Current perspectives on inhibitory SMAD7 in health and disease

Transforming growth factor β (TGF-β) family members play an extensive role in cellular communication that orchestrates both early development and adult tissue homeostasis. Aberrant TGF-β family signaling is associated with a pathological outcome in numerous diseases, and in-depth understanding of molecular and cellular processes could result in therapeutic benefit for patients. Canonical TGF-β signaling is mediated by receptor-regulated SMADs (R-SMADs), a single co-mediator SMAD (Co-SMAD), and inhibitory SMADs (I-SMADs). SMAD7, one of the I-SMADs, is an essential negative regulator of the pleiotropic TGF-β and bone morphogenetic protein (BMP) signaling pathways. In a negative feedback loop, SMAD7 inhibits TGF-β signaling by providing competition for TGF-β type-1 receptor (TβRI), blocking phosphorylation and activation of SMAD2. Moreover, SMAD7 recruits E3 ubiquitin SMURF ligases to the type I receptor to promote ubiquitin-mediated proteasomal degradation. In addition to its role in TGF-β and BMP signaling, SMAD7 is regulated by and implicated in a variety of other signaling pathways and functions as a mediator of crosstalk. This review is focused on SMAD7, its function in TGF-β and BMP signaling, and its role as a downstream integrator and crosstalk mediator. This crucial signaling molecule is tightly regulated by various mechanisms. We provide an overview of the ways by which SMAD7 is regulated, including noncoding RNAs (ncRNAs) and post-translational modifications (PTMs). Finally, we discuss its role in diseases, such as cancer, fibrosis, and inflammatory bowel disease (IBD).

RNF43 truncations trap CK1 to drive niche-independent self-renewal in cancer

Wnt/β-catenin signaling is a primary pathway for stem cell maintenance during tissue renewal and a frequent target for mutations in cancer. Impaired Wnt receptor endocytosis due to loss of the ubiquitin ligase RNF43 gives rise to Wnt-hypersensitive tumors that are susceptible to anti-Wnt-based therapy. Contrary to this paradigm, we identify a class of RNF43 truncating cancer mutations that induce β-catenin-mediated transcription, despite exhibiting retained Wnt receptor downregulation. These mutations interfere with a ubiquitin-independent suppressor role of the RNF43 cytosolic tail that involves Casein kinase 1 (CK1) binding and phosphorylation. Mechanistically, truncated RNF43 variants trap CK1 at the plasma membrane, thereby preventing β-catenin turnover and propelling ligand-independent target gene transcription. Gene editing of human colon stem cells shows that RNF43 truncations cooperate with p53 loss to drive a niche-independent program for self-renewal and proliferation. Moreover, these RNF43 variants confer decreased sensitivity to anti-Wnt-based therapy. Our data demonstrate the relevance of studying patient-derived mutations for understanding disease mechanisms and improved applications of precision medicine.

Linear ubiquitination at a glance

Ubiquitination (also known as ubiquitylation) is a post-translational modification that creates versatility in cell signalling and regulates a multitude of cellular processes. Its versatility lies in the capacity to form eight different inter-ubiquitin linkages through the seven lysine residues of ubiquitin and through its N-terminal methionine (M1). The latter, referred to as linear or M1 linkage, is created by the linear ubiquitin chain assembly complex (LUBAC), the only E3 ligase known to date that is capable of forming linear ubiquitin chains de novo Linear ubiquitin chains are crucial modulators of innate and adaptive immune responses, and act by regulating inflammatory and cell death signalling. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge on the role of LUBAC and linear ubiquitination in immune signalling and human physiology. We specifically focus on the role for LUBAC in signalling that is induced by the cytokine tumour necrosis factor (TNF) and its role in inflammation, gene activation and cell death. Furthermore, we highlight the roles of deubiquitinases (DUBs) that cleave M1 linkages and add an additional layer in the control of LUBAC-mediated immune signalling.

Tales from the crypt: intestinal niche signals in tissue renewal, plasticity and cancer

Rapidly renewing tissues such as the intestinal epithelium critically depend on the activity of small-sized stem cell populations that continuously generate new progeny to replace lost and damaged cells. The complex and tightly regulated process of intestinal homeostasis is governed by a variety of signalling pathways that balance cell proliferation and differentiation. Accumulating evidence suggests that stem cell control and daughter cell fate determination is largely dictated by the microenvironment. Here, we review recent developments in the understanding of intestinal stem cell dynamics, focusing on the roles, mechanisms and interconnectivity of prime signalling pathways that regulate stem cell behaviour in intestinal homeostasis. Furthermore, we discuss how mutational activation of these signalling pathways endows colorectal cancer cells with niche-independent growth advantages during carcinogenesis.

The linear ubiquitin chain assembly complex regulates TRAIL-induced gene activation and cell death

The linear ubiquitin chain assembly complex (LUBAC) is the only known E3 ubiquitin ligase which catalyses the generation of linear ubiquitin linkages de novo LUBAC is a crucial component of various immune receptor signalling pathways. Here, we show that LUBAC forms part of the TRAIL-R-associated complex I as well as of the cytoplasmic TRAIL-induced complex II In both of these complexes, HOIP limits caspase-8 activity and, consequently, apoptosis whilst being itself cleaved in a caspase-8-dependent manner. Yet, by limiting the formation of a RIPK1/RIPK3/MLKL-containing complex, LUBAC also restricts TRAIL-induced necroptosis. We identify RIPK1 and caspase-8 as linearly ubiquitinated targets of LUBAC following TRAIL stimulation. Contrary to its role in preventing TRAIL-induced RIPK1-independent apoptosis, HOIP presence, but not its activity, is required for preventing necroptosis. By promoting recruitment of the IKK complex to complex I, LUBAC also promotes TRAIL-induced activation of NF-κB and, consequently, the production of cytokines, downstream of FADD, caspase-8 and cIAP1/2. Hence, LUBAC controls the TRAIL signalling outcome from complex I and II, two platforms which both trigger cell death and gene activation.

Optimal interleukin-7 receptor-mediated signaling, cell cycle progression and viability of T-cell acute lymphoblastic leukemia cells rely on casein kinase 2 activity

Interleukin-7 and interleukin-7 receptor are essential for normal T-cell development and homeostasis, whereas excessive interleukin-7/interleukin-7 receptor-mediated signaling promotes leukemogenesis. The protein kinase, casein kinase 2, is overexpressed and hyperactivated in cancer, including T-cell acute lymphoblastic leukemia. Herein, we show that while interleukin-7 had a minor but significant positive effect on casein kinase 2 activity in leukemia T-cells, casein kinase 2 activity was mandatory for optimal interleukin-7/interleukin-7 receptor-mediated signaling. Casein kinase 2 pharmacological inhibition impaired signal transducer and activator of transcription 5 and phosphoinositide 3-kinase/v-Akt murine thymoma viral oncogene homolog 1 pathway activation triggered by interleukin-7 or by mutational activation of interleukin-7 receptor. By contrast, forced expression of casein kinase 2 augmented interleukin-7 signaling in human embryonic kidney 293T cells reconstituted with the interleukin-7 receptor machinery. Casein kinase 2 inactivation prevented interleukin-7-induced B-cell lymphoma 2 upregulation, maintenance of mitochondrial homeostasis and viability of T-cell acute lymphoblastic leukemia cell lines and primary leukemia cells collected from patients at diagnosis. Casein kinase 2 inhibition further abrogated interleukin-7-mediated cell growth and upregulation of the transferrin receptor, and blocked cyclin A and E upregulation and cell cycle progression. Notably, casein kinase 2 was also required for the viability of mutant interleukin-7 receptor expressing leukemia T-cells. Overall, our study identifies casein kinase 2 as a major player in the effects of interleukin-7 and interleukin-7 receptor in T-cell acute lymphoblastic leukemia. This further highlights the potential relevance of targeting casein kinase 2 in this malignancy.

Activation of bile salt nuclear receptor FXR is repressed by pro-inflammatory cytokines activating NF-κB signaling in the intestine

Hyperactivation of NF-κB is a key factor in the pathophysiology of inflammatory bowel disease (IBD). We previously showed that the bile salt nuclear Farnesoid X Receptor (FXR) counter-regulates intestinal inflammation, possibly via repression of NF-κB. Here, we examine whether mutual antagonism between NF-κB and FXR exists. FXR and its target genes IBABP and FGF15/19 expression were determined in HT29 colon carcinoma cells and ex vivo in intestinal specimens of wild type (WT) and Fxr-ko mice, treated with/without FXR ligands (GW4064/INT-747) and inflammatory stimuli (TNFα/IL-1β). In addition, FXR activation was studied in vivo in WT and Fxr-ko mice with DSS-colitis. The involvement of NF-κB in decreasing FXR activity was investigated by reporter assays and Glutathione S-transferase pulldown assays. FXR target gene expression was highly reduced by inflammatory stimuli in all model systems, while FXR mRNA expression was unaffected. In line with these results, reporter assays showed reduced FXR transcriptional activity upon TNFα/IL-1β stimulation. We show that this reduction in FXR activity is probably mediated by NF-κB, since overexpression of NF-κB subunits p50 and/or p65 also lead to inhibition of FXR activity. Finally, we report that p65 and p50 physically interact with FXR in vitro.

CONCLUSIONS

Together, these results indicate that intestinal inflammation strongly reduces FXR activation, probably via NF-κB-dependent tethering of FXR. Therefore, FXR not only inhibits inflammation, but also is targeted by the inflammatory response itself. This could result in a vicious cycle where reduced FXR activity results in less repression of inflammation, contributing to development of chronic intestinal inflammation. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Rectal hydrocortisone during stress in patients with adrenal insufficiency

OBJECTIVE

Patients with glucocorticoid deficiency need lifelong glucocorticoid replacement treatment. During acute stressful events, steroid dosage must be increased several times, which is often problematical in children. This study investigated the reliability of rectal hydrocortisone administration as an alternative to the intramuscular route.

STUDY DESIGN

Serum cortisol was assessed during stress in normal children to determine the concentration that should be achieved after rectal hydrocortisone. Subsequently, serum cortisol concentrations were measured three hours after administering a suppository containing hydrocortisone 100 mg/m2 to 57 patients with adrenocortical insufficiency. In eight patients, the time dependency of the cortisol rise after rectal administration was established.

RESULTS

In 51 previously healthy children admitted to hospital with an acute stressful condition, the mean serum cortisol concentration was 1092 nmol/l. Rectal hydrocortisone in patients with adrenocortical insufficiency resulted in a mean serum cortisol concentration of 1212 nmol/l three hours after insertion of the suppository containing hydrocortisone. In 14 of 57 children, serum cortisol was < 1000 nmol/l and in eight children it was below 600 nmol/l. One hour after administration, the mean cortisol concentration had reached 1000 nmol/l. This was sustained for more than four hours.

CONCLUSION

Rectal hydrocortisone is a safe alternative to parenteral administration in the self management of Addisonian prone conditions. However, because eight of 57 children did not achieve concentrations > 600 nmol/l, its use is recommended only after previously documenting an adequate serum cortisol concentration three hours after receiving a test dose.

Heterozygotes for 17 alpha-hydroxylase deficiency can be detected with a short ACTH test

A short ACTH test was performed in the six parents, and four siblings, of three cases with 17 alpha-hydroxylase deficiency. Baseline steroid levels were all normal in female heterozygotes but in males 17 alpha-hydroxyprogesterone levels were elevated. After ACTH-stimulation, plasma levels of corticosterone were elevated in five obligate heterozygotes and 18-hydroxydeoxycorticosterone levels were increased in four of them. Two of the four siblings had biochemical signs of heterozygosity. The short ACTH test appears to be an efficient method for detecting heterozygosity, but the abnormalities found are more heterogeneous than previously suggested.