No abstract

Activation of Ras induces a variety of cellular responses depending on the specific effector activated and the intensity and amplitude of this activation. We have previously shown that calmodulin is an essential molecule in the down-regulation of the Ras/Raf/MEK/extracellularly regulated kinase (ERK) pathway in cultured fibroblasts and that this is due at least in part to an inhibitory effect of calmodulin on Ras activation. Here we show that inhibition of calmodulin synergizes with diverse stimuli (epidermal growth factor, platelet-derived growth factor, bombesin, or fetal bovine serum) to induce ERK activation. Moreover, even in the absence of any added stimuli, activation of Ras by calmodulin inhibition was observed. To identify the calmodulin-binding protein involved in this process, calmodulin affinity chromatography was performed. We show that Ras and Raf from cellular lysates were able to bind to calmodulin. Furthermore, Ras binding to calmodulin was favored in lysates with large amounts of GTP-bound Ras, and it was Raf independent. Interestingly, only one of the Ras isoforms, K-RasB, was able to bind to calmodulin. Furthermore, calmodulin inhibition preferentially activated K-Ras. Interaction between calmodulin and K-RasB is direct and is inhibited by the calmodulin kinase II calmodulin-binding domain. Thus, GTP-bound K-RasB is a calmodulin-binding protein, and we suggest that this binding may be a key element in the modulation of Ras signaling.

The Rho GTPase family member RhoE regulates actin filaments partly by binding to and inhibiting ROCK I, a serine/threonine kinase that induces actomyosin contractility. Here, we show that ROCK I can phosphorylate multiple residues on RhoE in vitro. In cells, ROCK Iphosphorylated RhoE localizes in the cytosol, whereas unphosphorylated RhoE is primarily associated with membranes. Phosphorylation has no effect on RhoE binding to ROCK I, but instead increases RhoE protein stability. Using phospho-specific antibodies, we show that ROCK phosphorylates endogenous RhoE at serine 11 upon cell stimulation with platelet-derived growth factor, and that this phosphorylation requires an active protein kinase C signalling pathway. In addition, we demonstrate that phosphorylation of RhoE correlates with its activity in inducing stress fibre disruption and inhibiting Rasinduced transformation. This is the first demonstration of an endogenous Rho family member being phosphorylated in vivo and indicates that phosphorylation is an important mechanism to control the stability and function of this GTPase-deficient Rho protein.

Rho GTPases are major regulators of cytoskeletal dynamics, but they also affect cell proliferation, transformation, and oncogenesis. RhoE, a member of the Rnd subfamily that does not detectably hydrolyze GTP, inhibits RhoA/ROCK signaling to promote actin stress fiber and focal adhesion disassembly. We have generated fibroblasts with inducible RhoE expression to investigate the role of RhoE in cell proliferation. RhoE expression induced a loss of stress fibers and cell rounding, but these effects were only transient. RhoE induction inhibited cell proliferation and serum-induced S-phase entry. Neither ROCK nor RhoA inhibition accounted for this response. Consistent with its inhibitory effect on cell cycle progression, RhoE expression was induced by cisplatin, a DNA damage-inducing agent. RhoE-expressing cells failed to accumulate cyclin D1 or p21 cip1 protein or to activate E2F-regulated genes in response to serum, although ERK, PI3-K/Akt, FAK, Rac, and cyclin D1 transcription was activated normally. The expression of proteins that bypass the retinoblastoma (pRb) family cell cycle checkpoint, including human papillomavirus E7, adenovirus E1A, and cyclin E, rescued cell cycle progression in RhoE-expressing cells. RhoE also inhibited Ras-and Raf-induced fibroblast transformation. These results indicate that RhoE inhibits cell cycle progression upstream of the pRb checkpoint.Rho GTPases are major regulators of cytoskeleton dynamics in eukaryotic cells and consequently have a crucial role in biological processes involving the cytoskeleton, such as the control of cell shape and motility (11). In mammalian cells, the best-characterized Rho family members are RhoA, Rac1, and Cdc42. Like other small GTPases, these proteins are molecular switches that, except for members of the Rnd subfamily (Rnd1, Rnd2, and RhoE/Rnd3) and RhoH/TTF, cycle between an active GTP-bound state and an inactive GDP-bound state. Once activated, they bind to and activate many downstream effectors, most of which are directly implicated in cytoskeletal regulation, leading to their characteristic effects, namely, the formation of actin stress fibers downstream of RhoA and the induction of actin-containing protrusions such as lamellipodia and membrane ruffles or filopodia downstream of Rac1 and Cdc42 (22,[32][33][34].In addition to regulating cytoskeletal dynamics, Rho GTPases affect other cellular responses, such as transcriptional regulation, cell proliferation, and transformation (15,40). The ectopic expression of activated mutants of RhoA, Rac1, and Cdc42 has been shown to promote cell cycle entry in quiescent fibroblasts, and the inhibition of their function with dominant-interfering mutants or inhibitors leads to cell cycle arrest (24, 50). Furthermore, activated mutants of these Rho GTPases positively contribute to cell transformation, and conversely, dominant-negative mutants have an inhibitory effect on Ras-and Raf-induced transformation (27-29). On the whole, the notion that Rho GTPases are positive regulators of both cell cycle progression ...

Resistance to TGF-b1 occurred in pancreatic cancer cells suggesting that inactivation of TGF-b inhibitory signaling pathways may play an important role in human pancreatic cancer. The aim of our study was to determine the presence of alterations in the main putative components of the TGF-b inhibitory signaling pathways (p15, Smad4, Smad2, TGFb-RII, CDC25A). A panel of human carcinomas of the exocrine pancreas orthotopically implanted and perpetuated in nude mice and pancreatic cancer cell lines were studied. p15 gene alterations, mainly homozygous deletions that involved exons 1 and/or 2, were found in the 62.5% (5 of 8) of pancreatic xenografts whereas Smad4 gene aberrations were found in one of eight xenografts and in two of seven cell lines. Additional aberrations in these genes were acquired during in vivo perpetuation and distal dissemination. Paradoxically, TGFb-RII overexpression and a decrease in CDC25A protein levels were found in all tumors and cell lines. In one cell line, resistance to TGFb1 occurred in the absence of alterations in the genes analysed so far. We conclude that all human pancreatic tumor cells analysed herein have non-functional TGF-b pathways. The majority of cells harbor alterations in at least one of the putative components of TGF-b pathways, mainly in p15 and Smad4 genes. These results suggest that inactivation of TGF-b signaling pathways plays an important role in human pancreatic tumorigenesis.

Rnd3/RhoE has two distinct functions, regulating the actin cytoskeleton and cell proliferation. This might explain why its expression is often altered in cancer and by multiple stimuli during development and disease. Rnd3 together with its relatives Rnd1 and Rnd2 are atypical members of the Rho GTPase family in that they do not hydrolyse GTP. Rnd3 and Rnd1 both antagonise RhoA/ROCK-mediated actomyosin contractility, thereby regulating cell migration, smooth muscle contractility and neurite extension. In addition, Rnd3 has been shown to have a separate role in inhibiting cell cycle progression by reducing translation of cell cycle regulators, including cyclin D1 and Myc. We propose that Rnd3 could act as a tumour suppressor to limit proliferation, but when mutations bypass this activity of Rnd3, it can promote cancer invasion through its effects in the actin cytoskeleton.

We previously showed that K-Ras is a calmodulin-binding protein. Involvement of this interaction in anterograde and retrograde transport of K-Ras was then suggested. To test this we have analyzed here the domains of K-Ras essential for the interaction with calmodulin. At least three different regions in the K-Ras molecule were important; they are the hypervariable region, the ␣-helix between amino acids 151 and 166, and the Switch II. Within the hypervariable region, both the hydrophobic farnesyl group and the positive-charged amino acids were essential for the interaction between K-Ras and calmodulin in cellular extracts. Consistently, K-Ras S181D, which mimics phosphorylation of Ser-181 of K-Ras, also completely abolished binding to calmodulin. K-Ras mutants correctly farnesylated that did not bind calmodulin were all located at plasma membrane, showing that calmodulin interaction was not required for the transport of K-Ras to plasma membrane. In NIH3T3 cells, K-Ras and calmodulin colocalized mainly in the plasma membrane even after the addition of Ca 2؉ ionophore, indicating that interaction did not directly lead to K-Ras internalization. Furthermore, using a K-Ras with impaired binding to calmodulin but with membrane localization, we could demonstrate in striatal neurones that interaction between K-Ras and calmodulin was not required for Golgi K-Ras translocation induced by Ca 2؉

Cisplatin is a conventional chemotherapeutic agent that binds covalently to purine DNA bases and mediates cellular apoptosis. A better understanding of the downstream cellular targets of cisplatin will provide information on its mechanism of action and help to understand the mechanism of drug resistance. In this study, we have investigated the effects of cisplatin in a panel of colon carcinoma cell lines and the involvement of the phosphoinositide-3-kinase/forkhead/winged helix box class O (FOXO) pathway in cisplatin action and resistance. Cisplatin-sensitive and cisplatin-resistant cell lines have been characterized in cell viability, flow cytometry, and clonogenic assays. The main components of the phosphoinositide-3-kinase/protein kinase B pathway, particularly FOXO3a, have been analyzed in sensitive and resistant cells on cisplatin treatment. Interestingly, in sensitive cells, cisplatin induces FOXO3a dephosphorylation and nuclear translocation, and expression of its target genes, whereas in resistant cells the effect of cisplatin on FOXO3a is incomplete. Consistent with this, protein kinase B/FOXO signaling axis modulators triciribine and psammaplysene A sensitize the resistant HT29 cells to cisplatin treatment. Critically, knockdown of FOXO3a expression using small interfering RNA rescues sensitive SW620 cells from cisplatin-induced short-and long-term cell death. Together, our findings suggest that FOXO3a is a relevant mediator of the cytotoxic effects of cisplatin in colon cancer cells. [Mol Cancer Ther 2008;7(10):3237 -46]