Monophosphothreonyl extracellular signal-regulated kinases 1 and 2 (ERK1/2) are formed endogenously in intact cardiac myocytes and are enzymically active.

ERK1 and ERK2 (ERK1/2) are central to the regulation of cell division, growth and survival. They are activated by phosphorylation of the Thr- and the Tyr- residues in their Thr-Glu-Tyr activation loops. The dogma is that dually-phosphorylated ERK1/2 constitute the principal activities in intact cells. We previously showed that, in neonatal rat cardiac myocytes, endothelin-1 and phorbol 12-myristate 13-acetate (PMA) powerfully and rapidly (maximal at ~5 min) activate ERK1/2. Here, we show that dually-phosphorylated ERK1/2 rapidly (< 2 min) appear in the nucleus following stimulation with endothelin-1. We characterized the active ERK1/2 species in myocytes exposed to endothelin-1 or PMA using MonoQ FPLC. Unexpectedly, two peaks of ERK1 and two peaks of ERK2 activity were resolved using in vitro kinase assays. One of each of these represented the dually-phosphorylated species. The other two represented activities for ERK1 or ERK2 which were phosphorylated solely on the Thr- residue. Monophosphothreonyl ERK1/2 represented maximally ~30% of total ERK1/2 activity after stimulation with endothelin-1 or PMA, and their k(cat) values were estimated to be minimally ~30% of the dually-phosphorylated species. Appearance of monophosphothreonyl ERK1/2 was rapid but delayed in comparison with dually-phosphorylated ERK1/2. Of 10 agonists studied, endothelin-1 and PMA were most effective in terms of ERK1/2 activation and in stimulating the appearance of monophosphothreonyl and dually-phosphorylated ERK1/2. Thus, enzymically active monophosphothreonyl ERK1/2 are formed endogenously following activation of the ERK1/2 cascade and we suggest that monophosphothreonyl ERK1/2 arise by protein tyrosine phosphatase-mediated dephosphorylation of dually-phosphorylated ERK1/2.

Structural and mechanistic insights into ras association domains of phospholipase C epsilon.

Ras proteins signal to a number of distinct pathways by interacting with diverse effectors. Studies of ras/effector interactions have focused on three classes, Raf kinases, ral guanylnucleotide-exchange factors, and phosphatidylinositol-3-kinases. Here we describe ras interactions with another effector, the recently identified phospholipase C epsilon (PLCepsilon). We solved structures of PLCepsilon RA domains (RA1 and RA2) by NMR and the structure of the RA2/ras complex by X-ray crystallography. Although the similarity between ubiquitin-like folds of RA1 and RA2 proves that they are homologs, only RA2 can bind ras. Some of the features of the RA2/ras interface are unique to PLCepsilon, while the ability to make contacts with both switch I and II regions of ras is shared only with phosphatidylinositol-3-kinase. Studies of PLCepsilon regulation suggest that, in a cellular context, the RA2 domain, in a mode specific to PLCepsilon, has a role in membrane targeting with further regulatory impact on PLC activity.

RalGDS is required for tumor formation in a model of skin carcinogenesis.

To investigate the role of signaling by the small GTPase Ral, we have generated mice deficient for RalGDS, a guanine nucleotide exchange factor that activates Ral. We show that RalGDS is dispensable for mouse development but plays a substantial role in Ras-induced oncogenesis. Lack of RalGDS results in reduced tumor incidence, size, and progression to malignancy in multistage skin carcinogenesis, and reduced transformation by Ras in tissue culture. RalGDS does not appear to participate in the regulation of cell proliferation, but instead controls survival of transformed cells. Experiments performed in cells isolated from skin tumors suggest that RalGDS mediates cell survival through the activation of the JNK/SAPK pathway. These studies identify RalGDS as a key component in Ras-dependent carcinogenesis in vivo.

Cdc42-MRCK and Rho-ROCK signalling cooperate in myosin phosphorylation and cell invasion.

Actomyosin contractility is a mechanism by which cells exert locomotory force against their environment. Signalling downstream of the small GTPase Rho increases contractility through Rho-kinase (ROCK)-mediated regulation of myosin-II light chain (MLC2) phosphorylation. Cdc42 signalling has been shown to control cell polarity. Tumour cells can move through a three-dimensional matrix with either a rounded morphology characterized by Rho-ROCK dependence or with an elongated morphology characterized by Rho-ROCK independence. Here we show that contractility necessary for elongated morphology and invasion can be generated by Cdc42-MRCK signalling. MRCK (myotonic dystrophy kinase-related Cdc42-binding kinase) cooperates with ROCK in the maintenance of elongated morphology and invasion and either MRCK or ROCK is sufficient for MLC2 phosphorylation, through the inhibitory phosphorylation of myosin phosphatase. By contrast, in rounded ROCK-dependent movement, where MLC2 phosphorylation is higher, MRCK has a smaller role. Our data show that a Cdc42-MRCK signal mediates myosin-dependent cell motility and highlight convergence between Rho and Cdc42 signalling.

Signaling properties and expression in normal and tumor tissues of two phospholipase C epsilon splice variants.

Phospholipase Cepsilon (PLCepsilon) is a novel member of phosphoinositide-specific phospholipase C enzymes with a unique regulatory link to Ras GTP-ases. In the present studies, we establish existence of two splice variants (PLCepsilon1a and PLCepsilon1b) derived from human PLCepsilon1 gene. When expressed in COS or HEK293 cells, PLCepsilon1a and PLCepsilon1b have similar potential to be stimulated by diverse signaling pathways via tyrosine kinase and G-protein coupled receptors and share the ability to function as an effector of Ras. The expression pattern shows broader mRNA expression of PLCepsilon1a in normal tissues; furthermore, in most cell lines expressing PLCepsilon, PLCepsilon1a is the only splice variant present. Analysis of normal/tumor matched pairs derived from colon and rectum demonstrates greatly reduced expression levels in tumor tissues. Further studies in a colorectal tumor cell line lacking PLCepsilon show restoration of transcription of PLCepsilon1a and PLCepsilon1b by demethylating agent 5-aza-2'-deoxycytidine, suggesting epigenetic silencing through hypermethylation. In addition, expression of exogenous PLCepsilon in this cell line demonstrates inhibitory effects of PLCepsilon on cell viability and proliferation. Taken together, our findings suggest that regulatory mechanisms controlling expression of PLCepsilon, broadened by diversity introduced by splice variants, could play important role in PLCepsilon regulation in normal and tumor cells.

High-affinity binding sites for heparin generated on leukocytes during apoptosis arise from nuclear structures segregated during cell death.

During cell death of human cultured leukocytes (Jurkat, HL-60, THP-1, U937) and freshly prepared leukocytes, we observed a greater than 100-fold increase in the affinity of apoptotic and necrotic cells for fluorescein isothiocyanate (FITC)-heparin in comparison with live cells. Binding of FITC-heparin was reversed in the presence of high ionic strength, unlabeled heparan sulfate, and heparin and pentosan polysulfate, but not in the presence of chondroitin and dermatan sulfates. During the course of cell death, the increase in the percentage of cells positive for annexin V binding correlated with the increase in the population positive for binding FITC-heparin. Confocal microscopy demonstrated that heparin binding to dead cells was restricted to 1 or 2 small domains on the surfaces of apoptotic cells and to larger, but still discrete, areas that did not localize with chromatin on ruptured necrotic cells. The heparin-binding domains originated from the nucleus and may correspond to the ribonucleoprotein-containing structures that have recently been shown to segregate within the nucleus of cells and to move onto the cell membrane. We observed that phagocytosis of dead Jurkat cells by monocyte-derived macrophages was blocked when the heparin-binding capacity of the dead cells was saturated by the addition of pentosan polysulfate. From this we concluded that the ability of dead cells to bind to heparan sulfate proteoglycans on the surfaces of macrophages may assist in phagocytic clearance.