Chemical reverse transformation of CHO-K1 cells induces changes in expression of a candidate tumour suppressor and of a gene not previously characterised as transformation related

KRIT1 in vascular biology and beyond

KRIT1 is a 75 kDa scaffolding protein which regulates endothelial cell phenotype by limiting the response to inflammatory stimuli and maintaining a quiescent and stable endothelial barrier. Loss-of-function mutations in KRIT1 lead to the development of cerebral cavernous malformations (CCM), a disease marked by the formation of abnormal blood vessels which exhibit a loss of barrier function, increased endothelial proliferation, and altered gene expression. While many advances have been made in our understanding of how KRIT1, and the functionally related proteins CCM2 and PDCD10, contribute to the regulation of blood vessels and the vascular barrier, some important open questions remain. In addition, KRIT1 is widely expressed and KRIT1 and the other CCM proteins have been shown to play important roles in non-endothelial cell types and tissues, which may or may not be related to their role as pathogenic originators of CCM. In this review, we discuss some of the unsettled questions regarding the role of KRIT1 in vascular physiology and discuss recent advances that suggest this ubiquitously expressed protein may have a role beyond the endothelial cell.

Advances in the Structural and Physiological Functions of SHARPIN

SHARPIN was initially found as a SHANK-associated protein. SHARPIN can be used as an important component to form the linear ubiquitin chain assembly complex (LUBAC) with HOIL-1L, HOIP to produce a linear ubiquitin chain connected N-terminal Met1, playing a critical role in various cellular processes including NF-κB signaling, inflammation, embryogenesis and apoptosis. SHARPIN alone can also participate in many critical physiological activities and cause various disorders such as chronic dermatitis, tumor, and Alzheimer’s disease. Mice with spontaneous autosomal recessive mutations in the SHARPIN protein mainly exhibit chronic dermatitis and immunodeficiency with elevated IgM. Additionally, SHARPIN alone also plays a key role in various cellular events, such as B cells activation and platelet aggregation. Structural studies of the SHARPIN or LUBAC have been reported continuously, advancing our understanding of it at the molecular level. However, the full-length structure of the SHARPIN or LUBAC was lagging, and the molecular mechanism underlying these physiological processes is also unclear. Herein, we summarized the currently resolved structure of SHARPIN as well as the emerging physiological role of SHARPIN alone or in LUBAC. Further structural and functional study of SHARPIN will provide insight into the role and underlying mechanism of SHARPIN in disease, as well as its potential application in therapeutic.

miR-21 coordinates tumor growth and modulates KRIT1 levels

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miR-21 targets KRIT1.

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miR-21 and KRIT1 expression anticorrelate in human breast tumors.

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KRIT1 is involved in miR-21-mediated tumor cell growth.

miR-21 is overexpressed in tumors and it displays oncogenic activity. Here, we show that expression of miR-21 in primary tumors anticorrelates with KRIT1/CCM1, an interacting partner of the Ras-like GTPase Rap1, involved in Cerebral Cavernous Malformations (CCM). We present evidences that miR-21 silences KRIT1 by targeting its mRNA 3′UTR and that this interaction is involved in tumor growth control. In fact, miR-21 over-expression or KRIT1 knock-down promote anchorage independent tumor cell growth compared to controls, whereas the opposite is observed when anti-miR-21 or KRIT1 overexpression are employed. Our findings suggest that miR-21 promotes tumor cell growth, at least in part, by down-modulating the potential tumor suppressor KRIT1.

Rap1 and its effector KRIT1/CCM1 regulate beta-catenin signaling

KRIT1, also called CCM1, is a member of a multiprotein complex that contains the products of the CCM2 and PDCD10 (also known as CCM3) loci. Heterozygous loss of any of the genes that encode these proteins leads to cerebral cavernous malformations (CCM), which are vascular lesions that are found in around 0.5% of humans. KRIT1 mediates the stabilization of beta-catenin-containing endothelial cell-cell junctions downstream of the Rap1 GTPase. Here, we report that Rap1 and KRIT1 are negative regulators of canonical beta-catenin signaling in mice and that hemizygous Krit1 deficiency exacerbates beta-catenin-driven pathologies. Depletion of endothelial KRIT1 caused beta-catenin to dissociate from vascular endothelial (VE)-cadherin and to accumulate in the nucleus with consequent increases in beta-catenin-dependent transcription. Activation of Rap1 inhibited beta-catenin-dependent transcription in confluent endothelial cells; this effect required the presence of intact cell-cell junctions and KRIT1. These effects of KRIT1 were not limited to endothelial cells; the KRIT1 protein was expressed widely and its depletion increased beta-catenin signaling in epithelial cells. Moreover, a reduction in KRIT1 expression also increased beta-catenin signaling in vivo. Hemizygous deficiency of Krit1 resulted in a ~1.5-fold increase in intestinal polyps in the Apc(Min/+) mouse, which was associated with increased beta-catenin-driven transcription. Thus, KRIT1 regulates beta-catenin signaling, and Krit1(+/-) mice are more susceptible to beta-catenin-driven intestinal adenomas.