receptor 4 (EP4) in MSC mediated cell proliferation under hypoxia, we manipulated the levels of EP4 in HCC cells via small interfering RNA (siRNA), EP4 antagonist or agonist.

RESULTS

We found that MSC under hypoxia condition (hypo-MSC) could promote proliferation of HCC cell lines and tumor growth in xenograft model. Hypoxia increased COX2 expression in MSC and promoted the secretion of prostaglandin E2 (PGE2), which then activated YAP in HCC cells and led to increased cell proliferation. Meanwhile, YAP activation enhanced lipogenesis in HCC cell lines by upregulating AKT/mTOR/SREBP1 pathway. Knockdown or overexpression of YAP significantly decreased or increased lipogenesis. Finally, EP4 was found to mediate the effects of hypo-MSC on YAP activation and lipogenesis of HCC cells.

CONCLUSIONS

Hypo-MSC can promote HCC progression by activating YAP and the YAP mediated lipogenesis through COX2/PGE2/EP4 axis. The communication between MSC and cancer cells may be a potential therapeutic target for inhibiting cancer growth.

MST1 Regulates Neuronal Cell Death via JNK/Casp3 Signaling Pathway in HFD Mouse Brain and HT22 Cells

Oxidative stress has been considered as the main mediator in neurodegenerative diseases. A high-fat diet (HFD) and metabolic diseases result in oxidative stress generation, leading to various neurodegenerative diseases via molecular mechanisms that remain largely unknown. Protein kinases play an important role in the homeostasis between cell survival and cell apoptosis. The mammalian sterile 20-like kinase-1 (MST1) protein kinase plays an important role in cellular apoptosis in different organ systems, including the central nervous system. In this study, we evaluated the MST1/c-Jun N-terminal kinase (JNK) dependent oxidative damage mediated cognitive dysfunction in HFD-fed mice and stress-induced hippocampal HT22 (mice hippocampal) cells. Our Western blot and immunofluorescence results indicate that HFD and stress-induced hippocampal HT22 cells activate MST1/JNK/Caspase-3 (Casp-3) signaling, which regulates neuronal cell apoptosis and beta-amyloid-cleaving enzyme (BACE1) expression and leads to impaired cognition. Moreover, MST1 expression inhibition by shRNA significantly reduced JNK/Casp-3 signaling. Our in vivo and in vitro experiments mimicking metabolic stress, such as a high-fat diet, hyperglycemia, and an inflammatory response, determined that MST1 plays a key regulatory role in neuronal cell death and cognition, suggesting that MST1 could be a potential therapeutic target for numerous neurodegenerative diseases.

Dysregulation of the Hippo pathway signaling in aging and cancer

Human beings are facing emerging degenerative and cancer diseases, in large part, as a consequence of increased life expectancy. In the near future, researchers will have to put even more effort into fighting these new challenges, one of which will be prevention of cancer while continuing to improve the aging process through this increased life expectancy. In the last few decades, relevance of the Hippo pathway on cancer has become an important study since it is a major regulator of organ size control and proliferation. However, its deregulation can induce tumors throughout the body by regulating cell proliferation, disrupting cell polarity, releasing YAP and TAZ from the Scribble complexes and facilitating survival gene expression via activation of TEAD transcription factors. This pathway is also involved in some of the most important mechanisms that control the aging processes, such as the AMP-activated protein kinase and sirtuin pathways, along with autophagy and oxidative stress response/antioxidant defense. This could be the link between two tightly connected processes that could open a broader range of targeted molecular therapies to fight aging and cancer. Therefore, available knowledge of the processes involved in the Hippo pathway during aging and cancer must necessarily be well understood.

The MEK-ERK-MST1 Axis Potentiates the Activation of the Extrinsic Apoptotic Pathway during GDC-0941 Treatment in Jurkat T Cells

The discrete activation of individual caspases is essential during T-cell development, activation, and apoptosis. Humans carrying nonfunctional caspase-8 and caspase-8 conditional knockout mice exhibit several defects in the progression of naive CD4⁺ T cells to the effector stage. MST1, a key kinase of the Hippo signaling pathway, is often presented as a substrate of caspases, and its cleavage by caspases potentiates its activity. Several studies have focused on the involvement of MST1 in caspase activation and also reported several defects in the immune system function caused by MST1 deficiency. Here, we show the rapid activation of the MEK-ERK-MST1 axis together with the cleavage and activation of caspase-3, -6, -7, -8, and -9 after PI3K signaling blockade by the selective inhibitor GDC-0941 in Jurkat T cells. We determined the phosphorylation pattern of MST1 using a phosphoproteomic approach and identified two amino acid residues phosphorylated in an ERK-dependent manner after GDC-0941 treatment together with a novel phosphorylation site at S21 residue, which was extensively phosphorylated in an ERK-independent manner during PI3K signaling blockade. Using caspase inhibitors and the inhibition of MST1 expression using siRNA, we identified an exclusive role of the MEK-ERK-MST1 axis in the activation of initiator caspase-8, which in turn activates executive caspase-3/-7 that finally potentiate MST1 proteolytic cleavage. This mechanism forms a positive feed-back loop that amplifies the activation of MST1 together with apoptotic response in Jurkat T cells during PI3K inhibition. Altogether, we propose a novel MEK-ERK-MST1-CASP8-CASP3/7 apoptotic pathway in Jurkat T cells and believe that the regulation of this pathway can open novel possibilities in systemic and cancer therapies.

Recombinant asialoerythropoetin protects HL-1 cardiomyocytes from injury via suppression of Mst1 activation

Background

Recombinant human erythropoietin (rhuEPO) and asialoerythropoietin (asialo-rhuEPO) are cardioprotective. However, the protective effects of rhuEPO could not be translated into clinical practice because of its hematopoiesis-associated side effects while non-erythropoietic asialo-rhuEPO is unavailable in large quantities for clinical studies. This study was designed to investigate the cardiomyocyte protective potential of plant-produced asialo-rhuEPO (asialo-rhuEPO^P) against staurosporine (STS)-induced injury in HL-1 murine cardiomyocytes and identify cellular pathway(s) responsible for its cardioprotection.

Methods

HL-1 cardiomyocytes were simultaneously treated with STS and asialo-rhuEPO^P. Cellular injury, apoptosis, and cell viabilities were measured by LDH assay, Hoechst staining and trypan blue exclusion method, respectively while western blotting was used to study its effects on apoptosis and autophagy hallmarks.

Results

Our results showed that 20 IU/ml asialo-rhuEPO^P provided 39% protection to cardiomyocytes compared to STS-treated cells, which is 2-fold better than that of mammalian cell-produce rhuEPO (rhuEPO^M). Asialo-rhuEPO^P was found to suppress activation of proapoptotic kinase Mst1 (mammalian Sterile-20-like kinase 1) and FOXO3, leading to inhibition of apoptotic pathway and restoration of autophagy as indicated by the reduction of fragmented/condensed nuclei, altered ratios of Bax/Bcl2, p-Bad/Bad, cytosol/mitochondrial cyt c and caspase-3 activation, and the restored levels of autophagy markers Beclin1, p62 and LC3B-II. Additionally, Akt was found to be activated and FOXO3 was phosphorylated on Ser253, suggesting inhibition of FOXO3 transcriptional function.

Conclusions

Asialo-rhuEPO^P-mediated cardioprotection occurs through activation of PI3K/Akt pathway leading to suppression of Mst1 activation and promoting cardiomyocyte survival.

General significance

Asialo-rhuEPO^P could be used to modulate Mst1 activity elevated under numerous pathological states.

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Recombinant asialo-rhuEPO protect HL-1 cardiomyocytes against STS-induced injury.

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Protective effect of recombinant asialo-rhuEPO is superior to sialylated EPO.

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Asialo-rhuEPO suppresses activation of proapoptotic kinase MSt1 by activating Akt.

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Asialo-rhuEPO restores autophagy and inhibits apoptosis to promote cell survival.

mTORC2 Signaling: A Path for Pancreatic β Cell's Growth and Function

The mechanistic target of rapamycin (mTOR) signaling pathway is an evolutionary conserved pathway that senses signals from nutrients and growth factors to regulate cell growth, metabolism and survival. mTOR acts in two biochemically and functionally distinct complexes, mTOR complex 1 (mTORC1) and 2 (mTORC2), which differ in terms of regulatory mechanisms, substrate specificity and functional outputs. While mTORC1 signaling has been extensively studied in islet/β-cell biology, recent findings demonstrate a distinct role for mTORC2 in the regulation of pancreatic β-cell function and mass. mTORC2, a key component of the growth factor receptor signaling, is declined in β cells under diabetogenic conditions and in pancreatic islets from patients with type 2 diabetes. β cell-selective mTORC2 inactivation leads to glucose intolerance and acceleration of diabetes as a result of reduced β-cell mass, proliferation and impaired glucose-stimulated insulin secretion. Thereby, many mTORC2 targets, such as AKT, PKC, FOXO1, MST1 and cell cycle regulators, play an important role in β-cell survival and function. This indicates mTORC2 as important pathway for the maintenance of β-cell homeostasis, particularly to sustain proper β-cell compensatory response in the presence of nutrient overload and metabolic demand. This review summarizes recent emerging advances on the contribution of mTORC2 and its associated signaling on the regulation of glucose metabolism and functional β-cell mass under physiological and pathophysiological conditions in type 2 diabetes.

Hippo-mediated suppression of IRS2/AKT signaling prevents hepatic steatosis and liver cancer

Nonalcoholic fatty liver disease (NAFLD) is a major risk factor for liver cancer; therefore, its prevention is an important clinical goal. Ablation of phosphatase and tensin homolog (PTEN) or the protein kinase Hippo signaling pathway induces liver cancer via activation of AKT or the transcriptional regulators YAP/TAZ, respectively; however, the potential for crosstalk between the PTEN/AKT and Hippo/YAP/TAZ pathways in liver tumorigenesis has thus far remained unclear. Here, we have shown that deletion of both PTEN and SAV1 in the liver accelerates the development of NAFLD and liver cancer in mice. At the molecular level, activation of YAP/TAZ in the liver of Pten-/- Sav1-/- mice amplified AKT signaling through the upregulation of insulin receptor substrate 2 (IRS2) expression. Both ablation of YAP/TAZ and activation of the Hippo pathway could rescue these phenotypes. A high level of YAP/ TAZ expression was associated with a high level of IRS2 expression in human hepatocellular carcinoma (HCC). Moreover, treatment with the AKT inhibitor MK-2206 or knockout of IRS2 by AAV-Cas9 successfully repressed liver tumorigenesis in Pten-/- Sav1-/- mice. Thus, our findings suggest that Hippo signaling interacts with AKT signaling by regulating IRS2 expression to prevent NAFLD and liver cancer progression and provide evidence that impaired crosstalk between these 2 pathways accelerates NAFLD and liver cancer.

The Hippo Signaling Pathway in Pancreatic β-Cells: Functions and Regulations

Hippo signaling is an evolutionarily conserved pathway that critically regulates development and homeostasis of various tissues in response to a wide range of extracellular and intracellular signals. As an emerging important player in many diseases, the Hippo pathway is also involved in the pathophysiology of diabetes on the level of the pancreatic islets. Multiple lines of evidence uncover the importance of Hippo signaling in pancreas development as well as in the regulation of β-cell survival, proliferation, and regeneration. Hippo therefore represents a potential target for therapeutic agents designed to improve β-cell function and survival in diabetes. In this review, we summarize recent data on the regulation of the Hippo signaling pathway in the pancreas/in pancreatic islets, its functions on β-cell homeostasis in physiology and pathophysiology, and its contribution toward diabetes progression. The current knowledge related to general mechanisms of action and the possibility of exploiting the Hippo pathway for therapeutic approaches to block β-cell failure in diabetes is highlighted.

Cholesterol and Cholesterol-Rich Membranes in Prostate Cancer: An Update

Cells maintain normal structure and function by responding appropriately to cues from the surrounding milieu. Extracellular stimuli are transduced from the surface through the plasma membrane by a complex series of interactions between ligands, their receptors and intracellular signaling partners (e.g., kinases, G proteins). Cholesterol-enriched membrane microdomains, generally referred to as “lipid rafts”, exist within the lipid bilayer of all mammalian cells and play an important role in signaling from the cell surface to various subcellular compartments. Lipid rafts have also been implicated in tumor growth and aggressiveness. Epidemiological evidence suggests that the modern Western diet, which contains substantial levels of cholesterol and other fatty substances, promotes prostate cancer progression. Consistent with this idea, prolonged inhibition of the cholesterol synthesis pathway by pharmacologic intervention in men has recently been associated with reduction in risk of advanced prostate cancer. In this review, we discuss the possibility that membrane cholesterol promotes prostate cancer progression by a mechanism that involves dysregulation of lipid raft-resident signaling complexes. This hypothesis provides new avenues for mechanistic studies as well as therapeutic intervention.

A Peroxidase Peroxiredoxin 1-Specific Redox Regulation of the Novel FOXO3 microRNA Target let-7

Precision in redox signaling is attained through posttranslational protein modifications such as oxidation of protein thiols. The peroxidase peroxiredoxin 1 (PRDX1) regulates signal transduction through changes in thiol oxidation of its cysteines. We demonstrate here that PRDX1 is a binding partner for the tumor suppressive transcription factor FOXO3 that directly regulates the FOXO3 stress response. Heightened oxidative stress evokes formation of disulfide-bound heterotrimers linking dimeric PRDX1 to monomeric FOXO3. Absence of PRDX1 enhances FOXO3 nuclear localization and transcription that are dependent on the presence of Cys31 or Cys150 within FOXO3. Notably, FOXO3-T32 phosphorylation is constitutively enhanced in these mutants, but nuclear translocation of mutant FOXO3 is restored with PI3K inhibition. Here we show that on H₂O₂ exposure, transcription of tumor suppressive miRNAs let-7b and let-7c is regulated by FOXO3 or PRDX1 expression levels and that let-7c is a novel target for FOXO3. Conjointly, inhibition of let-7 microRNAs increases let-7-phenotypes in PRDX1-deficient breast cancer cells. Altogether, these data ascertain the existence of an H₂O₂-sensitive PRDX1-FOXO3 signaling axis that fine tunes FOXO3 activity toward the transcription of gene targets in response to oxidative stress. Antioxid. Redox Signal. 28, 62-77.

Mapping the STK4/Hippo signaling network in prostate cancer cell

Dysregulation of MST1/STK4, a key kinase component of the Hippo-YAP pathway, is linked to the etiology of many cancers with poor prognosis. However, how STK4 restricts the emergence of aggressive cancer remains elusive. Here, we investigated the effects of STK4, primarily localized in the cytoplasm, lipid raft, and nucleus, on cell growth and gene expression in aggressive prostate cancer. We demonstrated that lipid raft and nuclear STK4 had superior suppressive effects on cell growth in vitro and in vivo compared with cytoplasmic STK4. Using RNA sequencing and bioinformatics analysis, we identified several differentially expressed (DE) genes that responded to ectopic STK4 in all three subcellular compartments. We noted that the number of DE genes observed in lipid raft and nuclear STK4 cells were much greater than cytoplasmic STK4. Our functional annotation clustering showed that these DE genes were commonly associated with oncogenic pathways such as AR, PI3K/AKT, BMP/SMAD, GPCR, WNT, and RAS as well as unique pathways such as JAK/STAT, which emerged only in nuclear STK4 cells. These findings indicate that MST1/STK4/Hippo signaling restricts aggressive tumor cell growth by intersecting with multiple molecular pathways, suggesting that targeting of the STK4/Hippo pathway may have important therapeutic implications for cancer.

Common and Distinctive Functions of the Hippo Effectors Taz and Yap in Skeletal Muscle Stem Cell Function

Hippo pathway downstream effectors Yap and Taz play key roles in cell proliferation and regeneration, regulating gene expression especially via Tead transcription factors. To investigate their role in skeletal muscle stem cells, we analyzed Taz in vivo and ex vivo in comparison with Yap. Small interfering RNA knockdown or retroviral‐mediated expression of wild‐type human or constitutively active TAZ mutants in satellite cells showed that TAZ promoted proliferation, a function shared with YAP. However, at later stages of myogenesis, TAZ also enhanced myogenic differentiation of myoblasts, whereas YAP inhibits such differentiation. Functionally, while muscle growth was mildly affected in Taz (gene Wwtr1^–/–) knockout mice, there were no overt effects on regeneration. Conversely, conditional knockout of Yap in satellite cells of Pax7^Cre‐ERT2/+: Yap^fl°^x/fl°^x:Rosa26^Lacz mice produced a regeneration deficit. To identify potential mechanisms, microarray analysis showed many common TAZ/YAP target genes, but TAZ also regulates some genes independently of YAP, including myogenic genes such as Pax7, Myf5, and Myod1 (ArrayExpress–E‐MTAB‐5395). Proteomic analysis revealed many novel binding partners of TAZ/YAP in myogenic cells, but TAZ also interacts with proteins distinct from YAP that are often involved in myogenesis and aspects of cytoskeleton organization (ProteomeXchange–PXD005751). Neither TAZ nor YAP bind members of the Wnt destruction complex but both regulated expression of Wnt and Wnt‐cross talking genes with known roles in myogenesis. Finally, TAZ operates through Tead4 to enhance myogenic differentiation. In summary, Taz and Yap have overlapping functions in promoting myoblast proliferation but Taz then switches to enhance myogenic differentiation. Stem Cells2017;35:1958–1972

Hippo vs. Crab: tissue-specific functions of the mammalian Hippo pathway

The Hippo signaling pathway is a vital suppressor of tumorigenesis that is often inactivated in human cancers. In normal cells, the Hippo pathway is triggered by external forces such as cell crowding, or changes to the extracellular matrix or cell polarity. Once activated, Hippo signaling down-regulates transcription supported by the paralogous cofactors YAP1 and TAZ. The Hippo pathway's functions in normal and cancer biology have been dissected by studies of mutant mice with null or conditional tissue-specific mutations of Hippo signaling elements. In this review, we attempt to systematically summarize results that have been gleaned from detailed in vivo characterizations of these mutants. Our goal is to describe the physiological roles of Hippo signaling in several normal organ systems, as well as to emphasize how disruption of the Hippo pathway, and particularly hyperactivation of YAP1/TAZ, can be oncogenic.

Dissecting major signaling pathways in prostate cancer development and progression: Mechanisms and novel therapeutic targets

Prostate cancer (PCa) is the most frequently diagnosed non-cutaneous malignancy and leading cause of cancer mortality in men. At the initial stages, prostate cancer is dependent upon androgens for their growth and hence effectively combated by androgen deprivation therapy (ADT). However, most patients eventually recur with an androgen deprivation-resistant phenotype, referred to as castration-resistant prostate cancer (CRPC), a more aggressive form for which there is no effective therapy presently available. The current review is an attempt to cover and establish an understanding of some major signaling pathways implicated in prostate cancer development and castration-resistance, besides addressing therapeutic strategies that targets the key signaling mechanisms.

Dendritic cell MST1 inhibits Th17 differentiation

Although the differentiation of CD4⁺T cells is widely studied, the mechanisms of antigen-presenting cell-dependent T-cell modulation are unclear. Here, we investigate the role of dendritic cell (DC)-dependent T-cell differentiation in autoimmune and antifungal inflammation and find that mammalian sterile 20-like kinase 1 (MST1) signalling from DCs negatively regulates IL-17 producing-CD4⁺T helper cell (Th17) differentiation. MST1 deficiency in DCs increases IL-17 production by CD4⁺T cells, whereas ectopic MST1 expression in DCs inhibits it. Notably, MST1-mediated DC-dependent Th17 differentiation regulates experimental autoimmune encephalomyelitis and antifungal immunity. Mechanistically, MST1-deficient DCs promote IL-6 secretion and regulate the activation of IL-6 receptor α/β and STAT3 in CD4⁺T cells in the course of inducing Th17 differentiation. Activation of the p38 MAPK signal is responsible for IL-6 production in MST1-deficient DCs. Thus, our results define the DC MST1–p38MAPK signalling pathway in directing Th17 differentiation.

The differentiation of Th17 cells is central to infection and autoimmunity. Here, the authors show that expression of MST1 by dendritic cells limits IL-6 production and thereby controls Th17 differentiation in immunity to fungal infection and experimental autoimmune encephalomyelitis.

MST1/MST2 Protein Kinases: Regulation and Physiologic Roles

The MST1 and MST2 protein kinases comprise the GCK-II subfamily of protein kinases. In addition to their amino-terminal kinase catalytic domain, related to that of the Saccharomyces cerevisiae protein kinase Ste20, their most characteristic feature is the presence near the carboxy terminus of a unique helical structure called a SARAH domain; this segment allows MST1/MST2 to homodimerize and to heterodimerize with the other polypeptides that contain SARAH domains, the noncatalytic polypeptides RASSF1-6 and Sav1/WW45. Early studies emphasized the potent ability of MST1/MST2 to induce apoptosis upon being overexpressed, as well as the conversion of the endogenous MST1/MST2 polypeptides to constitutively active, caspase-cleaved catalytic fragments during apoptosis initiated by any stimulus. Later, the cleaved, constitutively active form of MST1 was identified in nonapoptotic, quiescent adult hepatocytes as well as in cells undergoing terminal differentiation, where its presence is necessary to maintain those cellular states. The physiologic regulation of full length MST1/MST2 is controlled by the availability of its noncatalytic SARAH domain partners. Interaction with Sav1/WW45 recruits MST1/MST2 into a tumor suppressor pathway, wherein it phosphorylates and activates the Sav1-bound protein kinases Lats1/Lats2, potent inhibitors of the Yap1 and TAZ oncogenic transcriptional regulators. A constitutive interaction with the Rap1-GTP binding protein RASSF5B (Nore1B/RAPL) in T cells recruits MST1 (especially) and MST2 as an effector of Rap1's control of T cell adhesion and migration, a program crucial to immune surveillance and response; loss of function mutation in human MST1 results in profound immunodeficiency. MST1 and MST2 are also regulated by other protein kinases, positively by TAO1 and negatively by Par1, SIK2/3, Akt, and cRaf1. The growing list of candidate MST1/MST2 substrates suggests that the full range of MST1/MST2's physiologic programs and contributions to pathophysiology remains to be elucidated.

MST1: a promising therapeutic target to restore functional beta cell mass in diabetes

The loss of insulin-producing beta cells by apoptosis is a hallmark of all forms of diabetes mellitus. Strategies to prevent beta cell apoptosis and dysfunction are urgently needed to restore the insulin-producing cells and to prevent severe diabetes progression. We recently identified the serine/threonine kinase known as mammalian sterile 20-like kinase 1 (MST1) as a critical regulator of apoptotic beta cell death and dysfunction. MST1 activates several apoptotic signalling pathways, which further stimulate its own cleavage, leading to a vicious cycle of cell death. This led us to hypothesise that MST1 signalling is central to the initiation of beta cell death in diabetes. We found that MST1 is strongly activated in a diabetic beta cell and induces not only its death but also directly impairs insulin secretion through promoting proteasomal degradation of key beta cell transcription factor, pancreatic and duodenal homeobox 1 (PDX1), which is critical for insulin production.Pre-clinical studies in various animal models of diabetes have reported that MST1 deficiency remarkably restores normoglycaemia and beta cell function and prevents the development of diabetes. Importantly, MST1 deficiency can revert fully diabetic beta cells to a non-diabetic state. MST1 may serve as a target for the development of novel therapies for diabetes that trigger the cause of the disease, namely, the destruction of the beta cells. The major current focus of our investigation is to identify and test the efficacy of potent inhibitors of this death signalling pathway to protect beta cells against the effects of autoimmune attack in type 1 diabetes and to preserve beta cell mass and function in type 2 diabetes. This review summarises a presentation given at the 'Can we make a better beta cell?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Heiko Lickert and colleagues, DOI: 10.1007/s00125-016-3949-9 , and by Harry Heimberg and colleagues, DOI: 10.1007/s00125-016-3879-6 ) and a commentary by the Session Chair, Shanta Persaud (DOI: 10.1007/s00125-016-3870-2 ).

Molecular chaperone Hsp27 regulates the Hippo tumor suppressor pathway in cancer

Heat shock protein 27 (Hsp27) is a molecular chaperone highly expressed in aggressive cancers, where it is involved in numerous pro-tumorigenic signaling pathways. Using functional genomics we identified for the first time that Hsp27 regulates the gene signature of transcriptional co-activators YAP and TAZ, which are negatively regulated by the Hippo Tumor Suppressor pathway. The Hippo pathway inactivates YAP by phosphorylating and increasing its cytoplasmic retention with the 14.3.3 proteins. Gain and loss of function experiments in prostate, breast and lung cancer cells showed that Hsp27 knockdown induced YAP phosphorylation and cytoplasmic localization while overexpression of Hsp27 displayed opposite results. Mechanistically, Hsp27 regulates the Hippo pathway by accelerating the proteasomal degradation of ubiquitinated MST1, the core Hippo kinase, resulting in reduced phosphorylation/activity of LATS1 and MOB1, its downstream effectors. Importantly, our in vitro results were supported by data from human tumors; clinically, high expression of Hsp27 in prostate tumors is correlated with increased expression of YAP gene signature and reduced phosphorylation of YAP in lung and invasive breast cancer clinical samples. This study reveals for the first time a link between Hsp27 and the Hippo cascade, providing a novel mechanism of deregulation of this tumor suppressor pathway across multiple cancers.

The MST/Hippo Pathway and Cell Death: A Non-Canonical Affair

The MST/Hippo signalling pathway was first described over a decade ago in Drosophila melanogaster and the core of the pathway is evolutionary conserved in mammals. The mammalian MST/Hippo pathway regulates organ size, cell proliferation and cell death. In addition, it has been shown to play a central role in the regulation of cellular homeostasis and it is commonly deregulated in human tumours. The delineation of the canonical pathway resembles the behaviour of the Hippo pathway in the fly where the activation of the core kinases of the pathway prevents the proliferative signal mediated by the key effector of the pathway YAP. Nevertheless, several lines of evidence support the idea that the mammalian MST/Hippo pathway has acquired new features during evolution, including different regulators and effectors, crosstalk with other essential signalling pathways involved in cellular homeostasis and the ability to actively trigger cell death. Here we describe the current knowledge of the mechanisms that mediate MST/Hippo dependent cell death, especially apoptosis. We include evidence for the existence of complex signalling networks where the core proteins of the pathway play a central role in controlling the balance between survival and cell death. Finally, we discuss the possible involvement of these signalling networks in several human diseases such as cancer, diabetes and neurodegenerative disorders.