Pals1 prevents Rac1-dependent colorectal cancer cell metastasis by inhibiting Arf6.

Loss of apical-basal polarity and downregulation of cell-cell contacts is a critical step during the pathogenesis of cancer. Both processes are regulated by the scaffolding protein Pals1, however, it is unclear whether the expression of Pals1 is affected in cancer cells and whether Pals1 is implicated in the pathogenesis of the disease.Using mRNA expression data and immunostainings of cancer specimen, we show that Pals1 is frequently downregulated in colorectal cancer, correlating with poorer survival of patients. We further found that Pals1 prevents cancer cell metastasis by controlling Rac1-dependent cell migration through inhibition of Arf6, which is independent of the canonical binding partners of Pals1. Loss of Pals1 in colorectal cancer cells results in increased Arf6 and Rac1 activity, enhanced cell migration and invasion in vitro and increased metastasis of transplanted tumor cells in mice. Thus, our data reveal a new function of Pals1 as a key inhibitor of cell migration and metastasis of colorectal cancer cells. Notably, this new function is independent of the known role of Pals1 in tight junction formation and apical-basal polarity.

Membrane-binding and activation of LKB1 by phosphatidic acid is essential for development and tumour suppression.

The serine/threonine kinase LKB1 regulates various cellular processes such as cell proliferation, energy homeostasis and cell polarity and is frequently downregulated in various tumours. Many downstream pathways controlled by LKB1 have been described but little is known about the upstream regulatory mechanisms. Here we show that targeting of the kinase to the membrane by a direct binding of LKB1 to phosphatidic acid is essential to fully activate its kinase activity. Consequently, LKB1 mutants that are deficient for membrane binding fail to activate the downstream target AMPK to control mTOR signalling. Furthermore, the in vivo function of LKB1 during development of Drosophila depends on its capacity to associate with membranes. Strikingly, we find LKB1 to be downregulated in malignant melanoma, which exhibit aberrant activation of Akt and overexpress phosphatidic acid generating Phospholipase D. These results provide evidence for a fundamental mechanism of LKB1 activation and its implication in vivo and during carcinogenesis.

Pals1 Haploinsufficiency Results in Proteinuria and Cyst Formation.

The nephron is the basic physiologic subunit of the mammalian kidney and is made up of several apicobasally polarized epithelial cell types. The process of apicobasal polarization in animal cells is controlled by the evolutionarily conserved Crumbs (CRB), Partitioning-defective, and Scribble protein complexes. Here, we investigated the role of protein associated with LIN-7 1 (Pals1, also known as Mpp5), a core component of the apical membrane-determining CRB complex in the nephron. Pals1 interacting proteins, including Crb3 and Wwtr1/Taz, have been linked to renal cyst formation in mice before. Immunohistologic analysis revealed Pals1 expression in renal tubular cells and podocytes of human kidneys. Mice lacking one Pals1 allele (functionally haploid for Pals1) in nephrons developed a fully penetrant phenotype, characterized by cyst formation and severe defects in renal barrier function, which led to death within 6-8 weeks. In Drosophila nephrocytes, deficiency of the Pals1 ortholog caused alterations in slit-diaphragm-like structures. Additional studies in epithelial cell culture models revealed that Pals1 functions as a dose-dependent upstream regulator of the crosstalk between Hippo- and TGF-ß-mediated signaling. Furthermore, Pals1 haploinsufficiency in mouse kidneys associated with the upregulation of Hippo pathway target genes and marker genes of TGF-ß signaling, including biomarkers of renal diseases. These findings support a link between apical polarity proteins and renal diseases, especially renal cyst diseases. Further investigation of the Pals1-linked networks is required to decipher the mechanisms underlying the pathogenesis of these diseases.

Lead ions abrogate lipopolysaccharide-induced nitric monoxide toxicity by reducing the expression of STAT1 and iNOS.

Lead is a widespread environmental pollutant and the highly poisonous metal compromises multiple organs in the body. Among other tissues and cells, lead ions (Pb(2+)) can affect macrophages and microglia cells. The present study observed a concentration-dependent protection of BV-2 microglia and RAW 264.7 macrophages by Pb(2+) against lipopolysaccharide (LPS)-induced toxicity. Both cell lines are potent producers of two substances that have previously been shown to mediate cytotoxic effects of LPS. These are the pro-inflammatory cytokine tumor necrosis factor (TNF)-α and nitric monoxide (NO), which creates nitrosative stress, hampering the distribution of invading pathogens and tumor cells. While the expression of TNF-α was unaffected by Pb(2+), the production of NO was significantly inhibited. Moreover, blocking NO synthesis by low molecular weight inhibitors prevented LPS-mediated toxicity, confirming the role of NO in these events. Pb(2+) exposure led to a downregulation of LPS-induced expression of the transcription factor STAT1, which is involved in iNOS transcription. Moreover, iNOS mRNA and protein levels were reduced in the presence of Pb(2+), explaining the reduced formation of NO and a subsequent increase of cellular viability in vitro. In vivo, the effect might limit collateral damage caused by excessive NO production, but also impair the efficiency of NO as a central mediator of the defense against various pathogens.