Regulation of cell morphology and adhesion by the tuberous sclerosis complex (TSC1/2) gene products in human kidney epithelial cells through increased E-cadherin/beta-catenin activity

Phosphoproteomic analysis of human embryonic stem cells

Protein phosphorylation, while critical to cellular behavior, has been undercharacterized in pluripotent cells. Therefore, we performed phosphoproteomic analyses of human embryonic stem cells (hESCs) and their differentiated derivatives. A total of 2546 phosphorylation sites were identified on 1602 phosphoproteins; 389 proteins contained more phosphorylation site identifications in undifferentiated hESCs, whereas 540 contained more such identifications in differentiated derivatives. Phosphoproteins in receptor tyrosine kinase (RTK) signaling pathways were numerous in undifferentiated hESCs. Cellular assays corroborated this observation by showing that multiple RTKs cooperatively supported undifferentiated hESCs. In addition to bFGF, EGFR, VEGFR, and PDGFR activation was critical to the undifferentiated state of hESCs. PDGF-AA complemented a subthreshold bFGF concentration to maintain undifferentiated hESCs. Also consistent with phosphoproteomics, JNK activity participated in maintenance of undifferentiated hESCs. These results support the utility of phosphoproteomic data, provide guidance for investigating protein function in hESCs, and complement transcriptomics/epigenetics for broadening our understanding of hESC fate determination.

Tuberin: a stimulus-regulated tumor suppressor protein controlled by a diverse array of receptor tyrosine kinases and G protein-coupled receptors

Tuberin, a tumor suppressor protein, is involved in various cellular functions including survival, proliferation, and growth. It has emerged as an important effector regulated by receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs). Regulation of tuberin by RTKs and GPCRs is highly complex and dependent on the type of receptors and their associated signaling molecules. Apart from Akt, the first kinase recognized to phosphorylate and inactivate tuberin upon growth factor stimulation, an increasing number of kinases upstream of tuberin have been identified. Furthermore, recruitment of different scaffolding adaptor components to the activated receptors appears to play an important role in the regulation of tuberin activity. More recently, the differential regulation of tuberin by various G protein family members have also been intensively studied, it appears that G proteins can both facilitate (e.g., G(i/o)) as well as inhibit (e.g., G(q)) tuberin phosphorylation. In the present review, we attempt to summarize our emerging understandings of the roles of RTKs, GPCRs, and their cross-talk on the regulation of tuberin.

Hypoxia-inducible factor-1alpha polymorphisms and TSC1/2 mutations are complementary in head and neck cancers

Background

Polymorphisms or mutations in hypoxia inducible factor-1 alpha (HIF-1alpha) that increases its activity and stability under normoxia have recently been identified. Likewise, disruption of the TSC1/TSC2 complex through loss of TSC1 or TSC2 has been shown to result in abnormal accumulation of HIF-1α. Here, we investigate the novel polymorphisms in exon 12, that approximate the oxygen-dependent degradation domain of HIF-1alpha in five cell lines and 28 patients with oral squamous carcinomas. Moreover, we assess for the presence of polymorphisms and mutations in TSC1 and TSC2, to ascertain if dysregulation of such might complement HIF-1alpha expression.

Results

Denaturing high pressure liquid chromatography (DHPLC) analysis on PCR fragments in exon 12 of HIF-1alpha from 28 patients with OSCC revealed that 6 of 28 patients had mismatched heteroduplex patterns. Genomic DNA was extracted from peripheral blood leukocytes and direct sequencing showed that in 5 of the six cases these changes represented polymorphisms while, one case was a somatic mutation. Analyses of TSC1 and TSC2 revealed heteroduplexes in exons: TSC1 exon 17; TSC2 exons 36,40, and 41. The relative levels of HIF-1alpha were significantly greater for tumors possessing a HIF-1alpha polymorphism or mutation within exon 12, whereas tumors possessing a deletion or polymorphism in TSC1/TSC2 displayed a trend for higher levels of HIF-1alpha. Western blot analyses for HIF-1alpha, TSC1 and TSC2 in five SCC cell lines revealed high levels of HIF-1alpha in SCC cells possessing TSC1 and/or TSC2 mutations. Wild-type TSC2 cells targeted with siRNA to TSC2 exhibited increased levels of HIF-1alpha. Transfection of a HIF-1alpha mutant produced higher levels of HIF-1alpha in TSC1/TSC2 mutant cell lines than in wild type cells. TSC1/TSC2 mutant cell lines administered Rapamycin blocked S6 phorphorylation and diminished the levels of HIF-1alpha to those observed in cell lines with wild type TSC1/TSC2.

Conclusion

Dysregulation of the TSC1/TSC2 complex by mutation compliments HIF-1α polymorphisms in the expression of HIF-1alpha in SCC of the head and neck, and may provide biomarkers to predict responses to specific therapies and overall disease prognosis.

Isolated unilateral tuberous sclerosis-associated renal cystic disease in a neonate

We present a male infant with antenatally detected, focal, unilateral apparently isolated renal cystic disease with morphological features of renal involvement in tuberous sclerosis. Only one previous case with similar presentation has been described. Most affected children present with either diffuse bilateral renal cystic disease or extrarenal manifestations. The major genes involved in tuberous sclerosis are now well described, and early onset of severe renal cystic disease in affected children often is related to the presence of a contiguous gene deletion syndrome involving TSC2 and PKD1 on chromosome 16.

TSC2 modulates actin cytoskeleton and focal adhesion through TSC1-binding domain and the Rac1 GTPase

Tuberous sclerosis complex (TSC) 1 and TSC2 are thought to be involved in protein translational regulation and cell growth, and loss of their function is a cause of TSC and lymphangioleiomyomatosis (LAM). However, TSC1 also activates Rho and regulates cell adhesion. We found that TSC2 modulates actin dynamics and cell adhesion and the TSC1-binding domain (TSC2-HBD) is essential for this function of TSC2. Expression of TSC2 or TSC2-HBD in TSC2−/− cells promoted Rac1 activation, inhibition of Rho, stress fiber disassembly, and focal adhesion remodeling. The down-regulation of TSC1 with TSC1 siRNA in TSC2−/− cells activated Rac1 and induced loss of stress fibers. Our data indicate that TSC1 inhibits Rac1 and TSC2 blocks this activity of TSC1. Because TSC1 and TSC2 regulate Rho and Rac1, whose activities are interconnected in a reciprocal fashion, loss of either TSC1 or TSC2 function may result in the deregulation of cell motility and adhesion, which are associated with the pathobiology of TSC and LAM.

Molecular genetic basis of tuberous sclerosis complex: from bench to bedside

Tuberous sclerosis complex is an autosomal dominant disease of benign tumors occurring in multiple organ systems of the body. Either of two genes, TSC1 or TSC2, can be mutated, resulting in the tuberous sclerosis complex phenotype. The protein products of the tuberous sclerosis complex genes, hamartin (TSC1) and tuberin (TSC2), have been discovered to play important roles in several cell-signaling pathways. Knowledge regarding the function of the tuberin-hamartin complex has led to therapeutic intervention trials. Numerous pathogenic mutations have been elucidated in individuals affected with tuberous sclerosis complex. Information on the type and distribution of nearly 1000 mutations in the two genes is discussed. Mosaicism for tuberous sclerosis complex mutations has been documented, complicating provision of genetic counseling to families. Emerging genotype-phenotype correlations should provide guidance for better medical care of individuals with tuberous sclerosis complex.

IGF-1 induces growth, survival and morphological change of primary hepatocytes on a galactose-bared polymer through both MAPK and beta-catenin pathways

PVLA poly-(N-p-vinylbenzyl-O-beta-D-galactopyranosyl-D-gluconamide) is a glycopolymer composed of hydrophilic carbohydrate side chain and hydrophobic styrene polymer. The hydrophilic carbohydrate residue of PVLA can be recognized as a ligand for hepatocytes asialoglycoprotein receptor (ASGP-R), which is abundant on the hepatocyte cell surface. Adhering to the PVLA coated dishes, hepatocytes try to form aggregates that have a long time survival and also cells in these aggregates exhibit better maintenance of specific hepatocyte functions. Stimulation of the cells with IGF-1 in this culture condition results in the formation of lower aggregates. In addition to the morphological influences of IGF-1 to these cells, we have also found that IGF-1 transmits growth stimulatory responses to hepatocytes on PVLA through both mitogen activated protein kinase (MAPK) pathway and beta-catenin pathways. The phosphorylation of MAPK can take place within 5 min of stimulation with IGF-1 and within at least 10 ng/ml of IGF-1 concentration. Inhibition of MAPK activation by MEK-1 inhibitor PD98059 reduces IGF-1 induced MAPK phosphorylation, and also IGF-1 stimulated DNA synthesis. Similarly, the use of PI3-K inhibitor LY294002 also inhibits IGF-1 stimulated DNA synthesis. IGF-1 stimulation enhances the migration of beta-catenin from the cytoskeleton and cell membrane to the cytoplasm which also is the reason behind formation of spheroids and lower aggregates. IGF-1 stimulation also shows increased translocalization of beta-catenin to the nucleus that is essentially important to produce beta-catenin responsive effects to the cells. These studies thus suggest that IGF-1 can stimulate the growth and survival of hepatocytes on PVLA through both MAPK and beta-catenin signaling pathways, and that the activation of beta-catenin signaling pathway produces the morphological changes of IGF-1 induced cells.

E-cadherin-mediated adhesion inhibits ligand-dependent activation of diverse receptor tyrosine kinases

E-cadherin is an essential adhesion protein as well as a tumor suppressor that is silenced in many cancers. Its adhesion-dependent regulation of signaling has not been elucidated. We report that E-cadherin can negatively regulate, in an adhesion-dependent manner, the ligand-dependent activation of divergent classes of receptor tyrosine kinases (RTKs), by inhibiting their ligand-dependent activation in association with decreases in receptor mobility and in ligand-binding affinity. E-cadherin did not regulate a constitutively active mutant RTK (Neu*) or the ligand-dependent activation of LPA receptors or muscarinic receptors, which are two classes of G protein-coupled receptors. EGFR regulation by E-cadherin was associated with complex formation between EGFR and E-cadherin that depended on the extracellular domain of E-cadherin but was independent of beta-catenin binding or p120-catenin binding. Transfection of E-cadherin conferred negative RTK regulation to human melanoma and breast cancer lines with downregulated endogenous E-cadherin. Abrogation of E-cadherin regulation may contribute to the frequent ligand-dependent activation of RTK in tumors.