Tumor cell migration and invasion are enhanced by depletion of Rap1 GTPase-activating protein (Rap1GAP).

The functional significance of the widespread down-regulation of Rap1 GTPase-activating protein (Rap1GAP), a negative regulator of Rap activity, in human tumors is unknown. Here we show that human colon cancer cells depleted of Rap1GAP are endowed with more aggressive migratory and invasive properties. Silencing Rap1GAP enhanced the migration of confluent and single cells.

RAP1GAP inhibits cytoskeletal remodeling and motility in thyroid cancer cells.

The functional significance of decreased RAP1GAP protein expression in human tumors is unclear. To identify targets of RAP1GAP downregulation in the thyroid gland, RAP1 and RAP2 protein expression in human thyroid cells and in primary thyroid tumors were analyzed. RAP1GAP and RAP2 were co-expressed in normal thyroid follicular cells.

Rap1GAP impairs cell-matrix adhesion in the absence of effects on cell-cell adhesion.

The significance of the widespread downregulation of Rap1GAP in human tumors is unknown. In previous studies we demonstrated that silencing Rap1GAP expression in human colon cancer cells resulted in sustained increases in Rap activity, enhanced spreading on collagen and the weakening of cell-cell contacts. The latter finding was unexpected based on the role of Rap1 in strengthening cell-cell adhesion and reports that Rap1GAP impairs cell-cell adhesion.

Histone deacetylase inhibitors upregulate Rap1GAP and inhibit Rap activity in thyroid tumor cells.

Increases in Rap activity have been associated with tumor progression. Although activating mutations in Rap have not been described, downregulation of Rap1GAP is frequent in human tumors including thyroid carcinomas. In this study, we explored whether endogenous Rap1GAP expression could be restored to thyroid tumor cells.

Downregulation of Rap1GAP in human tumor cells alters cell/matrix and cell/cell adhesion.

Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas.

Protein kinase A and B-Raf mediate extracellular signal-regulated kinase activation by thyrotropin.

Thyrotropin (TSH) regulates thyroid cell proliferation and function through cAMP-mediated signaling pathways that activate protein kinase A (PKA) and Epac/Rap1. The respective roles of PKA versus Epac/Rap1 in TSH signaling remain unclear. We set out to determine whether PKA and/or Rap1 mediate extracellular signal-regulated kinase (ERK) activation by TSH.

Loss of Rap1GAP in papillary thyroid cancer.

Context: Rap1 GTPase-activating protein (GAP) regulates the activity of Rap1, a putative oncogene. We previously reported Rap1GAP was highly expressed in normal human thyroid cells and decreased in five papillary thyroid carcinomas (PTCs).

Objectives: To confirm the significance of these findings, we analyzed Rap1GAP expression in a larger set of benign tumors (adenomas and hyperplastic nodules) and PTCs.

TULA proteins bind to ABCE-1, a host factor of HIV-1 assembly, and inhibit HIV-1 biogenesis in a UBA-dependent fashion.

TULA, a recently identified UBA- and SH3-containing protein, has previously been shown to regulate cell signaling through protein tyrosine kinases. In order to search for novel functions of TULA, we identified, using mass spectrometry, proteins associated with TULA. ABCE-1 also known as RLI and HP68, a host factor of HIV-1 assembly, was found among TULA-associated proteins in these experiments.

Downregulation of Rap1GAP contributes to Ras transformation.

Although abundant in well-differentiated rat thyroid cells, Rap1GAP expression was extinguished in a subset of human thyroid tumor-derived cell lines. Intriguingly, Rap1GAP was downregulated selectively in tumor cell lines that had acquired a mesenchymal morphology. Restoring Rap1GAP expression to these cells inhibited cell migration and invasion, effects that were correlated with the inhibition of Rap1 and Rac1 activity.

Ras induces chromosome instability and abrogation of the DNA damage response.

Ras mutations are frequent in thyroid tumors, the most common endocrine malignancy. The ability of Ras to transform thyroid cells is thought to rely on its mitogenic activity. Unexpectedly, acute expression of activated Ras in normal rat thyroid cells induced a DNA damage response, followed by apoptosis.

Ras triggers ataxia-telangiectasia-mutated and Rad-3-related activation and apoptosis through sustained mitogenic signaling.

Genetic evidence indicates that Ras plays a critical role in the initiation and progression of human thyroid tumors. Paradoxically, acute expression of activated Ras in normal rat thyroid cells induced deregulated cell cycle progression and apoptosis. We investigated whether cell cycle progression was required for Ras-stimulated apoptosis.

Interference with 3',5'-cyclic adenosine monophosphate response element binding protein stimulates apoptosis through aberrant cell cycle progression and checkpoint activation.

We previously reported that protein kinase A activity is an important determinant of thyroid cell survival. Given the important role of cAMP response element binding protein (CREB) in mediating the transcriptional effects of protein kinase A, we explored whether interference with CREB family members impaired thyroid cell survival. Expression of A-CREB, a dominant-negative CREB mutant that inhibits CREB DNA binding activity, induced apoptosis in rat thyroid cells.

Protein kinase C delta stimulates apoptosis by initiating G1 phase cell cycle progression and S phase arrest.

Overexpression of protein kinase C delta (PKCdelta) stimulates apoptosis in a wide variety of cell types through a mechanism that is incompletely understood. PKCdelta-deficient cells are impaired in their response to DNA damage-induced apoptosis, suggesting that PKCdelta is required to mount an appropriate apoptotic response under conditions of stress. The mechanism through which it does so remains elusive.

Thyrotropin-stimulated DNA synthesis and thyroglobulin expression in normal and hyperthyroid feline thyrocytes in monolayer culture.

Feline hyperthyroidism is a common, spontaneous disease in older cats that is similar clinically and histopathologically to human toxic multinodular goiter (TNG). In this study, the functional response of feline normal thyroid (NT) and hyperthyroid (HT) cells grown in monolayer culture to thyrotropin (TSH) was determined. Basal levels of DNA synthesis were similar in NT and HT cells.

Thyrotropin and serum regulate thyroid cell proliferation through differential effects on p27 expression and localization.

Thyroid cell proliferation is regulated by the concerted action of TSH/cAMP and serum growth factors. The specific contributions of cAMP-dependent vs. -independent signals to cell cycle progression are not well understood.

Histone deacetylase is a target of valproic acid-mediated cellular differentiation.

Valproic acid (VPA), a well-established therapy for seizures and bipolar disorder, has recently been shown to inhibit histone deacetylases (HDACs). Similar to more widely studied HDAC inhibitors, VPA can cause growth arrest and induce differentiation of transformed cells in culture. Whether this effect of VPA is through inhibition of HDACs or modulation of another target of VPA has not been tested.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability.

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated.

Neurofilament RNA causes neurodegeneration with accumulation of ubiquitinated aggregates in cultured motor neurons.

The mechanisms whereby mutant gene expression triggers neurodegeneration are poorly understood but have generally been attributed to translated gene products. We now demonstrate direct neuropathic effects of untranslated RNA on cultured motor neurons. We show that expression of untranslated light neurofilament (NF-L) RNA sequence in the 3'UTR of an EGFP transgene (pEGFP/NF-L RNA) or in a separate expression vector (pRc/NF-L RNA) causes dose-dependent, neuron-specific motor neuron degeneration.

Ras stimulates aberrant cell cycle progression and apoptosis in rat thyroid cells.

Abundant evidence supports the ability of Ras to stimulate thyroid cell proliferation. Stable expression of activated Ras enhances the sensitivity of thyroid cells to apoptosis. We report that apoptosis is a primary and general response of rat thyroid cells to acute expression of activated Ras in the absence or presence of thyrotropin, insulin, and serum, survival factors for thyroid cells.

Lysophosphatidic acid promotes survival and differentiation of rat Schwann cells.

Lysophosphatidic acid (LPA; 1-acyl-sn-glycerol-3-phosphate), an abundant constituent of serum, mediates multiple biological responses via G protein-coupled serpentine receptors. Schwann cells express the LPA receptors (Edg receptors), which, once activated, have the potential to signal through G(alphai) to activate p21(ras) and phosphatidylinositol 3-kinase, through G(alphaq) to activate phospholipase C, or through G(q12/13) to activate the Rho pathway. We found that the addition of serum or LPA to serum-starved Schwann cells rapidly (10 min) induced the appearance of actin stress fibers via a Rho-mediated pathway.

Role of cAMP, PKA and Rap1A in thyroid follicular cell survival.

Cyclic AMP (cAMP) rescues cells from apoptosis stimulated by diverse insults. We examined the role of cAMP as a survival factor, and the signaling pathways through which cAMP affords protection. Rat thyroid cells were selected for these studies given the predominant role of cAMP in thyrotropin (TSH)-stimulated proliferation and as an oncogene in thyroid cells.

Differential signaling of cyclic AMP: opposing effects of exchange protein directly activated by cyclic AMP and cAMP-dependent protein kinase on protein kinase B activation.

The recent discovery of Epac, a novel cAMP receptor protein, opens up a new dimension in studying cAMP-mediated cell signaling. It is conceivable that many of the cAMP functions previously attributed to cAMP-dependent protein kinase (PKA) are in fact also Epac-dependent. The finding of an additional intracellular cAMP receptor provides an opportunity to further dissect the divergent roles that cAMP exerts in different cell types.