Unphosphorylated and tyrosine-phosphorylated forms of a focal adhesion protein, paxillin, are substrates for calpain II in vitro: implications for the possible involvement of calpain II in mitosis-specific degradation of paxillin

The hyaluronic acid inhibitor 4-methylumbelliferone is an NSMase2 activator-role of Ceramide in MU anti-tumor activity

Increased synthesis of hyaluronic acid (HA) is often associated with increased metastatic potential and invasivity of tumor cells. 4-Methylumbelliferone (MU) is an inhibitor of HA synthesis, and has been studied as a potential anti-tumor drug to inhibit the growth of primary tumors and distant metastasis of tumor cells. Although several studies reported that the anticancer effects of MU are mediated by inhibition of HA signaling, the mechanism still needs to be clarified. In a previous study we demonstrated the regulation of HA synthesis by ceramide, and now show how MU activated neutral sphingomyelinase2 (NSMase2) generates ceramides and mediates MU induced inhibition of HA synthesis, cell migration and invasion, and apoptosis of tumor cells. Using a HA enriched mouse oligodendroglioma cell line G26-24 we found that MU elevated the activity of NSMase2 and increased ceramide levels, which in turn increased phosphatase PP2A activity. Further, the activated PP2A reduced phosphorylation of Akt, decreased activities of HA synthase2 (HAS2) and calpains, and inhibited both the synthesis of HA, and the migration and invasion of G26-24 tumor cells. In addition, MU mediated ceramide stimulated activation of p53 and caspase-3, reduced SIRT1 expression and decreased G26-24 viability. The mechanism of the MU anticancer therefore initially involves NSMase2/ceramide/PP2A/AKT/HAS2/caspase-3/p53/SIRT1 and the calpain signaling pathway, suggesting that ceramides play a key role in the ability of a tumor to become aggressively metastatic and grow.

Calpain in Breast Cancer: Role in Disease Progression and Treatment Response

The calpains are a family of intracellular cysteine proteases that function in a wide array of cellular activities, including cytoskeletal remodelling, survival and apoptosis. The ubiquitously expressed micro (µ)-calpain and milli (m)-calpain are archetypal family members that require calcium for function and can be inhibited by their endogenous inhibitor calpastatin. This review describes the role of the calpain system in the prognosis of breast cancer and disease progression, in addition to the role of the calpain system in the response to breast cancer treatments, including chemotherapeutic, endocrine and targeted therapies.

Calpain-2 expression is associated with response to platinum based chemotherapy, progression-free and overall survival in ovarian cancer

Ovarian cancer is routinely treated with surgery and platinum-based chemotherapy. Resistance is a major obstacle in the efficacy of this chemotherapy regimen and the ability to identify those patients at risk of developing resistance is of considerable clinical importance. The expression of calpain-1, calpain-2 and calpastatin were determined using standard immunohistochemistry on a tissue microarray of 154 primary ovarian carcinomas from patients subsequently treated with platinum-based adjuvant chemotherapy. High levels of calpain-2 expression was significantly associated with platinum resistant tumours (P = 0.031). Furthermore, high expression of calpain-2 was significantly associated with progression-free (P = 0.049) and overall survival (P = 0.006) in this cohort. The association between calpain-2 expression and overall survival remained significant in multivariate analysis accounting for tumour grade, stage, optimal debulking and platinum sensitivity (hazard ratio = 2.174; 95% confidence interval = 1.144–4.130; P = 0.018). The results suggest that determining calpain-2 expression in ovarian carcinomas may allow prognostic stratification of patients treated with surgery and platinum-based chemotherapy. The findings of this study warrant validation in a larger clinical cohort.

Effects of lysophosphatidic acid on calpain-mediated proteolysis of focal adhesion kinase in human prostate cancer cells

BACKGROUND

Calcium-mediated proteolysis plays an important role in cell migration. Lysophosphatidic acid (LPA), a lipid mediator present in serum, enhances migration of carcinoma cells. The effects of LPA on calpain-mediated proteolysis were, therefore, examined in PC-3, a human prostate cancer cell line.

METHODS

Cultured PC-3 cells were used in studies utilizing pharmacologic interventions, immunoblotting, and confocal immunolocalization.

RESULTS

Focal adhesion kinase (FAK), a tyrosine kinase involved in cell adhesion, is rapidly proteolyzed in serum-starved PC-3 cells exposed to the calcium ionophore, ionomycin; Nck, p130CAS, PKCα, and Ras-GAP are also degraded. Thapsigargin, which causes more moderate increases in intracellular calcium, induces partial proteolysis of these proteins. Calpain inhibitors block the proteolytic responses to ionomycin and thapsigargin. Ionomycin does not induce proteolysis in cells maintained in serum, suggesting a protective role for growth factors contained in serum. LPA causes minor FAK proteolysis when added alone, but protects against ionomycin-induced proteolysis in a time-dependent manner. LPA also protects against the cell detachment that eventually follows ionomycin treatment. The response to LPA is blocked by an LPA receptor antagonist. A similar effect of LPA is observed in ionomycin-treated Rat-1 fibroblasts. In PC-3 cells, the protective effects of LPA and serum are correlated with phosphorylation and redistribution of paxillin, suggesting roles for phosphorylation-mediated protein-protein interactions.

CONCLUSIONS

The complex effects of LPA on calpain-mediated proteolysis of FAK and other adhesion proteins are likely to play a role in the ability of LPA to promote attachment, migration, and survival of prostate cancer cells.

The calpain system and cancer

The calpains are a conserved family of cysteine proteinases that catalyse the controlled proteolysis of many specific substrates. Calpain activity is implicated in several fundamental physiological processes, including cytoskeletal remodelling, cellular signalling, apoptosis and cell survival. Calpain expression is altered during tumorigenesis, and the proteolysis of numerous substrates, such as inhibitors of nuclear factor-κB (IκB), focal adhesion proteins (including, focal adhesion kinase and talin) and proto-oncogenes (for example, MYC), has been implicated in tumour pathogenesis. Recent evidence indicates that the increased expression of certain family members might influence the response to cancer therapies, providing justification for the development of novel calpain inhibitors.

Paxillin comes of age

Paxillin is a multi-domain scaffold protein that localizes to the intracellular surface of sites of cell adhesion to the extracellular matrix. Through the interactions of its multiple protein-binding modules, many of which are regulated by phosphorylation, paxillin serves as a platform for the recruitment of numerous regulatory and structural proteins that together control the dynamic changes in cell adhesion, cytoskeletal reorganization and gene expression that are necessary for cell migration and survival. In particular, paxillin plays a central role in coordinating the spatial and temporal action of the Rho family of small GTPases, which regulate the actin cytoskeleton, by recruiting an array of GTPase activator, suppressor and effector proteins to cell adhesions. When paxillin was first described 18 years ago, the amazing complexity of cell-adhesion organization, dynamics and signaling was yet to be realized. Herein we highlight our current understanding of how the multiple protein interactions of paxillin contribute to the coordination of cell-adhesion function.

Membrane depolarization inhibits spiral ganglion neurite growth via activation of multiple types of voltage sensitive calcium channels and calpain

The effect of membrane electrical activity on spiral ganglion neuron (SGN) neurite growth remains unknown despite its relevance to cochlear implant technology. We demonstrate that membrane depolarization delays the initial formation and inhibits the subsequent extension of cultured SGN neurites. This inhibition depends directly on the level of depolarization with higher levels of depolarization causing retraction of existing neurites. Cultured SGNs express subunits for L-type, N-type, and P/Q type voltage-gated calcium channels (VGCCs) and removal of extracellular Ca(2+) or treatment with a combination of L-type, N-type, and P/Q-type VGCC antagonists rescues SGN neurite growth under depolarizing conditions. By measuring the fluorescence intensity of SGNs loaded with the fluorogenic calpain substrate t-butoxy carbonyl-Leu-Met-chloromethylaminocoumarin (20 microM), we demonstrate that depolarization activates calpains. Calpeptin (15 microM), a calpain inhibitor, prevents calpain activation by depolarization and rescues neurite growth in depolarized SGNs suggesting that calpain activation contributes to the inhibition of neurite growth by depolarization.

Serine phosphorylation regulates paxillin turnover during cell migration

Background

Paxillin acts as an adaptor protein that localizes to focal adhesion. This protein is regulated during cell migration by phosphorylation on tyrosine, serine and threonine residues. Most of these phosphorylations have been implicated in the regulation of different steps of cell migration. The two major phosphorylation sites of paxillin in response to adhesion to an extracellular matrix are serines 188 and 190. However, the function of this phosphorylation event remains unknown. The purpose of this work was to determine the role of paxillin phosphorylation on residues S188 and S190 in the regulation of cell migration.

Results

We used NBT-II epithelial cells that can be induced to migrate when plated on collagen. To examine the role of paxillin serines 188/190 in cell migration, we constructed an EGFP-tagged paxillin mutant in which S188/S190 were mutated into unphosphorylatable alanine residues. We provide evidence that paxillin is regulated by proteasomal degradation following polyubiquitylation of the protein. During active cell migration on collagen, paxillin is protected from proteasome-dependent degradation. We demonstrate that phosphorylation of serines 188/190 is necessary for the protective effect of collagen. In an effort to understand the physiological relevance of paxillin protection from degradation, we show that cells expressing the paxillin S188/190A interfering mutant spread less, have reduced protrusive activity but migrate more actively.

Conclusion

Our data demonstrate for the first time that serine-regulated degradation of paxillin plays a key role in the modulation of membrane dynamics and consequently, in the control of cell motility.

Regulation of paxillin family members during epithelial-mesenchymal transformation: a putative role for paxillin delta

Epithelial-mesenchymal transformation (EMT) and the resulting induction of cell motility are essential components of tissue remodeling during embryonic development and wound repair, as well as tumor progression to an invasive metastatic phenotype. Paxillin, a multi-domain adaptor and phosphoprotein has previously been implicated in integrin signaling and cell motility. In this report we characterize a novel paxillin gene product, paxillin delta, generated from an evolutionarily conserved internal translation initiation site within the full-length paxillin mRNA. Paxillin delta, which lacks the key phosphorylation sites Y31 and Y118 as well as the ILK and actopaxin binding LD1 motif, exhibits a restricted distribution to epithelial cell types and is downregulated during TGF-beta1-induced EMT of normal murine mammary gland (NMuMG) epithelial cells. Interestingly, Hic-5, a paxillin superfamily member, exhibits a reciprocal protein expression profile to paxillin delta. In addition, paxillin delta expression is maintained following NMuMG differentiation in a 3D collagen I gel while other focal adhesion components are downregulated. Paxillin delta protein expression coincided with reduced paxillin tyrosine phosphorylation in NMuMG cells and paxillin delta overexpression in CHO.K1 cells inhibited adhesion-mediated tyrosine phosphorylation of paxillin. Forced expression of paxillin delta in NMuMG cells suppressed cell migration whereas Hic-5 overexpression stimulated motility. Together our data support a role for paxillin delta as a naturally occurring functional antagonist of paxillin signaling potentially through suppression of a Crk-mediated pathway during processes associated with cell migration.

Cyclin E Both Regulates and Is Regulated by Calpain 2, a Protease Associated with Metastatic Breast Cancer Phenotype

Overexpression of cyclin E in breast tumors is associated with a poor response to tamoxifen therapy, greater genomic instability, more aggressive behavior, and a poor clinical prognosis. These tumors also express low molecular weight isoforms of cyclin E that are associated with higher kinase activity and increased metastatic potential. In the current study, we show that cyclin E overexpression in MCF7 cells transactivates the expression of calpain 2, leading to proteolysis of cyclin E as well as several known calpain substrates including focal adhesion kinase (FAK), calpastatin, pp60src, and p53. In vivo inhibition of calpain activity in MCF7-cyclin E cells impedes cyclin E proteolysis, whereas in vivo induction of calpain activity promotes cyclin E proteolysis. An analysis of human breast tumors shows that high levels of cyclin E are coincident with the expression of the low molecular weight isoforms, high levels of calpain 2 protein, and proteolysis of FAK. Lastly, studies using a mouse model of metastasis reveal that highly metastatic tumors express proteolyzed cyclin E and FAK when compared to tumors with a low metastatic potential. Our results suggest that cyclin E–dependent deregulation of calpain may be pivotal in modifying multiple cellular processes that are instrumental in the etiology and progression of breast cancer.

Paxillin: adapting to change

Molecular scaffold or adaptor proteins facilitate precise spatiotemporal regulation and integration of multiple signaling pathways to effect the optimal cellular response to changes in the immediate environment. Paxillin is a multidomain adaptor that recruits both structural and signaling molecules to focal adhesions, sites of integrin engagement with the extracellular matrix, where it performs a critical role in transducing adhesion and growth factor signals to elicit changes in cell migration and gene expression.

SRC-mediated phosphorylation of focal adhesion kinase couples actin and adhesion dynamics to survival signaling

Integrin-associated focal adhesions not only provide adhesive links between cellular actin and extracellular matrix but also are sites of signal transmission into the cell interior. Many cell responses signal through focal adhesion kinase (FAK), often by integrin-induced autophosphorylation of FAK or phosphorylation by Src family kinases. Here, we used an interfering FAK mutant (4-9F-FAK) to show that Src-dependent FAK phosphorylation is required for focal adhesion turnover and cell migration, by controlling assembly of a calpain 2/FAK/Src/p42ERK complex, calpain activation, and proteolysis of FAK. Expression of 4-9F-FAK in FAK-deficient fibroblasts also disrupts F-actin assembly associated with normal adhesion and spreading. In addition, we found that FAK's ability to regulate both assembly and disassembly of the actin and adhesion networks may be linked to regulation of the protease calpain. Surprisingly, we also found that the same interfering 4-9F-FAK mutant protein causes apoptosis of serum-deprived, transformed cells and suppresses anchorage-independent growth. These data show that Src-mediated phosphorylation of FAK acts as a pivotal regulator of both actin and adhesion dynamics and survival signaling, which, in turn, control apparently distinct processes such as cell migration and anchorage-independent growth. This also highlights that dynamic regulation of actin and adhesions (which include the integrin matrix receptors) is critical to signaling output and biological responses.

Activation of m-calpain is required for chromosome alignment on the metaphase plate during mitosis

Calpains form a superfamily of Ca(2+)-dependent intracellular cysteine proteases with various isoforms. Two isoforms, micro- and m-calpains, are ubiquitously expressed and known as conventional calpains. It has been previously shown that the mammalian calpains are activated during mitosis by transient increases in cytosolic Ca(2+) concentration. However, it is still unknown whether the activation of calpains contributes to particular events in mitosis. With the use of RNA interference (RNAi), we investigated the roles of calpains in mitosis. Cells reduced the levels of m-calpain, but not mu-calpain, arrested at prometaphase and failed to align their chromosomes at the spindle equator. Specific peptidyl calpain inhibitors also induced aberrant mitosis with chromosome misalignment. Although both m-calpain RNAi and calpain inhibitors affected neither the separation of centrosomes nor the assembly of bipolar spindles, Mad2 was detected on the kinetochores of the misaligned chromosomes, indicating that the prometaphase arrest induced by calpain inhibition is due to activation of the spindle assembly checkpoint. Furthermore, when calpain activity was inhibited in cells having monopolar spindles, chromosomes were clustered adjacent to the centrosome, suggesting that calpain activity is involved in a polar ejection force for metaphase alignment of chromosomes. Based on these findings, we propose that activation of m-calpain during mitosis is required for cells to establish the chromosome alignment by regulating some molecules that generate polar ejection force.

Glu-Leu-Arg-negative CXC chemokine interferon gamma inducible protein-9 as a mediator of epidermal-dermal communication during wound repair

Normal wound healing is a complex, highly regulated dynamic process that requires co-ordinate responses of both epidermal and dermal compartments. To accomplish the healing process several growth factors, chemokines, and matrix elements signal both cell proliferation and migration during the inflammatory and reparative phases and limit these responses during the remodeling phase. We have found that the Glu-Leu-Arg-negative CXC chemokines interferon gamma inducible protein 10, monokine induced by interferon gamma, and platelet factor 4, limit fibroblast responsiveness to growth factors, but the functioning of these factors in wound healing remains uncertain. We hypothesized that the keratinocyte-derived member of this Glu-Leu-Arg-negative CXC family, interferon gamma inducible protein 9 (IP-9) CXCL11 (also known as I-TAC, beta-R1, and H-174) signals to the dermal compartment to synchronize the re-epithelialization process. Interferon gamma inducible protein 9 was produced after mechanical wounding of a keratinocyte monolayer, suggesting for the first time that this could be a wound response factor. Interferon gamma inducible protein 9 limited epidermal growth factor (EGF)-induced fibroblast motility (57+/-7%) by the same protein kinase A (KA)-mediated inhibition of calpain activation and cell de-adhesion as described for interferon gamma inducible protein 10. Surprisingly, interferon gamma inducible protein 9 enhanced growth factor-induced motility in undifferentiated keratinocytes (137+/-19%) as determined in a two-dimensional in vitro wound healing assay, and interferon gamma inducible protein 9 alone promoted motility in undifferentiated keratinocytes (49+/-10% of epidermal growth factor-induced motility). A stimulated keratinocyte/target cell coculture system revealed that interferon gamma inducible protein 9 acts as a soluble keratinocyte-derived paracrine factor for both fibroblasts and keratinocytes. Further, we found that in both fibroblasts and undifferentiated keratinocytes, interferon gamma inducible protein 9 exerted its action through modulation of a cytosolic protease, calpain. Interestingly, interferon gamma inducible protein 9 increased calpain activity in undifferentiated keratinocytes, whereas the same chemokine inhibited the calpain activity in fibroblasts. This provides for a model whereby redifferentiated basal keratinocytes could limit fibroblast repopulation of the dermis underlying healed wounds while simultaneously promoting re-epithelialization of the remaining provisional wound.

Inhibition of calpain is a component of nitric oxide-induced down-regulation of human mast cell adhesion

Nitric oxide is an important messenger that regulates mast cell activity by modifications to gene expression and intracellular pathways associated with exocytosis and adhesion. Integrin interactions with extracellular matrix components modulate an array of cell activities, including mediator production and secretion. To investigate the molecular mechanisms underlying NO regulation of mast cell function, we studied its effects on adhesion of a human mast cell line (HMC-1) to fibronectin (FN). The NO donors S-nitrosoglutathione and S-nitroso-N-acetylpenicillamine strongly down-regulated the adhesion of HMC-1 to FN. Inhibitors of soluble guanylate cyclase and protein kinase G did not alter the response of cells to NO. A peroxynitrite scavenger did not affect modulation of adhesion by NO, nor could the effect of NO be mimicked by the peroxynitrite-producing compound 3-morpholinosydnonimine. NO donors inhibited the cysteine protease, calpain, while calpain inhibitors mimicked the effect of NO and led to a decrease in the ability of HMC-1 cells to adhere to FN. Thus, NO is an effective down-regulator of human mast cell adhesion. The mechanism for this action does not involve peroxynitrite or activation of soluble guanylate cyclase. Instead, a portion of NO-induced down-regulation of adhesion may be attributed to inhibition of the cysteine protease, calpain, an enzyme that has been associated with control of integrin activation in other cell types. The inhibition of calpain is most likely mediated via nitrosylation of its active site thiol group. Calpain may represent a novel therapeutic target for the regulation of mast cell activity in inflammatory disorders.

Extracellular matrix enhances heregulin-dependent BRCA1 phosphorylation and suppresses BRCA1 expression through its C terminus

Germ line mutations in the breast cancer susceptibility gene BRCA1 account for the increased risk of early onset of familial breast cancer, whereas overexpression of the ErbB family of receptor tyrosine kinases has been linked to the development of nonfamilial or sporadic breast cancer. To analyze whether there is a link between these two regulatory molecules, we studied the effects of ErbB-2 activation by heregulin (HRG) on BRCA1 function. It was previously demonstrated that HRG induced the phosphorylation of BRCA1, which was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Since altered interaction between cells and the surrounding extracellular matrix (ECM) is a common feature in a variety of tumors and since ECM modulates intracellular signaling, we hypothesized that ECM may affect the expression and HRG-dependent phosphorylation of BRCA1. Following stimulation by HRG, a strong increase in [(3)H]thymidine incorporation was observed in human T47D breast cancer cells seeded on plastic (PL). When T47D cells were seeded on laminin (LAM) or Matrigel, HRG induced a significantly higher proliferation than it did in cells seeded on PL. T47D cells seeded on poly-L-lysine had an abrogated mitogenic response, indicating the involvement of integrins in this process. HRG treatment induced a transient phosphorylation of BRCA1 that was enhanced in T47D cells grown on LAM. LAM-enhanced BRCA1 phosphorylation was mediated through alpha(6) integrin upon HRG stimulation. Accordingly, T47D cells grown on LAM had the greatest increase in ErbB-2 activation, PI3K activity, and phosphorylation of Akt. A similar pattern of BRCA1 mRNA expression was observed when T47D cells were seeded on PL, LAM, or COL4. There was a significant decrease in the steady state of the BRCA1 mRNA level on both the LAM and COL4 matrices compared to that for cells seeded on PL. In addition, HRG stimulation caused a significant decrease in BRCA1 mRNA expression that was dependent on protein synthesis. Pretreatment with both the calpain inhibitor ALLN (N-acetyl-Leu-Leu-norleucinal) and the proteosome inhibitor lactacystin inhibited the HRG-induced down-regulation of BRCA1 mRNA expression. Likewise, there was a strong decrease in the protein level of BRCA1 in T47D cells 4 h after treatment with HRG compared to its level in control nontreated T47D cells. Pretreatment with the proteosome inhibitors ALLN, lactacystin, and PSI [N-benzyloxycarbonyl-Ile-Glu-(O-t-butyl)-Ala-leucinal] inhibited also the HRG-induced down-regulation of BRCA1 protein in breast cancer cells. Interestingly, BRCA1 mRNA expression in HCC-1937 breast cancer cells, which express C-terminally truncated BRCA1, was not affected by either LAM or CL4. No phosphorylation of BRCA1 from HCC-1937 cells was observed in response to HRG. While Cdk4 phosphorylated wild-type BRCA1 in response to HRG in T47D cells, Cdk4 failed to phosphorylate the truncated form of BRCA1 in HCC-1937 cells. Furthermore, overexpression of wild-type BRCA1 in HCC-1937 cells resulted in the phosphorylation of BRCA1 and decreased BRCA1 expression upon HRG stimulation while overexpression of truncated BRCA1 in T47D cells resulted in a lack of BRCA1 phosphorylation and restoration of BRCA1 expression. These findings suggest that ECM enhances HRG-dependent BRCA1 phosphorylation and that ECM and HRG down-regulate BRCA1 expression in breast cancer cells. Furthermore, ECM suppresses BRCA1 expression through the C terminus of BRCA1.

Calpain II colocalizes with detergent-insoluble rafts on human and Jurkat T-cells

Calpain, a calcium-dependent cysteine protease, is known to associate with the T-cell plasma membrane and subsequently cleave a number of cytoskeletal-associated proteins. In this study, we report the novel observation that calpain II, but not calpain I, associates with membrane lipid rafts on human peripheral blood T-cells and Jurkat cells. Raft-associated calpain activity is enhanced with exogenous calcium and inhibited with calpeptin, a specific inhibitor of calpain activity. In addition, we demonstrate that calpain cleaves the cytoskeletal-associated protein, talin, during the first 30-min after cell stimulation. We propose that lipid raft associated-calpain II could function in early TCR signaling to facilitate immune synapse formation through cytoskeletal remodeling mechanisms. Hence, we demonstrate that the positioning of calpain II within T-cell lipid rafts strategically places it in close proximity to known calpain substrates that are cleaved during Ag-specific T-cell signaling and immune synapse formation.

Motility is rate-limiting for invasion of bladder carcinoma cell lines

Induced migration of tumor cells is generally considered to be one critical step in cancer progression to the invasive and metastatic stage. The implicit caveat of studies that show this is that other, unknown, signaling pathways and biophysical events are actually the operative rate-limiting steps, and not motility per se. Thus, to examine the hypothesis that motility is a single, but overall rate-limiting function required for invasion, disparate motility processes need be blocked with concordant effects on tumor invasion. Recently, we and others have described two signaling pathways that are critical to growth factor-induced motility but not mitogenesis. The key molecular switches are phospholipase C-gamma (PLCgamma) and calpain for cytoskeletal reorganization and rear detachment, respectively. We examined this hypothesis in a highly invasive tumor, bladder carcinoma. Three different human tumor cell lines, 253J-B-V, UMUC and T-24, were tested for invasiveness in vitro by transmigration of a Matrigel barrier. Inhibiting PLCgamma with the pharmacologic agent U73122 or the molecular dominant-negative PLCz construct reduced both invasiveness and motility. The same was noted when calpain was blocked using calpain inhibitor I (ALLN). These results demonstrate that one interventional target for limiting invasion is not necessarily an individual motility pathway but rather cell migration per se.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

Reduced cell migration and disruption of the actin cytoskeleton in calpain-deficient embryonic fibroblasts

The physiological functions and substrates of the calcium-dependent protease calpain remain only partly understood. The mu- and m-calpains consist of a mu- or m-80-kDa large subunit (genes Capn1 and Capn2), and a common 28-kDa small subunit (Capn4). To assess the role of calpain in migration, we used fibroblasts obtained from Capn4(-/-) mouse embryos. The cells lacked calpain activity on casein zymography and did not generate the characteristic calpain-generated spectrin breakdown product that is observed in wild-type cells. Capn4(-/-) cells had decreased migration rates and abnormal organization of the actin cytoskeleton with a loss of central stress fibers. Interestingly, these cells extended numerous thin projections and displayed delayed retraction of membrane protrusions and filopodia. The number of focal adhesions was decreased in Capn4(-/-) cells, but the cells had prominent vinculin-containing focal complexes at the cell periphery. The levels of the focal adhesion proteins, alpha-actinin, focal adhesion kinase (FAK), spectrin, talin, and vinculin, were the same in Capn4(+/+) and Capn4(-/-) cells. FAK, alpha-actinin, and vinculin were not cleaved in either cell type plated on fibronectin. However, proteolysis of the focal complex component, talin, was detected in the wild-type cells but not in the Capn4(-/-) cells, suggesting that calpain cleavage of talin is important during cell migration. Moreover, talin cleavage was again observed when calpain activity was partially restored in Capn4(-/-) embryonic fibroblasts by stable transfection with a vector expressing the rat 28-kDa calpain small subunit. The results demonstrate unequivocally that calpain is a critical regulator of cell migration and of the organization of the actin cytoskeleton and focal adhesions.

Calpain 3 mRNA expression in mice after denervation and during muscle regeneration

Lack of functional calpain 3 in humans is a cause of limb girdle muscular dystrophy, but the function(s) of calpain 3 remain(s) unknown. Special muscle conditions in which calpain 3 is downregulated could yield valuable clues to the understanding of its function(s). We monitored calpain 3 mRNA amounts by quantitative RT-PCR and compared them with those of α-skeletal actin mRNA in mouse leg muscles for different types of denervation and muscle injury. Intact muscle denervation reduced calpain 3 mRNA expression by a factor of 5 to 10, while α-skeletal actin mRNA was reduced in a slower and less extensive manner. Muscle injury (denervation-devascularization), which leads to muscle degeneration and regeneration, induced a 20-fold decrease in the mRNA level of both calpain 3 and α-skeletal actin. Furthermore, whereas in normal muscle and intact denervated muscle, the full-length transcript is the major calpain 3 mRNA, in injured muscle, isoforms lacking exon 6 are predominant during the early regeneration process. These data suggest that muscle condition determines the specific calpain 3 isoform pattern of expression and that calpain 3 expression is downregulated by denervation.

Cleavage of focal adhesion kinase by different proteases during SRC-regulated transformation and apoptosis. Distinct roles for calpain and caspases

Integrin-associated focal adhesion complexes provide the main adhesive links between the cellular actin cytoskeleton and the surrounding extracellular matrix. In vitro, cells utilize a complex temporal and spatially regulated mechanism of focal adhesion assembly and disassembly required for cell migration. Recent studies indicate that members of both calpain and caspase protease families can promote limited proteolytic cleavage of several components of focal adhesions leading to disassembly of these complexes. Such mechanisms that influence cell adhesion may be deregulated under pathological conditions characterized by increased cell motility, such as tumor invasion. v-Src-induced oncogenic transformation is associated with loss of focal adhesion structures and transition to a less adherent, more motile phenotype, while inactivating temperature-sensitive v-Src in serum-deprived transformed cells leads to detachment and apoptosis. In this report, we demonstrate that v-Src-induced disassembly of focal adhesions is accompanied by calpain-dependent proteolysis of focal adhesion kinase. Furthermore, inhibitors of calpain repress v-Src-induced focal adhesion disruption, loss of substrate adhesion, and cell migration. In contrast, focal adhesion loss during detachment and apoptosis induced after switching off temperature-sensitive v-Src in serum-deprived transformed cells is accompanied by caspase-mediated proteolysis of focal adhesion kinase. Thus, calpain and caspase differentially regulate focal adhesion turnover during Src-regulated cell transformation, motility, and apoptosis.

Beta1 integrin-mediated T cell adhesion and cell spreading are regulated by calpain

To investigate the function of calpain in T cells, we sought to determine the role of this protease in cellular events mediated by beta1 integrins. T cell receptor cross-linked or phorbol ester-stimulated T cells binding to immobilized fibronectin induce the translocation of calpain to the cytoskeletal/membrane fraction of these cells. Such translocation of calpain is associated with proteolytic modification of protein tyrosine phosphatase 1B, increased cellular adhesion, and dramatic alterations in cellular morphology. However, affinity-related increases in T cell adhesion induced by the anti-beta1 integrin antibody 8A2 occur in a calpain-independent manner and in the absence of morphological shape changes. Furthermore, calpain undergoes activation in response to either alpha4beta1 or alpha5beta1 integrin binding to fibronectin in appropriately stimulated T cells, and calpain II as well as protein tyrosine phosphatase 1B accumulates at sites of focal contact formation. Inhibition of calpain activity not only inhibits the proteolytic modification of protein tyrosine phosphatase 1B, but also decreases the ability of T cells to adhere to and spread on immobilized fibronectin. Thus, we describe a potential regulatory role for calpain in beta1 integrin-mediated signaling events associated with T cell adhesion and cell spreading on fibronectin.

Adhesion complexes implicated in intestinal epithelial cell-matrix interactions

This article review summarizes data on cell-substratum adhesion complexes involved in the regulation of cellular functions in the intestine. We first focus on the molecular composition of the two main adhesion structures-the beta1 integrin-adhesion complex and the hemidesmosome-found in vivo and in two human intestinal cell lines. We also report the key findings on the cellular behavior and response to the extracellular matrix that involve integrins, the main transmembrane anchors of these complexes. How the dynamics of cell/extracellular matrix interactions contribute to cell migration, proliferation, differentiation, and tumorigenicity is discussed in the light of the data provided by the human intestinal cells.

Tumor invasion: role of growth factor-induced cell motility

Cancer progression to the invasive and metastatic stage represents the most formidable barrier to successful treatment. To develop rational therapies, we must determine the molecular bases of these transitions. Cell motility is one of the defining characteristics of invasive tumors, enabling tumors to migrate into adjacent tissues or transmigrate limiting basement membranes and extracellular matrices. Invasive tumor cells have been demonstrated to present dysregulated cell motility in response to extracellular signals from growth factors and cytokines. Recent findings suggest that this growth factor receptor-mediated motility is one of the most common aberrations in tumor cells leading to invasiveness and represents a cellular behavior distinct from-adhesion-related haptokinetic and haptotactic migration. This review focuses on the emerging understanding of the biochemical and biophysical foundations of growth factor-induced cell motility and tumor cell invasiveness, and the implications for development of targeted agents, with particular emphasis on signaling from the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptors, as these have most often been associated with tumor invasion. The nascent models highlight the roles of various intracellular signaling pathways including phospholipase C-gamma (PLC gamma), phosphatidylinositol (PI)3'-kinase, mitogen-activated protein (MAP) kinase, and actin cytoskeleton-related events. Development of novel agents against tumor invasion will require not only a detailed appreciation of the biochemical regulatory elements of motility but also a paradigm shift in our approach to and assessment of cancer therapy.

Talin concentrates to the midbody region during mammalian cell cytokinesis

In this study we investigated the cellular distribution of talin, a cytoskeletal protein, during mammalian cell cytokinesis. Immunohistochemical experiments on various carcinoma cell lines and mesenchyme-derived cells reveal that talin displays a cell cycle-dependent cellular localization. During metaphase, talin is located in the centromeric region of the chromosome, like the TD-60 protein and intrinsic centromere components detected by a CREST serum. From anaphase to telophase, talin is present in the cleavage furrow. As the cells progress to cytokinesis, when the furrow is complete, talin is concentrated in the midbody structures, as assessed by immunofluorescence and confirmed by Western blot experiments on purified midbodies. Double staining experiments reveal that alpha-tubulin, TD-60 protein, and talin co-localize in the midbodies. These results suggest that talin, in addition to its implication in focal adhesion organization and signaling, may play a critical role in cytokinesis. (J Histochem Cytochem 47:1357-1367, 1999)

Degraded collagen fragments promote rapid disassembly of smooth muscle focal adhesions that correlates with cleavage of pp125(FAK), paxillin, and talin

Active matrix metalloproteinases and degraded collagen are observed in disease states, such as atherosclerosis. To examine whether degraded collagen fragments have distinct effects on vascular smooth muscle cells (SMC), collagenase-digested type I collagen was added to cultured human arterial SMC. After addition of collagen fragments, adherent SMC lose their focal adhesion structures and round up. Analysis of components of the focal adhesion complex demonstrates rapid cleavage of the focal adhesion kinase (pp125(FAK)), paxillin, and talin. Cleavage is suppressed by inhibitors of the proteolytic enzyme, calpain I. In vitro translated pp125(FAK) is a substrate for both calpain I- and II-mediated processing. Mapping of the proteolytic cleavage fragments of pp125(FAK) predicts a dissociation of the focal adhesion targeting (FAT) sequence and second proline-rich domain from the tyrosine kinase domain and integrin-binding sequence. Coimmunoprecipitation studies confirm that the ability of pp125(FAK) to associate with paxillin, vinculin, and p130cas is significantly reduced in SMC treated with degraded collagen fragments. Further, there is a significant reduction in the association of intact pp125(FAK) with the cytoskeletal fraction, while pp125(FAK) cleavage fragments appear in the cytoplasm in SMC treated with degraded collagen fragments. Integrin-blocking studies indicate that integrin-mediated signals are involved in degraded collagen induction of pp125(FAK) cleavage. Thus, collagen fragments induce distinct integrin signals that lead to initiation of calpain-mediated cleavage of pp125(FAK), paxillin, and talin and dissolution of the focal adhesion complex.

Calpain cleavage of integrin beta cytoplasmic domains

We showed previously that the calcium-dependent protease, calpain, cleaves the cytoplasmic domain of the integrin beta3 subunit. To investigate whether susceptibility to calpain is a common feature of all integrin beta subunits, and to map calpain cleavage sites in different integrin beta tails, we treated recombinant cytoplasmic domains of integrin beta1A, beta1D, beta2, beta3 and beta7 subunits with purified calpain in vitro. We found that the cytoplasmic domains of all these integrin chains were cleaved by calpain. HPLC followed by mass spectrometry was used to identify calpain cleavage sites. These sites were clustered in the C-terminal half of the integrin beta cytoplasmic domains in regions flanking the two NXXY motifs, suggesting the possibility that the structural framework provided by these motifs is recognized by calpain. We used the knowledge of these cleavage sites to develop cleavage site-specific antibodies and to demonstrate cleavage of the beta1A cytoplasmic domain in intact platelets stimulated with calcium ionophore or thrombin. Thus susceptibility to calpain cleavage is common to integrin beta subunits, can be induced in intact cells, and appears to favor regions surrounding two conserved NXXY motifs.

Expression of the focal adhesion protein paxillin in lung cancer and its relation to cell motility

Lung cancer can lead to abnormalities of the actin cytoskeleton structure which may be important in transformation. In this study, we have investigated the expression of the cytoskeletal associated protein paxillin in lung cancer. Paxillin is a 68 kDa focal adhesion protein, with four tandem LIM domains at the C-terminus, involved in growth factor receptor, integrin and oncogenic signaling such as v-src, BCR/ABL, and E6 of the papilloma virus. In non-small cell lung cancer (NSCLC) cell lines, paxillin localized to the focal adhesions. The possible role of paxillin in lung cancer cells was assessed by overexpressing green fluorescence protein (GFP)-paxillin construct in two separate NSCLC cell lines (Calu-1 and H661). Over the course of 48 h, GFP-paxillin consistently caused the cells to become round and to decrease cell motility as compared to normal controls, GFP-N-terminus paxillin, or GFP-LIM transfected cells. Because some lung cancers may be quite aggressive and metastasize quickly, which may be related to the cytoskeleton, we determined the expression of paxillin in NSCLC and small cell lung cancer (SCLC) cell lines and patient tumor tissues. Expression of paxillin in NSCLC and SCLC cell lines were determined by Northern blot and Western blot analysis. The expression of paxillin was consistently low in SCLC cell lines, whereas there was paxillin expression in NSCLC cell lines. There was a variability of expression of paxillin in NSCLC tumor tissue as compared to normal lung tissue. In contrast, by immunohistochemistry, we show that there was no detectable expression of paxillin in 5/5 SCLC patients. This data suggests that absence or low level of paxillin protein expression may cause certain lung cancers, such as SCLC, to be more motile and possibly more aggressive.

Epidermal growth factor receptor-mediated motility in fibroblasts

Cell motility is induced by many growth factors acting through cognate receptors with intrinsic tyrosine kinase activity (RPTK). However, most of the links between receptor activation and the biophysical processes of cell motility remain undeciphered. We have focused on the mechanisms by which the EGF receptor (EGFR) actuates fibroblast cell motility in an attempt to define this integrated process in one system. Our working model is that divergent, but interconnected pathways lead to the biophysical processes necessary for cell motility: cytoskeleton reorganization, membrane extension, formation of new adhesions to substratum, cell contraction, and release of adhesions at the rear. We postulate that for any given growth factor some of the pathways/processes will be actively signaled and rate-limiting, while others will be permissive due to background low-level activation. Certain couplings have been defined, such as PLCgamma and actin modifying proteins being involved in cytoskeletal reorganization and lamellipod extension and MEK being implicated in detachment from substratum. Others are suggested by complementary investigations in integrin-mediated motility, including rac in membrane protrusion, rho in new adhesions, myosin II motors in contraction, and calpain in detachment, but have yet to be placed in growth factor-induced motility. Our model postulates that many biochemical pathways will be shared between chemokinetic and haptokinetic motility but that select pathways will be activated only during RPTK-enhanced motility.

Evidence that DOCK180 up-regulates signals from the CrkII-p130(Cas) complex

DOCK180 is one of the two principal proteins bound to the SH3 domain of the adaptor protein CrkII. Here, we have studied the involvement of DOCK180 in integrin signaling. DOCK180 was neither phosphorylated nor bound to CrkII in quiescent NIH 3T3 cells and 3Y1 cells. We found that DOCK180 was phosphorylated and bound to CrkII in NIH 3T3 cells stimulated with integrin and also in 3Y1 cells transformed by v-src or v-crk. The binding of DOCK180 to CrkII correlated with the binding of CrkII to p130(Cas), which is a major CrkII SH2 domain-binding protein at focal adhesions. In a reconstitution experiment, expression of DOCK180 induced hyperphosphorylation of p130(Cas) and a concomitant increase in the amount of CrkII bound to p130(Cas). Similarly, binding of DOCK180 to CrkII was also enhanced by the coexpression of p130(Cas). Finally, we found that coexpression of p130(Cas) and CrkII with DOCK180 induced local membrane spreading and accumulation of DOCK180-CrkII-p130(Cas) complexes at focal adhesions. These findings suggest that DOCK180 positively regulates signaling from integrins to CrkII-p130(Cas) complexes at focal adhesions.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

Calcium-dependent signaling pathways in T cells. Potential role of calpain, protein tyrosine phosphatase 1b, and p130Cas in integrin-mediated signaling events

Engagement of beta1 integrin receptors initiates an increase in intracellular calcium concentrations in T cells, potentially affecting calcium-sensitive signaling pathways. The calcium-activated cysteine protease, calpain, regulates a variety of cell functions by calcium-dependent limited proteolysis. To investigate the function of calpain in T cells, we sought to determine the role of this protease in calcium-dependent signaling events. Subsequent to elevations in intracellular calcium concentrations induced by ionomycin or adherence to fibronectin, calpain activity translocated to the cytoskeletal/membrane fraction of T cells. In addition, stimulation of T cells with these agents initiated the proteolytic cleavage of protein tyrosine phosphatase 1B by calpain. Enzymatic cleavage of protein tyrosine phosphatase 1B occurs near the endoplasmic reticulum-targeting sequence and results in the generation of an enzymatically active form of the phosphatase. Furthermore, we show that both the native and the cleaved forms of protein tyrosine phosphatase 1B interact with p130(Cas) in T cells. This interaction may serve to relocate protein tyrosine phosphatase 1B to sites of focal contact resulting in potential interactions with substrates previously inaccessible to the endoplasmic reticulum-associated phosphatase. Thus, we describe a novel calcium-dependent signaling pathway in T cells that may mediate signals generated by beta1 integrin adherence to the extracellular matrix.

A new subfamily of vertebrate calpains lacking a calmodulin-like domain: implications for calpain regulation and evolution

Calpains are calcium-dependent intracellular nonlysosomal proteases that are believed to participate in signal transduction. In vertebrates, five different calpains have so far been identified, of which three, mu-, m-, and mu/m-calpain, are ubiquitously expressed while the other two, nCL-1 (p94) and nCL-2, exhibit a restricted tissue distribution. We have identified two new vertebrate calpain genes, Capn5 and Capn6. The human and mouse amino acid sequences of these new calpains are the most divergent of the vertebrate calpains identified. They possess most of the residues conserved in calpain family members but the C-terminal region lacks any homology to the calmodulin-like domain of other vertebrate calpains. They both exhibit significant homology over the entire coding region to the protein encoded by the gene tra-3, involved in nematode sex determination, and Capn5 may represent its vertebrate orthologue. The predicted Capn6 protein lacks critical active site residues and may not be proteolytically active. Both genes are differentially expressed in human tissues with highest RNA levels for Capn5 occurring in the testis, liver, trachea, colon, and kidney, while Capn6 is highly expressed only in the placenta sample of the 50 tissues examined. Phylogenetic analysis suggests that the vertebrate calpains arose through a series of gene duplication events that began before the initial divergence of the vertebrate and invertebrate lineages. The discovery of these two new calpains highlights a hitherto unknown complexity of the calpain family with subclasses perhaps possessing different modes of regulation.

Changes in calpain during meiosis in the rat egg

Resumption of meiosis at fertilization is mediated by increased levels of calcium which activate several calcium-dependent enzymes. Calpain, a neutral calcium-activated thiol protease, is present in the cytoplasm of many cells. Its activation is associated with limited autolysis and relocalization in the cell. Calpain is thought to participate in the regulation of mitosis and resumption of meiosis in Xenopus oocytes. In this study we followed the activation and localization of calpain during maturation and fertilization in rat eggs using a polyclonal antibody raised against chicken muscle calpain. A band of 80 kDa was detected in GV oocytes and its level increased in unfertilized MII eggs. At the early stages of fertilization, we observed a transient decrease in the level of calpain which was regained at the pronuclear stage. Adding Ca2+ to lysate of MII eggs resulted in an additional band, representing the degraded fragment of the activated protein. In eggs activated by ionomycin, calpain level decreased, followed by an increase in a dynamic similar to that observed in fertilized eggs. Egg activation also led to changes in calpain localization. A homogenous distribution was observed in GV and in MII eggs, while in activated eggs it was localized predominantly overlying the metaphase plate. In the current study we demonstrate the presence of calpain in the rat egg. During maturation, calpain level increases; however, during egg activation, in response to [Ca2+]i changes, calpain undergoes autolysis, translocaton, and fluctuation in its level. We therefore suggest a correlation between calpain activation and fertilization.

BCR/ABL induces multiple abnormalities of cytoskeletal function

The BCR/ABL oncogene causes human chronic myelogenous leukemia (CML), a myeloproliferative disease characterized by massive expansion of hematopoietic progenitor cells and cells of the granulocyte lineage. When transfected into murine hematopoietic cell lines, BCR/ABL causes cytokine-independence and enhances viability. There is also growing evidence that p210(BCR/ABL) affects cytoskeletal structure. p210(BCR/ABL) binds to actin, and several cytoskeletal proteins are tyrosine phosphorylated by this oncoprotein. Also, at least one aspect of cytoskeletal function is abnormal, in that the affinity of beta1 integrins for fibronectin is altered in CML cells. However, isolated changes in beta1 integrin function would be unlikely to explain the clinical phenotype of CML. We used time-lapse video microscopy to study cell motility and cell morphology on extracellular cell matrix protein-coated surfaces of a series of cell lines before and after transformation by BCR/ABL. BCR/ABL was associated with a striking increase in spontaneous motility, membrane ruffling, formation of long actin extensions (filopodia) and accelerated the rate of protrusion and retraction of pseudopodia on fibronectin-coated surfaces. Also, while untransformed cells were sessile for long periods, BCR/ABL-transformed cells exhibited persistent motility, except for brief periods during cell division. Using cell lines transformed by a temperature-sensitive mutant of BCR/ABL, these kinetic abnormalities of cytoskeletal function were shown to require BCR/ABL tyrosine kinase activity. Similar abnormalities of cytoskeletal function on fibronectin-coated surfaces were observed when hematopoietic progenitor cells purified by CD34 selection from patients with CML were compared with CD34 positive cells from normal individuals. Interestingly, alpha-interferon treatment was found to slowly revert the abnormal motility phenotype of BCR/ABL-transformed cells towards normal. The increase in spontaneous motility and other defects of cytoskeletal function described here will be useful biological markers of the functional effects of BCR/ABL in hematopoietic cells.

Angiotensin II signal transduction in vascular smooth muscle: role of tyrosine kinases

In this review, the role of tyrosine kinases in angiotensin II-mediated signal transduction pathways in vascular smooth muscle is discussed. Angiotensin II was isolated by virtue of its vasoconstrictor abilities and has long been thought to play a critical role in hypertension. However, recent studies indicate important roles for angiotensin II in inflammation, atherosclerosis, and congestive heart failure. The expanding role of angiotensin II indicates that multiple signal transduction pathways are likely to be activated in a tissue-specific manner. Exciting recent data show that angiotensin II directly stimulates tyrosine kinases, including pp60(c-src) kinase (c-Src), focal adhesion kinase (FAK), and Janus kinases (JAK2 and TYK2). Angiotensin II may activate receptor tyrosine kinases, such as Axl and platelet-derived growth factor, by as-yet-undefined autocrine mechanisms. Finally, unknown tyrosine kinases may mediate tyrosine phosphorylation of Shc, Raf, and phospholipase C-gamma after angiotensin II stimulation. These angiotensin II-regulated tyrosine kinases appear to be required for angiotensin II effects, such as vasoconstriction, proto-oncogene expression, and protein synthesis, on the basis of studies with tyrosine kinase inhibitors. Thus, understanding angiotensin II-stimulated signaling events, especially those related to tyrosine kinase activity, may form the basis for the development of new therapies for cardiovascular diseases.

Emerging components of the Crk oncogene product: the first identified adaptor protein

v-Crk, identified as an oncogene product of the CT10 retrovirus, became the first example of an adaptor protein. It consists mostly of the Src homology 2 (SH2) and Src homology 3 (SH3) domains. Two of the three major proteins bound to Crk SH2 have been identified as paxillin and p130Cas. Both paxillin and p130Cas are phosphorylated upon stimulation by integrin, suggesting that Crk transduces signals from integrin. The cloning of the complementary DNA of two major proteins bound to Crk SH3 was recently completed. Both cDNAs encoded novel proteins: C3G, a guanine nucleotide exchange protein for Rap1, and DOCK180, an SH3-containing protein of unknown function. The SH3 domain of Crk also binds to Sos, Abl, and Eps15. The variety of the proteins bound to Crk SH3 implies that Crk provides a set of effector proteins that are triggered together. Alternatively, other domains of the SH3-binding proteins enable Crk to specifically activate each of the SH3-binding proteins according to the particular form of stimulation.

MAP kinase activation by flow in endothelial cells. Role of beta 1 integrins and tyrosine kinases

Local alterations in the hemodynamic environment regulate endothelial cell function, but the signal-transduction mechanisms involved in this process remain unclear. We previously demonstrated that mitogen-activated protein (MAP) kinase is rapidly stimulated by flow in bovine aortic endothelial cells. Integrin receptors may act as mechanotransducers, as suggested by rapid remodeling of focal adhesion complexes in response to flow. To study the role of integrins in flow-mediated MAP kinase activation, we compared the effects of beta 1 integrin activation (with 8A2 antibody) and flow in cultured human umbilical vein endothelial cells (HUVECs). Both 8A2 (3 micrograms/mL) and flow (shear stress, 12 dynes/cm2) stimulated MAP kinase, although the flow response was faster and greater. To characterize flow-activated tyrosine kinases, tyrosine-phosphorylated proteins were immunoprecipitated and identified by Western blot. There was a time-dependent increase in phosphotyrosine content in 60- to 80-kD, 110-kD, 125- to 150-kD, and 180- to 190-kD proteins. A 125-kD protein was identified as focal adhesion kinase (FAK), suggesting that flow activates integrins. In comparison with flow, 8A2 caused less tyrosine phosphorylation of fewer proteins, although FAK was tyrosine phosphorylated. Concurrent stimulation of HUVECs with 8A2 and flow caused additive increases in MAP kinase. Antibody 8A2 increased binding of the beta 1 affinity-sensitive antibody, 15/7, while flow failed to increase binding of 15/7. In summary, both a beta 1-activating antibody and flow stimulate tyrosine kinases, leading to activation of FAK and MAP kinase signal-transduction pathways. However, the cellular responses elicited by 8A2 represent only a portion of those stimulated by flow, suggesting that "costimulatory" events such as calcium mobilization, in addition to integrin activation, mediate the HUVEC response to fluid shear stress.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Protein kinases as mediators of fluid shear stress stimulated signal transduction in endothelial cells: a hypothesis for calcium-dependent and calcium-independent events activated by flow

Fluid shear stress regulates endothelial cell function, but the signal transduction mechanisms involved in mechanotransduction remain unclear. Recent findings demonstrate that several intracellular kinases are activated by mechanical forces. In particular, members of the mitogen-activated protein (MAP) kinase family are stimulated by hyperosmolarity, stretch, and stress such as heat shock. We propose a model for mechanotransduction in endothelial cells involving calcium-dependent and calcium-independent protein kinase pathways. The calcium-dependent pathway involves activation of phospholipase C, hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), increases in intracellular calcium and stimulation of kinases such as calcium-calmodulin and C kinases (PKC). The calcium-independent pathway involves activation of a small GTP-binding protein and stimulation of calcium-independent PKC and MAP kinases. The calcium-dependent pathway mediates the rapid, transient response to fluid shear stress including activation of nitric oxide synthase (NOS) and ion transport. In contrast, the calcium-independent pathway mediates a slower response including the sustained activation of NOS and changes in cell morphology and gene expression. We propose that focal adhesion complexes link the calcium-dependent and calcium-independent pathways by regulating activity of phosphatidylinositol 4-phosphate (PIP) 5-kinase (which regulates PIP2 levels) and p125 focal adhesion kinase (FAK, which phosphorylates paxillin and interacts with cytoskeletal proteins). This model predicts that dynamic interactions between integrin molecules present in focal adhesion complexes and membrane events involved in mechanotransduction will be integrated by calcium-dependent and calcium-independent kinases to generate intracellular signals involved in the endothelial cell response to flow.

Calpain: novel family members, activation, and physiologic function

The current status of calpain research is summarized on the basis of the most recent results. The main points are as follows. (i) Calpain constitutes a large family. (ii) Ca2+ ions cause the dissociation of calpain into subunits and the resulting free 80 kDa subunit is the active form of the enzyme. This dissociation corresponds to the activation of calpain. (iii) Some powerful clues have been obtained that will be helpful for analyzing the physiological function.