Nerve growth factor stimulates the tyrosine phosphorylation of endogenous Crk-II and augments its association with p130Cas in PC-12 cells

Epidermal growth factor modulates tyrosine phosphorylation of p130Cas. Involvement of phosphatidylinositol 3'-kinase and actin cytoskeleton

Epidermal growth factor (EGF) treatment of Rat-1 cells expressing human EGF receptor results in the modification of the tyrosine phosphorylation of the p130 Crk-associated substrate (Cas), a novel signaling molecule residing in focal adhesions. At low, mitogenic concentrations (<10 ng/ml), EGF treatment induced a rapid and transient tyrosine phosphorylation of Cas and promoted the formation of a Cas-adapter protein Crk complex in intact cells. The increase in tyrosine phosphorylation of Cas paralleled an increase in the cellular content of actin stress fibers and occurred via a pathway that depended on the integrity of the cytoskeleton. Further, phosphatidylinositol 3'-kinase activity was found to be required for the EGF-stimulated Cas phosphorylation and actin polymerization. At high concentrations (>30 ng/ml), EGF treatment resulted in the tyrosine dephosphorylation of Cas in a time-dependent manner with a concomitant decrease in the length and number of actin stress fibers. Thus, Cas exhibits an unusual bell-shaped dose-response curve in response to EGF stimulation. These results demonstrate a novel signaling role for EGF in inducing changes in tyrosine phosphorylation of Cas and Cas-Crk complex formation and suggest that Cas could be a signaling component in EGF-mediated signal transduction.

Detection of distinct pools of the adapter protein p130CAS using a panel of monoclonal antibodies

Dynamic protein interactions are thought to play an important role in regulating a wide variety of signal transduction pathways. Adapter molecules that contribute to the assembly and disassembly of these protein complexes are likely to play a critical role in the regulation of these pathways. The function of one such adapter molecule, p130CAS (CAS), has been implicated in signaling pathways involving cell growth, adhesion, and differentiation. We report here the isolation and characterization of a panel of monoclonal antibodies that specifically recognize CAS. These antibodies are proving to be invaluable molecular reagents for defining the expression, phosphorylation, binding partners, and ultimately the function of CAS with respect to cell signaling. In addition to their utility as conventional reagents for protein isolation, a subset of these antibodies has also proven to be a sensitive tool for distinguishing between different tyrosine-phosphorylated pools of CAS in the cell. Because tyrosine phosphorylation of CAS provides a dynamic means with which to regulate protein-protein interactions, these antibodies may thus serve as molecular reagents that can discern the protein binding potential of CAS. Collectively, the antibodies described in this report provide the means with which to define specific roles for CAS in cell signaling that have been otherwise difficult to establish.

The proto-oncogene Crk-II enhances apoptosis by a Ras-dependent, Raf-1/MAP kinase-independent pathway

Human embryonic kidney 293 cells and 293 cells overexpressing different amounts of the adaptor protein Crk-II (ranging from 3- to 10-fold higher levels than the parental cell line) were examined for their ability to undergo apoptosis when maintained in control and serum-free (SF) medium. Parental 293 cells undergo apoptosis only when deprived of serum for prolonged periods of time (24-48 h). On the other hand, 293 cells overexpressing different levels of Crk-II present detectable levels of apoptosis as measured by DNA fragmentation when grown in control medium, with a marked increase when they are deprived of serum for 12-48 h. To determine the pathways involved in Crk-II-induced apoptosis, Crk-II overexpressing cells were transiently transfected with a dominant-negative Ras construct (N17-Ras). Compared to cells transfected with control vectors, the cells overexpressing N17-Ras presented lower levels of apoptosis when maintained in SF-medium. On the other hand, transient transfection of a dominant-negative Raf-1 construct (K375W-Raf-1) did not decrease apoptosis; slightly increasing DNA fragmentation levels were seen. Similar results were obtained when the cells were incubated in the presence of a MEK1 inhibitor. The results presented here suggest that overexpression of Crk-II induces apoptosis via a Ras-dependent, Raf-1/MEK1/ERK-independent pathway.

Downstream of Crk adaptor signaling pathway: activation of Jun kinase by v-Crk through the guanine nucleotide exchange protein C3G

Crk, which belongs to the adaptor family of proteins composed of Src homology 2 (SH2) and SH3 domains, has a putative role in signaling. However, the downstream events of Crk signaling remain unclear. In this study, we found that Jun kinase (JNK) is moderately activated by v-Crk in both NIH 3T3 cells and chicken embryo fibroblasts. Transient expression of v-Crk, c-Crk-I, or c-Crk-II activated JNK1 in human embryo kidney cells, 293T. Coexpression of a guanine nucleotide exchange protein C3G, which specifically binds to Crk's SH3 domain, further enhanced the JNK activity as well as growth rate and anchorage-independent growth of v-Crk NIH 3T3 cells. Furthermore, overexpression of a dominant-negative form of C3G lacking the guanine nucleotide exchange domain abolished both the JNK activity and the colony forming potential of v-Crk NIH 3T3 cells. The requirement for JNK activation in v-Crk induced transformation was demonstrated by the suppression of colony forming activity of v-Crk NIH 3T3 cells when a dominant-negative form of JNK kinase, Sek1/MKK4 is expressed in these cells. These data strongly suggest the existence of a novel signaling cascade involving an adaptor protein v-Crk, which transmits signals through C3G toward JNK activation.

Identification of p130(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST

PTP-PEST is a ubiquitously expressed, cytosolic, mammalian protein tyrosine phosphatase (PTP) which exhibits high specific activity in vitro. We have investigated the substrate specificity of PTP-PEST by a novel substrate-trapping approach in combination with in vitro dephosphorylation experiments. We initially identified a prominent 130-kDa tyrosine-phosphorylated protein in pervanadate-treated HeLa cell lysates which was preferentially dephosphorylated by PTP-PEST in vitro. In order to identify this potential substrate, mutant (substrate-trapping) forms of PTP-PEST were generated which lack catalytic activity but retain the ability to bind substrates. These mutant proteins associated in stable complexes exclusively with the same 130-kDa protein, which was identified as p130(cas) by immunoblotting. This exclusive association was observed in lysates from several cell lines and in transfected COS cells, but was not observed with other members of the PTP family, strongly suggesting that p130(cas) represents a major physiologically relevant substrate for PTP-PEST. Our studies suggest potential roles for PTP-PEST in regulation of p130(cas) function. These functions include mitogen- and cell adhesion-induced signalling events and probable roles in transformation by various oncogenes. These results provide the first demonstration of a PTP having an inherently restricted substrate specificity in vitro and in vivo. The methods used to identify p130(cas) as a specific substrate for PTP-PEST are potentially applicable to any PTP and should therefore prove useful in determining the physiological substrates of other members of the PTP family.