Lymphatic malformations: mechanistic insights and evolving therapeutic frontiers.

The lymphatic vascular system is gaining recognition for its multifaceted role and broad pathological significance. Once perceived as a mere conduit for interstitial fluid and immune cell transport, recent research has unveiled its active involvement in critical physiological processes and common diseases, including inflammation, autoimmune diseases, and atherosclerosis. Consequently, abnormal development or functionality of lymphatic vessels can result in serious health complications. Here, we discuss lymphatic malformations (LMs), which are localized lesions that manifest as fluid-filled cysts or extensive infiltrative lymphatic vessel overgrowth, often associated with debilitating, even life-threatening, consequences. Genetic causes of LMs have been uncovered, and several promising drug-based therapies are currently under investigation and will be discussed.

The Tumor Suppressor MIG6 Controls Mitotic Progression and the G2/M DNA Damage Checkpoint by Stabilizing the WEE1 Kinase.

MIG6 is an important tumor suppressor that binds to and negatively regulates epidermal growth factor receptor (EGFR). Here, we report an EGFR-independent function for MIG6 as an integral component of the cell cycle machinery. We found that depletion of MIG6 causes accelerated entry into and delayed exit from mitosis. This is due to premature and prolonged activation of CDK1, a key regulator of mitotic progression at the G2/M and meta- and anaphase transitions. Furthermore, MIG6 is required for inhibition of CDK1 upon DNA damage and subsequent G2/M cell cycle arrest. Mechanistically, we found that MIG6 depletion results in reduced phosphorylation of CDK1 on the inhibitory WEE1-targeted tyrosine-15 residue. MIG6 interacts with WEE1 and promotes its stability by interfering with the recruitment of the ßTrCP-SCF E3 ubiquitin ligase and consequent proteasomal degradation of WEE1. Our findings uncover a critical role of MIG6 in cell cycle progression that is likely to contribute to its potent tumor-suppressive properties.

Activated Cdc42-associated kinase 1 (Ack1) is required for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor recruitment to lipid rafts and induction of cell death.

TNF-related apoptosis-inducing ligand (TRAIL) holds promise for treatment of cancer due to its ability to selectively kill cancer cells while sparing normal cells. Ligand-induced translocation of TRAIL receptors (TRAIL-R) 1 and 2 (also called DR4 and DR5, respectively) into lipid raft membrane microdomains is required for TRAIL-induced cell death by facilitating receptor clustering and formation of the death-inducing signaling complex, yet the underlying regulatory mechanisms remain largely unknown. We show here that the non-receptor tyrosine kinase Ack1, previously implicated in the spatiotemporal regulation of the EGF receptor, is required for TRAIL-induced cell death in multiple epithelial cell lines. TRAIL triggered a transient up-regulation of Ack1 and its recruitment to lipid rafts along with TRAIL-R1/2. siRNA-mediated depletion of Ack1 disrupted TRAIL-induced accumulation of TRAIL-R1/2 in lipid rafts and efficient recruitment of caspase-8 to the death-inducing signaling complex. Pharmacological inhibition of Ack1 did not affect TRAIL-induced cell death, indicating that Ack1 acts in a kinase-independent manner to promote TRAIL-R1/2 accumulation in lipid rafts. These findings identify Ack1 as an essential player in the spatial regulation of TRAIL-R1/2.

Mig6 is a sensor of EGF receptor inactivation that directly activates c-Abl to induce apoptosis during epithelial homeostasis.

A fundamental aspect of epithelial homeostasis is the dependence on specific growth factors for cell survival, yet the underlying mechanisms remain obscure. We found an "inverse" mode of receptor tyrosine kinase signaling that directly links ErbB receptor inactivation to the induction of apoptosis. Upon ligand deprivation Mig6 dissociates from the ErbB receptor and binds to and activates the tyrosine kinase c-Abl to trigger p73-dependent apoptosis in mammary epithelial cells. Deletion of Errfi1 (encoding Mig6) and inhibition or RNAi silencing of c-Abl causes impaired apoptosis and luminal filling of mammary ducts. Mig6 activates c-Abl by binding to the kinase domain, which is prevented in the presence of epidermal growth factor (EGF) by Src family kinase-mediated phosphorylation on c-Abl-Tyr488. These results reveal a receptor-proximal switch mechanism by which Mig6 actively senses EGF deprivation to directly activate proapoptotic c-Abl. Our findings challenge the common belief that deprivation of growth factors induces apoptosis passively by lack of mitogenic signaling.

Mitogen-inducible gene-6 is a negative regulator of epidermal growth factor receptor signaling in hepatocytes and human hepatocellular carcinoma.

UNLABELLED: The mitogen-inducible gene-6 (mig-6) is a multi-adaptor protein implicated in the regulation of the HER family of receptor tyrosine kinases. We have reported recently that mig-6 is a negative regulator of epidermal growth factor receptor (EGFR)-dependent skin morphogenesis and tumor formation in vivo. In the liver, ablation of mig-6 leads to an increase in EGFR protein levels, suggesting that mig-6 is a negative regulator of EGFR function. In line with this observation, primary hepatocytes isolated from mig-6 knockout and wild-type control mice display sustained mitogenic signaling in response to EGF. In order to explore the role of mig-6 in the liver in vivo, we analyzed liver regeneration in mig-6 knockout and wild-type control mice. Interestingly, mig-6 knockout mice display enhanced hepatocyte proliferation in the initial phases after partial hepatectomy. This phenotype correlates with activation of endogenous EGFR signaling, predominantly through the protein kinase B pathway. In addition, mig-6 is an endogenous inhibitor of EGFR signaling and EGF-induced tumor cell migration in human liver cancer cell lines. Moreover, mig-6 is down-regulated in human hepatocellular carcinoma and this correlates with increased EGFR expression. CONCLUSION: Our data implicate mig-6 as a regulator of EGFR activity in hepatocytes and as a suppressor of EGFR signaling in human liver cancer.

Meiotic regulation of the CDK activator RINGO/Speedy by ubiquitin-proteasome-mediated processing and degradation.

Xenopus RINGO/Speedy (XRINGO) is a potent inducer of oocyte meiotic maturation that can directly activate Cdk1 and Cdk2. Here, we show that endogenous XRINGO protein accumulates transiently during meiosis I entry and then is downregulated. This tight regulation of XRINGO expression is the consequence of two interconnected mechanisms: processing and degradation. XRINGO processing involves recognition of at least three distinct phosphorylated recognition motifs by the SCF(betaTrCP) ubiquitin ligase, followed by proteasome-mediated limited degradation, resulting in an amino-terminal XRINGO fragment. XRINGO processing is directly stimulated by several kinases, including protein kinase A and glycogen synthase kinase-3beta, and may contribute to the maintenance of G2 arrest. On the other hand, XRINGO degradation after meiosis I is mediated by the ubiquitin ligase Siah-2, which probably requires phosphorylation of XRINGO on Ser 243 and may be important for the omission of S phase at the meiosis-I-meiosis-II transition in Xenopus oocytes.

Mig6 is a negative regulator of EGF receptor-mediated skin morphogenesis and tumor formation.

The growing number of recently identified negative feedback regulators of receptor tyrosine kinases (RTKs) highlights the importance of signal attenuation and modulation for correct signaling outcome. Mitogen-inducible gene 6 (Mig6 also known as RALT or Gene 33) is a multiadaptor protein thought to be involved in the regulation of RTK and stress signaling. Here, we show that deletion of the mouse gene encoding Mig6 (designated Errfi1, which stands for ERBB receptor feedback inhibitor 1) causes hyperactivation of endogenous epidermal growth factor receptor (EGFR) and sustained signaling through the mitogen-activated protein kinase (MAPK) pathway, resulting in overproliferation and impaired differentiation of epidermal keratinocytes. Furthermore, Errfi1-/- mice develop spontaneous tumors in various organs and are highly susceptible to chemically induced formation of skin tumors. A tumor-suppressive role for Mig6 is supported by our finding that MIG6 is downregulated in various human cancers. Inhibition of endogenous Egfr signaling with the Egfr inhibitor gefitinib (Iressa) or replacement of wild-type Egfr with the kinase-deficient protein encoded by the hypomorphic Egfr(wa2) allele completely rescued skin defects in Erffi1-/- mice. Carcinogen-induced tumors displayed by Errfi1-/- mice were highly sensitive to gefitinib. These results indicate that Mig6 is a specific negative regulator of Egfr signaling in skin morphogenesis and is a novel tumor suppressor of Egfr-dependent carcinogenesis.

Mitogen-inducible gene 6 is an endogenous inhibitor of HGF/Met-induced cell migration and neurite growth.

Hepatocyte growth factor (HGF)/Met signaling controls cell migration, growth and differentiation in several embryonic organs and is implicated in human cancer. The physiologic mechanisms that attenuate Met signaling are not well understood. Here we report a mechanism by which mitogen-inducible gene 6 (Mig6; also called Gene 33 and receptor-associated late transducer) negatively regulates HGF/Met-induced cell migration. The effect is observed by Mig6 overexpression and is reversed by Mig6 small interfering RNA knock-down experiments; this indicates that endogenous Mig6 is part of a mechanism that inhibits Met signaling. Mig6 functions in cells of hepatic origin and in neurons, which suggests a role for Mig6 in different cell lineages. Mechanistically, Mig6 requires an intact Cdc42/Rac interactive binding site to exert its inhibitory action, which suggests that Mig6 acts, at least in part, distally from Met, possibly by inhibiting Rho-like GTPases. Because Mig6 also is induced by HGF stimulation, our results suggest that Mig6 is part of a negative feedback loop that attenuates Met functions in different contexts and cell types.