Regulation of actin binding and actin bundling activities of fascin by caldesmon coupled with tropomyosin

Cytoskeletal organization in tropomyosin-mediated reversion of ras-transformation: Evidence for Rho kinase pathway

Tropomyosin (TM) family of cytoskeletal proteins is implicated in stabilizing actin microfilaments. Many TM isoforms, including tropomyosin-1 (TM1), are down-regulated in transformed cells. Previously we demonstrated that TM1 is a suppressor of the malignant transformation, and that TM1 reorganizes microfilaments in the transformed cells. To investigate how TM1 induces microfilament organization in transformed cells, we utilized ras-transformed NIH3T3 (DT) cells, and those transduced to express TM1, and/or TM2. Enhanced expression of TM1 alone, but not TM2, results in re-emergence of microfilaments; TM1, together with TM2 remarkably improves microfilament architecture. TM1 induced cytoskeletal reorganization involves an enhanced expression of caldesmon, but not vinculin, alpha-actinin, or gelsolin. In addition, TM1-induced cytoskeletal reorganization and the revertant phenotype appears to involve re-activation of RhoA controlled pathways in DT cells. RhoA expression, which is suppressed in DT cells, is significantly increased in TM1-expressing cells, without detectable changes in the expression of Rac or Cdc42. Furthermore, expression of a dominant negative Rho kinase, or treatment with Y-27632 disassembled microfilaments in normal NIH3T3 and in TM1 expressing cells. These data suggest that reactivation of Rho kinase directed pathways are critical for TM1-mediated microfilament assemblies.

Amlexanox reversibly inhibits cell migration and proliferation and induces the Src-dependent disassembly of actin stress fibers in vitro

Amlexanox binds S100A13 and inhibits the release of fibroblast growth factor 1 (FGF1). Because members of the S100 gene family are known to be involved with the function of the cytoskeleton, we examined the ability of amlexanox to modify the cytoskeleton and report that amlexanox induces a dramatic reduction in the presence of actin stress fibers and the appearance of a random, non-oriented distribution of focal adhesion sites. Correspondingly, amlexanox induces the complete and reversible non-apoptotic inhibition of cell migration and proliferation, and although amlexanox does not induce either the down-regulation of F-actin levels or the depolymerization of actin filaments, it does induce the tyrosine phosphorylation of cortactin, a Src substrate known to regulate actin bundling. In addition, a dominant negative form of Src is able to partially rescue cells from the effect of amlexanox on both the actin cytoskeleton and cell migration. In contrast, the inhibition of cell proliferation by amlexanox correlates with the inhibition of cyclin D1 expression without interference of the receptor tyrosine kinase/mitogen-activated protein kinase signaling pathway. Last, the ability of amlexanox to inhibit FGF1 release is reversible and correlates with the restoration of the actin cytoskeleton, suggesting a role for the actin cytoskeleton in the FGF1 release pathway.

Regulation of microfilament reorganization and invasiveness of breast cancer cells by kinase dead p21-activated kinase-1

Stimulation of growth factor signaling has been implicated in the development of invasive phenotype and p21-activated kinase (PAK1) activation in human breast epithelial cancer cells. To further explore the roles of PAK1 in the invasive behavior of breast cancer cells, in the present study we investigated the influence of inhibition of PAK1 activity on the reorganization of cytoskeleton components that control motility and invasiveness of cells, using a highly invasive breast cancer MDA-MB435 as a model system. Our results demonstrate that overexpression of a kinase dead K299R PAK1 mutant leads to suppression of motile phenotypes as well as invasiveness of cells both in the absence or presence of exogenous heregulin-beta1. In addition, these phenotypic changes were accompanied by a blockade of disassembly of focal adhesion points, stabilization of stress fibers, and enhanced cell spreading and were dependent on the presence of the kinase dead domain but independent of the presence of the Rac/cdc42 intact (Cdc42/Rac interactive binding) domain of PAK1. We also demonstrated that in K299R PAK1-expressing cells, F-actin filaments were stabilized by persistent co-localization with the actin-binding proteins tropomyosin and caldesmon. Extension of these studies to invasive breast cancer MDA-MB231 cells illustrated that conditional expression of kinase-defective K299R PAK1 was also accompanied by persistent cell spreading, multiple focal adhesion points, and reduced invasiveness. Furthermore, inhibition of PAK1 activity in breast cancer cells was associated with a reduction in c-Jun N-terminal kinase activity, inhibition of DNA binding activity of transcription factor AP-1, and suppression of in vivo transcription driven by AP-1 promoter (known to be involved in breast cancer invasion). These findings suggest that PAK1 downstream pathways have a role in the development and maintenance of invasive phenotypes in breast cancer cells.

Mitotic misregulation and human aging

Messenger RNA levels were measured in actively dividing fibroblasts isolated from young, middle-age, and old-age humans and humans with progeria, a rare genetic disorder characterized by accelerated aging. Genes whose expression is associated with age-related phenotypes and diseases were identified. The data also suggest that an underlying mechanism of the aging process involves increasing errors in the mitotic machinery of dividing cells in the postreproductive stage of life. We propose that this dysfunction leads to chromosomal pathologies that result in misregulation of genes involved in the aging process.

Parallel actin bundles and their multiple actin-bundling proteins

Parallel actin bundles are present in a diverse array of structures, where they are critical determinants of cellular shape and physiology. In the past 18 months, new findings have solidified the concept that parallel actin bundles are assembled in cells through the sequential action of multiple actin-bundling proteins and have begun to shed light on the roles played by the individual actin-bundling proteins.

Cell-matrix adhesions differentially regulate fascin phosphorylation

Cell adhesion to individual macromolecules of the extracellular matrix has dramatic effects on the subcellular localization of the actin-bundling protein fascin and on the ability of cells to form stable fascin microspikes. The actin-binding activity of fascin is down-regulated by phosphorylation, and we used two differentiated cell types, C2C12 skeletal myoblasts and LLC-PK1 kidney epithelial cells, to examine the hypothesis that cell adhesion to the matrix components fibronectin, laminin-1, and thrombospondin-1 differentially regulates fascin phosphorylation. In both cell types, treatment with the PKC activator 12-tetradecanoyl phorbol 13-acetate (TPA) or adhesion to fibronectin led to a diffuse distribution of fascin after 1 h. C2C12 cells contain the PKC family members alpha, gamma, and lambda, and PKCalpha localization was altered upon cell adhesion to fibronectin. Two-dimensional isoelectric focusing/SDS-polyacrylamide gels were used to determine that fascin became phosphorylated in cells adherent to fibronectin and was inhibited by the PKC inhibitors calphostin C and chelerythrine chloride. Phosphorylation of fascin was not detected in cells adherent to thrombospondin-1 or to laminin-1. LLC-PK1 cells expressing green fluorescent protein (GFP)-fascin also displayed similar regulation of fascin phosphorylation. LLC-PK1 cells expressing GFP-fascin S39A, a nonphosphorylatable mutant, did not undergo spreading and focal contact organization on fibronectin, whereas cells expressing a GFP-fascin S39D mutant with constitutive negative charge spread more extensively than wild-type cells. In contrast, C2C12 cells coexpressing S39A fascin with endogenous fascin remained competent to form microspikes on thrombospondin-1, and cells that expressed fascin S39D attached to thrombospondin-1 but did not form microspikes. Blockade of PKCalpha activity by TPA-induced down-regulation led to actin association of wild-type fascin in fibronectin-adherent C2C12 and LLC-PK1 cells but did not alter the distribution of S39A or S39D fascins. The association of fascin with actin in fibronectin-adherent cells was also evident in the presence of an inhibitory antibody to integrin alpha5 subunit. These novel results establish matrix-initiated PKC-dependent regulation of fascin phosphorylation at serine 39 as a mechanism whereby matrix adhesion is coupled to the organization of cytoskeletal structure.

Tyrosine phosphorylation of caldesmon is required for binding to the Shc.Grb2 complex

S3-v-erbB is a retroviral oncogene that encodes a ligand-independent, transforming mutant of the epidermal growth factor receptor. This oncogene has been shown to be sarcomagenic in vivo and to transform fibroblasts in vitro. Our previous studies (McManus, M. J., Lingle, W. L., Salisbury, J. L., and Maihle, N. J. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 11351-11356) showed that expression of S3-v-erbB in primary fibroblasts results in the tyrosine phosphorylation of caldesmon (CaD), an actin- and calmodulin-binding protein. This phosphorylation is transformation-associated, and the phosphorylated form of CaD is associated with a signaling complex consisting of Shc, Grb2, and Sos in transformed fibroblasts. To identify the tyrosine phosphorylation site(s) in the CaD molecule and to further elucidate the functional role of CaD tyrosine phosphorylation in S3-v-ErbB oncogenic signaling, we have generated a series of mutant CaDs in which one or more tyrosine residues have been replaced with phenylalanine. Using a CaD null cell line, DF1 cells (an immortalized chicken embryo fibroblast cell line), and transient transfection assays, we demonstrated that Tyr-27 and Tyr-393 are the major sites of tyrosine phosphorylation on CaD. Interestingly, Tyr-27 is located within the myosin binding domain of CaD, and Tyr-393 is adjacent to one of the major actin binding and actomyosin ATPase inhibitory domains. Our studies also show that the tyrosine phosphorylation of CaD enhances its binding to the Shc.Grb2 complex. Specifically, replacement of Tyr-27, but not of Tyr-165 or Tyr-393, significantly reduces the ability of CaD to interact with the Shc. Grb2 complex. Together, these studies demonstrate that the major sites of tyrosine phosphorylation on CaD are located in the myosin and actin binding domains of CaD and that Tyr-27 is the major tyrosine phosphorylation site through which CaD interacts with the Shc.Grb2 complex.