Grb2 regulation of the actin-based cytoskeleton is required for ligand-independent EGF receptor-mediated oncogenesis

Cyclin-Dependent Kinase 5 (CDK5) Controls Melanoma Cell Motility, Invasiveness, and Metastatic Spread-Identification of a Promising Novel therapeutic target

Despite considerable progress in recent years, the overall prognosis of metastatic malignant melanoma remains poor, and curative therapeutic options are lacking. Therefore, better understanding of molecular mechanisms underlying melanoma progression and metastasis, as well as identification of novel therapeutic targets that allow inhibition of metastatic spread, are urgently required. The current study provides evidence for aberrant cyclin-dependent kinase 5 (CDK5) activation in primary and metastatic melanoma lesions by overexpression of its activator protein CDK5R1/p35. Moreover, using melanoma in vitro model systems, shRNA-mediated inducible knockdown of CDK5 was found to cause marked inhibition of cell motility, invasiveness, and anchorage-independent growth, while at the same time net cell growth was not affected. In vivo, CDK5 knockdown inhibited growth of orthotopic xenografts as well as formation of lung and liver colonies in xenogenic injection models mimicking systemic metastases. Inhibition of lung metastasis was further validated in a syngenic murine melanoma model. CDK5 knockdown was accompanied by dephosphorylation and overexpression of caldesmon, and concomitant caldesmon knockdown rescued cell motility and proinvasive phenotype. Finally, it was found that pharmacological inhibition of CDK5 activity by means of roscovitine as well as by a novel small molecule CDK5-inhibitor, N-(5-isopropylthiazol-2-yl)-3-phenylpropanamide, similarly caused marked inhibition of invasion/migration, colony formation, and anchorage-independent growth of melanoma cells. Thus, experimental data presented here provide strong evidence for a crucial role of aberrantly activated CDK5 in melanoma progression and metastasis and establish CDK5 as promising target for therapeutic intervention.

Clinicopathologic significance of expression of EEF1A2 and GRB2 in pancreatic adenocarcinoma

AIM: To investigate the expression of eukaryotic elongation factor 1A2 (EEF1A2) and growth factor receptor-bound 2 (GRB2) in pancreatic adenocarcinoma (PA) and to analyze their clinicopathologic significance.

METHODS: Expression of EEF1A2 and GRB2 was examined by immunohistochemistry in 97 PA specimens and surrounding pancreatic tissues.

RESULTS: EEF1A2 expression was absent in normal pancreatic tissue. In contrast, EEF1A2 showed positive immunoreactivity in 77.8% (76/97) of PA cases. The increased eEF1A2 expression was significantly associated with the presence of nodal metastasis (χ² = 4.28, P = 0.039) and perineural invasion (χ² = 4.11, P = 0.043). The expression rate of GRB2 in 97 PA specimens and surrounding pancreatic tissues were 82.5% (80/97) and 30.2% (31/97), respectively. The expression level of GRB2 in PA was significantly higher than that in surrounding pancreatic tissues (χ² = 48.5, P < 0.001). The positive rate of GRB2 expression was significantly correlated with lymph node metastasis (χ² = 4.63, P = 0.031). There was a positive expression between the expression of EEF1A2 and that of GRB2 in PA (r_s = 0.451, P < 0.001).

CONCLUSION: The expression of GRB2 and EEF1A2 is closely correlated with the biological behavior of PA. The expression of GRB2 is significantly correlated with that of EEF1A2.

Proteomic analysis to identify the cellular responses induced by hydroquinone in human embryonic lung fibroblasts

Hydroquinone (HQ), a major metabolite of benzene, is used widely as a reagent in photographic developers, as an antioxidant in the manufacture of rubber, as a polymerization inhibitor for acrylic and vinyl acetate monomers, and in cosmetic products as a skin-lightening agent. But the mechanism of its effect on human cells is far from clear. In the present work, we studied the cellular response induced by HQ using proteomic approaches. Human embryonic lung fibroblasts (HLFs) were treated with 100 mu M HQ for 24 h. This dose of HQ was found in assays to significantly decrease cell viability. After treatment, two-dimensional electrophoresis was performed using the Amersham Bioscience 2DE system following the manufacturer's instructions. Proteins were visualized by staining with colloidal coomassie blue. Fifteen protein spots showed significant changes after HQ treatment. Eleven protein spots were identified by peptide mass fingerprinting using MALDI-TOF or by peptide sequence tagging using MALDI-TOF-TOF. Among them are transaldolase, growth factor receptor-bound protein 2, mutant beta -actin, gamma -actin, Lasp-1, TAR DNA-binding protein, and a protein similar to neural precursor cell-expressed protein. These include proteins involved in oxidative stress, cellular signaling, RNA splicing, and cytoskeleton reconstruction. Most of their involvements in the cellular responses to HQ have not been reported. Therefore, our findings may offer new insights into the mechanisms of HQ cytotoxicity and these proteins may serve as new biomarkers for detecting exposure of human populations to HQ. It is suggested that proteomic approaches may provide new strategies to evaluate the toxicity of xenobiotics.

Rhein inhibits invasion and migration of human nasopharyngeal carcinoma cells in vitro by down-regulation of matrix metalloproteinases-9 and vascular endothelial growth factor

Progression of cancer invasion is believed to be dependent on the remodeling of extracellular matrix induced by tumor cells. Rhein has been shown to inhibit the growth and proliferation of human nasopharyngeal carcinoma (NPC) cells. However, the molecular mechanism underlying rhein-induced inhibition of cancer invasion has not been explored. Herein, we show that rhein could inhibit the invasion and migration of NPC cells in vitro. Rhein inhibits invasion by reducing the expression of matrix metalloproteinase-9 (MMP-9) and vascular endothelial growth factor (VEGF). Moreover, we demonstrate that the pathway involved in rhein-inhibited invasion is presumably through the growth factor receptor bound protein 2/son of sevenless-Ras-mitogen-activated protein kinase (GRB2/SOS-Ras-MAPK) pathway, as shown by an decrease in the expression levels of GRB2, SOS-1 and Ras as well as led to suppression of the phosphorylation of extracellular signal-regulated kinase (ERK) and p38 MAPK. Further study has shown that rhein also inhibited activation of transcription factor nuclear factor kappaB (NF-kappaB), which is known to implicate the regulation of MMP-9 and VEGF gene expression in cancer invasion. Our findings suggest that rhein inhibits the invasion of NPC cells may be mediated in part through the suppression of MMP-9 and VEGF expression via the modulation of NF-kappaB signaling pathway.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Phosphorylation of caldesmon during smooth muscle contraction and cell migration or proliferation

The actin-binding protein caldesmon (CaD) exists both in smooth muscle (the heavy isoform, h-CaD) and non-muscle cells (the light isoform, l-CaD). In smooth muscles h-CaD binds to myosin and actin simultaneously and modulates the actomyosin interaction. In non-muscle cells l-CaD binds to actin and stabilizes the actin stress fibers; it may also mediate the interaction between actin and non-muscle myosins. Both h- and l-CaD are phosphorylated in vivo upon stimulation. The major phosphorylation sites of h-CaD when activated by phorbol ester are the Erk-specific sites, modification of which is attenuated by the MEK inhibitor PD98059. The same sites in l-CaD are also phosphorylated when cells are stimulated to migrate, whereas in dividing cells l-CaD is phosphorylated more extensively, presumably by cdc2 kinase. Both Erk and cdc2 are members of the MAPK family. Thus it appears that CaD is a downstream effector of the Ras signaling pathways. Significantly, the phosphorylatable serine residues shared by both CaD isoforms are in the C-terminal region that also contains the actin-binding sites. Biochemical and structural studies indicated that phosphorylation of CaD at the Erk sites is accompanied by a conformational change that partially dissociates CaD from actin. Such a structural change in h-CaD exposes the myosin-binding sites on the actin surface and allows actomyosin interactions in smooth muscles. In the case of non-muscle cells, the change in l-CaD weakens the stability of the actin filament and facilitates its disassembly. Indeed, the level of l-CaD modification correlates very well in a reciprocal manner with the level of actin stress fibers. Since both cell migration and cell division require dynamic remodeling of actin cytoskeleton that leads to cell shape changes, phosphorylation of CaD may therefore serve as a plausible means to regulate these processes. Thus CaD not only links the smooth muscle contractility and non-muscle motility, but also provides a common mechanism for the regulation of cell migration and cell proliferation.

Inhibition of angiogenesis by growth factor receptor bound protein 2-Src homology 2 domain bound antagonists

Growth factor receptor bound protein 2 (Grb2) is an intracellular adaptor protein that participates in the signal transduction cascades of several angiogenic factors, including hepatocyte growth factor, basic fibroblast growth factor, and vascular endothelial growth factor. We described previously the potent blockade of hepatocyte growth factor–stimulated cell motility, matrix invasion, and epithelial tubulogenesis by synthetic Grb2-Src homology 2 (SH2) domain binding antagonists. Here, we show that these binding antagonists block basic morphogenetic events required for angiogenesis, including hepatocyte growth factor–, vascular endothelial growth factor–, and basic fibroblast growth factor–stimulated endothelial cell proliferation and migration, as well as phorbol 12-myristate 13-acetate–stimulated endothelial cell migration and matrix invasion. The Grb2-SH2 domain binding antagonists also impair angiogenesis in vitro, as shown by the inhibition of cord formation by macrovascular endothelial cells on Matrigel. We further show that a representative compound inhibits angiogenesis in vivo as measured using a chick chorioallantoic membrane assay. These results suggest that Grb2 is an important mediator of key proangiogenic events, with potential application to pathologic conditions where neovascularization contributes to disease progression. In particular, the well-characterized role of Grb2 in signaling cell cycle progression together with our present findings suggests that Grb2-SH2 domain binding antagonists have the potential to act as anticancer drugs that target both tumor and vascular cell compartments.