Caspase-activated PAK-2 is regulated by subcellular targeting and proteasomal degradation

p21-activated protein kinases (PAKs) are a family of serine/threonine protein kinases that are activated by binding of the p21 G proteins Cdc42 or Rac. The ubiquitous PAK-2 (gamma-PAK) is unique among the PAK isoforms because it is also activated through proteolytic cleavage by caspases or caspase-like proteases. In response to stress stimulants such as tumor necrosis factor alpha or growth factor withdrawal, PAK-2 is activated as a full-length enzyme and as a proteolytic PAK-2p34 fragment. Activation of full-length PAK-2 stimulates cell survival, whereas proteolytic activation of PAK-2p34 is involved in programmed cell death. Here we provide evidence that the proapoptotic effect of PAK-2p34 is regulated by subcellular targeting and degradation by the proteasome. Full-length PAK-2 is localized in the cytoplasm, whereas the proteolytic PAK-2p34 fragment translocates to the nucleus. Subcellular localization of PAK-2 is regulated by nuclear localization and nuclear export signal motifs. A nuclear export signal motif within the regulatory domain prevents nuclear localization of full-length PAK-2. Proteolytic activation removes most of the regulatory domain and disrupts the nuclear export signal. The activated PAK-2p34 fragment contains a nuclear localization signal and translocates to the nucleus. However, levels of activated PAK-2p34 are tightly regulated through ubiquitination and degradation by the proteasome. Inhibition of degradation by blocking polyubiquitination results in significantly increased levels of PAK-2p34 and as a consequence, in stimulation of programmed cell death. Therefore, nuclear targeting and inhibition of degradation appear to be critical for stimulation of the cell death response by PAK-2p34.     

Identification and characterization of PS-GAP as a novel regulator of caspase-activated PAK-2

p21-activated protein kinase (PAK)-2 is a member of the PAK family of serine/threonine kinases. PAKs are activated by the p21 G-proteins Rac and Cdc42 in response to a variety of extracellular signals and act in pathways controlling cell growth, shape, motility, survival, and death. PAK-2 is unique among the PAK family members because it is also activated through proteolytic cleavage by caspase-3 or similar proteases to generate the constitutively active PAK-2p34 fragment. Activation of full-length PAK-2 by Rac or Cdc42 stimulates cell survival and protects cells from cell death, whereas caspase-activated PAK-2p34 induces a cell death response. Caspase-activated PAK-2p34 is rapidly degraded by the 26 S proteasome, but full-length PAK-2 is not. Stabilization of PAK-2p34 by preventing its polyubiquitination and degradation results in a dramatic stimulation of cell death. Although many proteins have been shown to interact with and regulate full-length PAK-2, little is known about the regulation of caspase-activated PAK-2p34. Here, we identify PS-GAP as a regulator of caspase-activated PAK-2p34. PS-GAP is a GTPase-activating protein for Cdc42 and RhoA that was originally identified by its interaction with the tyrosine kinase PYK-2. PS-GAP interacts specifically with caspase-activated PAK-2p34, but not active or inactive full-length PAK-2, through a region between the GAP and SH3 domains. The interaction with PS-GAP inhibits the protein kinase activity of PAK-2p34 and changes the localization of PAK-2p34 from the nucleus to the perinuclear region. Furthermore, PS-GAP decreases the stimulation of cell death induced by stabilization of PAK-2p34.     

c-Abl-binding protein interacts with p21-activated kinase 2 (PAK-2) to regulate PDGF-induced membrane ruffles

p21-Activated kinases (PAKs) are serine/threonine kinases involved in multiple cellular functions including cytoskeleton regulation, proliferation and apoptosis. We performed a screen for proteins interacting with PAK-2, a ubiquitously expressed kinase involved in apoptotic signaling. Among the PAK-2 interacting proteins were different members of the Abl-binding protein family. Abl-binding proteins bound to a proline-rich region of PAK-2 located in the regulatory N terminus. Moreover, active PAK-2 phosphorylated Abl-binding proteins in vitro. Interestingly, we show that PAK-2 also interacted with c-Abl but via a different domain than with the Abl-binding proteins. PAK-2 and Abi-1 co-localized in the cytoplasm and to membrane dorsal ruffles induced by PDGF treatment. Expression of mutant PAK-2 deficient in binding to Abl-binding proteins or silencing of PAK-2 expression prevented the formation of membrane dorsal ruffles in response to PDGF. Our findings define a new class of PAK-interacting proteins, which play an important role in actin cytoskeletal reorganization.     

Partial Correction of Abnormal Cardiac Development in Caspase-8-deficient Mice by Cardiomyocyte Expression of p35

Baculovirus p35 protein protects cells from apoptotic cell death by inhibiting caspase activation. We have established transgenic mouse lines specifically expressing p35 in cardiomyocytes, and primary cardiomyocytes isolated from these mice exhibit resistance to staurosporine-induced apoptosis. In a previous study, we observed defects in heart formation associated with abdominal hemorrhage and cardiomyocyte cell death in caspase-8-deficent animals. In order to better understand the etiology of the cardiac defects and embryonic lethality in caspase-8-deficient mice, we crossed these mice with the p35 transgenic animals. Although the newly generated mice still died in utero and exhibited some cardiac defects, cardiomyocyte apoptosis was suppressed and ventricular trabeculation was restored. Thus, cardiomyocyte expression of p35 prevented cell death induced by staurosporine or caspase-8 deficiency. Additionally, our data suggest that caspase-8 plays multiple roles in cardiac development.     

Caspase-2 deficiency promotes aberrant DNA-damage response and genetic instability

Caspase-2 is an initiator caspase, which has been implicated to function in apoptotic and non-apoptotic signalling pathways, including cell-cycle regulation, DNA-damage signalling and tumour suppression. We previously demonstrated that caspase-2 deficiency enhances E1A/Ras oncogene-induced cell transformation and augments lymphomagenesis in the EμMyc mouse model. Caspase-2(-/-) mouse embryonic fibroblasts (casp2(-/-) MEFs) show aberrant cell-cycle checkpoint regulation and a defective apoptotic response following DNA damage. Disruption of cell-cycle checkpoints often leads to genomic instability (GIN), which is a common phenotype of cancer cells and can contribute to cellular transformation. Here we show that caspase-2 deficiency results in increased DNA damage and GIN in proliferating cells. Casp2(-/-) MEFs readily escape senescence in culture and exhibit increased micronuclei formation and sustained DNA damage during cell culture and following γ-irradiation. Metaphase analyses demonstrated that a lack of caspase-2 is associated with increased aneuploidy in both MEFs and in EμMyc lymphoma cells. In addition, casp2(-/-) MEFs and lymphoma cells exhibit significantly decreased telomere length. We also noted that loss of caspase-2 leads to defective p53-mediated signalling and decreased trans-activation of p53 target genes upon DNA damage. Our findings suggest that loss of caspase-2 serves as a key function in maintaining genomic integrity, during cell proliferation and following DNA damage.     

DAP kinase activates a p19ARF/p53-mediated apoptotic checkpoint to suppress oncogenic transformation

DAP kinase is a pro-apoptotic calcium-regulated serine/threonine kinase, whose expression is frequently lost in human tumours. Here we show that DAP kinase counteracts oncogene-induced transformation by activating a p19ARF/p53-dependent apoptotic checkpoint. Ectopic expression of DAP kinase suppressed oncogenic transformation of primary embryonic fibroblasts by activating p53 in a p19ARF-dependent manner. Consequently, the fibroblasts underwent apoptosis, characterized by caspase activation and DNA fragmentation. In response to c-Myc or E2F-1, the endogenous DAP kinase protein was upregulated. Furthermore, functional or genetic inactivation of the endogenous DAP kinase reduced the extent of induction of p19ARF/p53 and weakened the subsequent apoptotic responses to c-Myc or E2F-1. These results establish a role for DAP kinase in an early apoptotic checkpoint designed to eliminate pre-malignant cells during cancer development.     

Cathepsin D mediates cytochrome c release and caspase activation in human fibroblast apoptosis induced by staurosporine

There is increasing evidence that proteases other than caspases, for example, the lysosomal cathepsins B, D and L, are involved in apoptotic cell death. In the present study, we present data that suggest a role for cathepsin D in staurosporine-induced apoptosis in human foreskin fibroblasts. Cathepsin D and cytochrome c were detected partially released to the cytosol after exposure to 0.1 muM staurosporine for 1 h. After 4 h, activation of caspase-9 and -3 was initiated and later caspase-8 activation and a decrease in full-length Bid were detected. Pretreatment of cells with the cathepsin D inhibitor, pepstatin A, prevented cytochrome c release and caspase activation, and delayed cell death. These results imply that cytosolic cathepsin D is a key mediator in staurosporine-induced apoptosis. Analysis of the relative sequence of apoptotic events indicates that, in this cell type, cathepsin D acts upstream of cytochrome c release and caspase activation.     

Binding of activated alpha2-macroglobulin to its cell surface receptor GRP78 in 1-LN prostate cancer cells regulates PAK-2-dependent activation of LIMK

Two characteristics of highly malignant cells are their increased motility and secretion of proteinases allowing these cells to penetrate surrounding basement membranes and metastasize. Activation of 21-kDa activated kinases (PAKs) is an important mechanism for increasing cell motility. Recently, we reported that binding of receptor-recognized forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*) to GRP78 on the cell surface of 1-LN human prostate cancer cells induces mitogenic signaling and cellular proliferation. In the current study, we have examined the ability of alpha2M* to activate PAK-1 and PAK-2. Exposure of 1-LN cells to alpha2M* caused a 2- to 3-fold increase in phosphorylated PAK-2 and a similar increase in its kinase activity toward myelin basic protein. By contrast, the phosphorylation of PAK-1 was only negligibly affected. Silencing the expression of the GRP78 gene, using either of two different mRNA sequences, greatly attenuated the appearance of phosphorylated PAK-2 in alpha2M*-stimulated cells. Treatment of 1-LN cells with alpha2M* caused translocation of PAK-2 in association with NCK to the cell surface as evidenced by the co-immunoprecipitation of PAK-2 and NCK in the GRP78 immunoprecipitate from plasma membranes. alpha2M*-induced activation of PAK-2 was inhibited by prior incubation of the cells with specific inhibitors of tyrosine kinases and phosphatidylinositol 3-kinase. PAK-2 activation was accompanied by significant increases in the levels of phosphorylated LIMK and phosphorylated cofilin. Silencing the expression of the PAK-2 gene greatly attenuated the phosphorylation of LIMK. In conclusion, we show for the first time the activation of PAK-2 in 1-LN prostate cancer cells by a proteinase inhibitor, alpha2-macroglobulin. These studies suggest a mechanism by which alpha2M* enhances the metastatic potential of these cells.     

Wuho/WDR4 deficiency inhibits cell proliferation and induces apoptosis via DNA damage in mouse embryonic fibroblasts

Wuho known as WDR4 encodes a highly conserved WD40-repeat protein, which has known homologues of WDR4 in human and mouse. Wuho-FEN1 interaction may have a critical role in the growth and development, and in the maintenance of genome stability. However, how Wuho gene deletion contributes to cell growth inhibition and apoptosis is still unknown. We utilized CAGGCre-ER transgenic mice have a tamoxifen-inducible cre-mediated recombination cassette to prepare primary mouse embryonic fibroblasts (MEFs) with Wuho deficiency. We have demonstrated that Wuho deficiency would induces γH2AX protein level elevation, heterochromatin relaxation and DNA damage down-stream sequences, including p53 activation, caspase-mediated apoptotic pathway, and p21-mediated G2/M cell cycle arrest.     

Loss of mitofusin 2 promotes endoplasmic reticulum stress

The outer mitochondrial membrane GTPase mitofusin 2 (Mfn2) is known to regulate endoplasmic reticulum (ER) shape in addition to its mitochondrial fusion effects. However, its role in ER stress is unknown. We report here that induction of ER stress with either thapsigargin or tunicamycin in mouse embryonic fibroblasts leads to up-regulation of Mfn2 mRNA and protein levels with no change in the expression of the mitochondrial shaping factors Mfn1, Opa1, Drp1, and Fis1. Genetic deletion of Mfn2 but not Mfn1 in mouse embryonic fibroblasts or cardiac myocytes in mice led to an increase in the expression of the ER chaperone proteins. Genetic ablation of Mfn2 in mouse embryonic fibroblasts amplified ER stress and exacerbated ER stress-induced apoptosis. Deletion of Mfn2 delayed translational recovery through prolonged eIF2α phosphorylation associated with decreased GADD34 and p58(IPK) expression and elevated C/EBP homologous protein induction at late time points. These changes in the unfolded protein response were coupled to increased cell death reflected by augmented caspase 3/7 activity, lactate dehydrogenase release from cells, and an increase in propidium iodide-positive nuclei in response to thapsigargin or tunicamycin treatment. In contrast, genetic deletion of Mfn1 did not affect ER stress-mediated increase in ER chaperone synthesis or eIF2α phosphorylation. Additionally, ER stress-induced C/EBP homologous protein, GADD34, and p58(IPK) induction and cell death were not affected by loss of Mfn1. We conclude that Mfn2 but not Mfn1 is an ER stress-inducible protein that is required for the proper temporal sequence of the ER stress response.     

Caspase-2 cleavage of BID is a critical apoptotic signal downstream of endoplasmic reticulum stress

The accumulation of misfolded proteins stresses the endoplasmic reticulum (ER) and triggers cell death through activation of the multidomain proapoptotic BCL-2 proteins BAX and BAK at the outer mitochondrial membrane. The signaling events that connect ER stress with the mitochondrial apoptotic machinery remain unclear, despite evidence that deregulation of this pathway contributes to cell loss in many human degenerative diseases. In order to "trap" and identify the apoptotic signals upstream of mitochondrial permeabilization, we challenged Bax-/- Bak-/- mouse embryonic fibroblasts with pharmacological inducers of ER stress. We found that ER stress induces proteolytic activation of the BH3-only protein BID as a critical apoptotic switch. Moreover, we identified caspase-2 as the premitochondrial protease that cleaves BID in response to ER stress and showed that resistance to ER stress-induced apoptosis can be conferred by inhibiting caspase-2 activity. Our work defines a novel signaling pathway that couples the ER and mitochondria and establishes a principal apoptotic effector downstream of ER stress.     

Caspase-dependent cleavage disrupts the ERK cascade scaffolding function of KSR1

Kinase suppressor of Ras 1 (KSR1) is a protein scaffold that facilitates ERK cascade activation at the plasma membrane, a critical step in the signal transduction process that allows cells to respond to survival, proliferative, and differentiative cues. Here, we report that KSR1 undergoes caspase-dependent cleavage in apoptotic cells and that cleavage destroys the scaffolding function of the full-length KSR1 protein and generates a stable C-terminal fragment that can inhibit ERK activation. KSR1 is cleaved in response to multiple apoptotic stimuli and occurs in vivo during the involution of mouse mammary tissues, a morphogenic process requiring cellular apoptosis. In addition, we find that in comparison with KSR1(-/-) mouse embryonic fibroblasts expressing wild type KSR1 (WT-KSR1), cells expressing a cleavage-resistant KSR1 protein (DEVA-KSR1) exhibit reduced apoptotic signaling in response to tumor necrosis factor-alpha/cycloheximide treatment. The effect of DEVA-KSR1 expression was found to correlate with increased levels of active phosphoERK and could be significantly reversed by treating cells with the MEK inhibitor U0126. In contrast, reduced phosphoERK levels and enhanced apoptotic signaling were observed in cells constitutively expressing the C-terminal KSR1 fragment (CTF-KSR1). Moreover, we find that cleavage of WT-KSR1 correlates with a dramatic reduction in active phosphoERK levels. These findings identify KSR1 as a caspase target and suggest that cleavage of the KSR1 scaffold represents another mechanism whereby caspases down-regulate ERK survival signaling to promote cellular apoptosis.     

A Bax/Bak-independent mechanism of cytochrome c release

Bax and Bak are multidomain pro-apoptotic members of the Bcl-2 family of proteins that regulate mitochondria-mediated apoptosis by direct modulation of mitochondrial membrane permeability. Since double-knock-out mouse embryonic fibroblasts with deficiency of Bax and Bak are resistant to multiple apoptotic stimuli, Bax and Bak are considered to be an essential gateway for various apoptotic signals. Here we showed that the combination of calcium ionophore A23187 and arachidonic acid induced cytochrome c release and caspase-dependent death of double-knock-out mouse embryonic fibroblasts, indicating that other mechanisms of cytochrome c release exist. Furthermore, A23187/arachidonic acid (ArA)-induced caspase-dependent death was significantly suppressed by the treatment of several serine protease inhibitors including 4-(2-aminoethyl)benzenesulfonylfluoride and l-1-chloro-3-(4-tosylamido)-4-phenyl-2-butanone but not the overexpression of anti-apoptotic Bcl-2 family of proteins or the inhibition of mitochondrial membrane permeability transition. These results indicate that there are at least two mechanisms of cytochrome c release leading to caspase activation, a Bax/Bak-dependent mechanism and a Bax/Bak-independent, but serine protease(s)-dependent, mechanism.     

Rapid reactive oxygen species (ROS) generation induced by curcumin leads to caspase-dependent and -independent apoptosis in L929 cells

Evidence that curcumin may have anticancer activities has renewed interest in its potential to prevent and treat disease. In this study, we show that curcumin-mediated rapid generation of reactive oxygen species (ROS) leads to apoptosis by modulating different apoptotic pathways in mouse fibroblast L929 cells. We show for the first time that curcumin-induced rapid ROS generation causes the release of apoptosis inducing factor (AIF) from the mitochondria to the cytosol and nucleus, hence, leading to caspase 3-independent apoptosis. However, our studies also show that curcumin induces the release of cytochrome c from mitochondria, causing activation of caspase 3, and concomitant PARP cleavage, which is the hallmark of caspase-dependent apoptosis. Furthermore, curcumin-induced ROS generation leads to the induction of the proapoptotic protein p53 and its effector protein p21 and down-regulation of cell cycle regulatory proteins such as Rb and cyclin D1 and D3. Both glutathione (GSH) and N-acetylcysteine (NAC) pretreatment resulted in the complete inhibition of curcumin-induced ROS generation, AIF release from mitochondria, and caspase activation. Additionally, pretreatment of L929 cells with these antioxidants completely blocked the induction of p53-dependent p21 accumulation. In conclusion, our data show that in addition to caspase 3 activation, curcumin-induced rapid ROS generation leads to AIF release, and the activation of the caspase-independent apoptotic pathway.     

Caspase-2 activation in the absence of PIDDosome formation

PIDD (p53-induced protein with a death domain [DD]), together with the bipartite adapter protein RAIDD (receptor-interacting protein-associated ICH-1/CED-3 homologous protein with a DD), is implicated in the activation of pro–caspase-2 in a high molecular weight complex called the PIDDosome during apoptosis induction after DNA damage. To investigate the role of PIDD in cell death initiation, we generated PIDD-deficient mice. Processing of caspase-2 is readily detected in the absence of PIDDosome formation in primary lymphocytes. Although caspase-2 processing is delayed in simian virus 40–immortalized pidd^−/− mouse embryonic fibroblasts, it still depends on loss of mitochondrial integrity and effector caspase activation. Consistently, apoptosis occurs normally in all cell types analyzed, suggesting alternative biological roles for caspase-2 after DNA damage. Because loss of either PIDD or its adapter molecule RAIDD did not affect subcellular localization, nuclear translocation, or caspase-2 activation in high molecular weight complexes, we suggest that at least one alternative PIDDosome-independent mechanism of caspase-2 activation exists in mammals in response to DNA damage.     

The molecular archaeology of a mitochondrial death effector: AIF in Drosophila

Apoptosis-inducing factor (AIF) is a phylogenetically conserved redox-active flavoprotein that contributes to cell death and oxidative phosphorylation in Saccharomyces cerevisiae, Caenorhabditis elegans, mouse and humans. AIF has been characterized as a caspase-independent death effector that is activated by its translocation from mitochondria to the cytosol and nucleus. Here, we report the molecular characterization of AIF in Drosophila melanogaster, a species in which most cell deaths occur in a caspase-dependent manner. Interestingly, knockout of zygotic D. melanogaster AIF (DmAIF) expression using gene targeting resulted in decreased embryonic cell death and the persistence of differentiated neuronal cells at late embryonic stages. Although knockout embryos hatch, they undergo growth arrest at early larval stages, accompanied by mitochondrial respiratory dysfunction. Transgenic expression of DmAIF misdirected to the extramitochondrial compartment (DeltaN-DmAIF), but not wild-type DmAIF, triggered ectopic caspase activation and cell death. DeltaN-DmAIF-induced death was not blocked by removal of caspase activator Dark or transgenic expression of baculoviral caspase inhibitor p35, but was partially inhibited by Diap1 overexpression. Knockdown studies revealed that DeltaN-DmAIF interacts genetically with the redox protein thioredoxin-2. In conclusion, we show that Drosophila AIF is a mitochondrial effector of cell death that plays roles in developmentally regulated cell death and normal mitochondrial function.     

Defining the DNA mismatch repair-dependent apoptotic pathway in primary cells: Evidence for p53-independence and involvement of centrosomal caspase 2

Many studies have shown that DNA mismatch repair (MMR) has a role beyond that of repair in response to several types of DNA damage, including ultraviolet radiation (UV). We have demonstrated previously that the MMR-dependent component of UVB-induced apoptosis is integral to the suppression of UVB-induced tumorigenesis. Here we demonstrate that Msh6-dependent UVB-induced apoptotic pathway is both activated via the mitochondria and p53-independent. In addition, we have shown for the first time that caspase 2, an initiator caspase, localizes to the centrosomes in mitotic primary mouse embryonic fibroblasts, irrespective of MMR status and UVB treatment.     

N‐terminal cleavage of Bax by calpain generates a potent proapoptotic 18‐kDa fragment that promotes Bcl‐2‐independent cytochrome C release and apoptotic cell death

Upon apoptosis induction, the proapoptotic protein Bax is translocated from the cytosol to mitochondria, where it promotes release of cytochrome c, a caspase‐activating protein. However, the molecular mechanisms by which Bax triggers cytochrome c release are unknown. Here we report that before the initiation of apoptotic execution by etoposide or staurosporin, an active calpain activity cleaves Bax at its N‐terminus, generating a potent proapoptotic 18‐kDa fragment (Bax/p18). Both the calpain‐mediated Bax cleavage activity and the Bax/p18 fragment were found in the mitochondrial membrane‐enriched fraction. Cleavage of Bax was followed by release of mitochondrial cytochrome c, activation of caspase‐3, cleavage of poly(ADP‐ribose) polymerase, and fragmentation of DNA. Unlike the full‐length Bax, Bax/p18 did not interact with the antiapoptotic Bcl‐2 protein in the mitochondrial fraction of drug‐treated cells. Pretreatment with a specific calpain inhibitor calpeptin inhibited etoposide‐induced calpain activation, Bax cleavage, cytochrome c release, and caspase‐3 activation. In contrast, transfection of a cloned Bax/p18 cDNA into multiple human cancer cell lines targeted Bax/p18 to mitochondria, which was accompanied by release of cytochrome c and induction of caspase‐3‐mediated apoptosis that was not blocked by overexpression of Bcl‐2 protein. Therefore, Bax/p18 has a cytochrome c–releasing activity that promotes cell death independent of Bcl‐2. Finally, Bcl‐2 overexpression inhibited etoposide‐induced calpain activation, Bax cleavage, cytochrome c release, and apoptosis. Our results suggest that the mitochondrial calpain plays an essential role in apoptotic commitment by cleaving Bax and generating the Bax/p18 fragment, which in turn mediates cytochrome c release and initiates the apoptotic execution. J. Cell. Biochem. 80:53–72, 2000. © 2000 Wiley‐Liss, Inc.     

TCR and CD28 are coupled via ZAP-70 to the activation of the Vav/Rac-1-/PAK-1/p38 MAPK signaling pathway.

CD28 costimulation amplifies TCR-dependent signaling in activated T cells, however, the biochemical mechanism(s) by which this occurs is not precisely understood. The small GTPase Rac-1 controls the catalytic activity of the mitogen-activated protein kinases (MAPKs) and cell cycle progression through G1. Rac-1 activation requires the phospho-tyrosine (p-Tyr)-dependent recruitment of the Vav GDP releasing factor (GRF) to the plasma membrane and assembly of GTPase/GRF complexes, an event critical for Ag receptor-triggered T cell activation. Here, we show that TCR/CD28 costimulation synergistically induces Rac-1 GDP/GTP exchange. Our findings, obtained by using ZAP-70-negative Jurkat T cells, indicate that CD28 costimulation augments TCR-mediated T cell activation by increasing the ZAP-70-mediated Tyr phosphorylation of Vav. This event regulates the Rac-1-associated GTP/GDP exchange activity of Vav and downstream pathway(s) leading to PAK-1 and p38 MAPK activation. CD28 amplifies TCR-induced ZAP-70 activity and association of Vav with ZAP-70 and linker for activation of T cells (LAT). These results favor a model in which ZAP-70 regulates the intersection of the TCR and CD28 signaling pathways, which elicits the coupling of TCR and CD28 to the Rac-1, PAK-1, and p38 MAPK effector molecules.