Participation of phospholipase D and alpha/beta-protein kinase C in growth factor-induced signalling in C3H10T1/2 fibroblasts

Deletion of CD74, a putative MIF receptor, in mice enhances osteoclastogenesis and decreases bone mass

CD74 is a type II transmembrane protein that can act as a receptor for macrophage migration inhibitory factor (MIF) and plays a role in MIF-regulated responses. We reported that MIF inhibited osteoclast formation and MIF knockout (KO) mice had decreased bone mass. We therefore examined if CD74 was involved in the ability of MIF to alter osteoclastogenesis in cultured bone marrow (BM) from wild-type (WT) and CD74-deficient (KO) male mice. We also measured the bone phenotype of CD74 KO male mice. Bone mass in the femur of 8-week-old mice was measured by micro-computed tomography and histomorphometry. Bone marrow cells from CD74 KO mice formed 15% more osteoclast-like cells (OCLs) with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) (both at 30 ng/mL) compared to WT. Addition of MIF to WT cultures inhibited OCL formation by 16% but had no effect on CD74KO cultures. The number of colony forming unit granulocyte-macrophage (CFU-GM) in the bone marrow of CD74 KO mice was 26% greater than in WT controls. Trabecular bone volume (TBV) in the femurs of CD74 KO male mice was decreased by 26% compared to WT. In addition, cortical area and thickness were decreased by 14% and 11%, respectively. Histomorphometric analysis demonstrated that tartrate-resistant acid phosphatase (TRAP)(+) osteoclast number and area were significantly increased in CD74 KO by 35% and 43%, respectively compared to WT. Finally, we examined the effect of MIF on RANKL-induced-signaling pathways in bone marrow macrophage (BMM) cultures. MIF treatment decreased RANKL-induced nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and c-Fos protein in BMM cultures by 70% and 41%, respectively. Our data demonstrate that CD74 is required for MIF to affect in vitro osteoclastogenesis. Further, the bone phenotype of CD74 KO mice is similar to that of MIF KO mice. MIF treatment of WT cultures suppressed RANKL-induced activator protein 1 (AP-1) expression, which resulted in decreased osteoclast differentiation in vitro. We propose that CD74 plays a critical role in the MIF inhibition of osteoclastogenesis.

Phospholipase D in human platelets: presence of isoenzymes and participation of autocrine stimulation during thrombin activation

Phospholipase D (PLD), which hydrolyzes phosphatidylcholine to phosphatidic acid (PA) and choline, is present in human platelets. Thrombin and other agonists have been shown to activate PLD but the precise mechanisms of activation and PLDs role in platelet activation remains unclear. We measured thrombin-stimulated PLD activity in platelets as formation of phosphatidylethanol. Since no specific PLD inhibitors exist, we investigated possible roles for PLD in platelets by correlating PLD activity with platelet responses such as thrombin-mediated secretion and F-actin formation (part of platelet shape change). Extracellular Ca2+ potentiated thrombin-stimulated PLD, but did not stimulate PLD in the absence of thrombin. Thrombin-induced PLD activity was enhanced by secreted ADP and binding of fibrinogen to its receptors. In contrast to others, we also found a basal PLD activity. Comparison of time courses and dose responses of platelets with PLD showed many points of correlation between PLD activation and lysosomal secretion and F-actin formation. The finding of different PLD activities suggested that different PLD isoenzymes exist in platelets as reported for other cells. Here we present evidence for the presence of both PLD1 and PLD2 in platelets by use of specific antibodies with immunoblotting and immunohistochemistry. Both isoforms were randomly localized in resting platelets, but became rapidly translocated to the proximity of the plasma membrane upon thrombin stimulation, thus indicating a role for PLD in platelet activation.

RACK1 regulates Ki-Ras-mediated signaling and morphological transformation of NIH 3T3 cells

Activating Ras mutations are involved in a significant fraction of human tumors. A suppressor screen using a retroviral mouse fibroblast cDNA library was performed to identify novel factors in Ras-mediated transformation. We identified a novel potent inhibitor of Ras-mediated morphological transformation encoded by a truncated version of the receptor for activated C-kinase (RACK1). The truncated protein, designated RACK1DeltaWD1, lacked the N-terminal 49 amino acids encoding the first of the 7 WD40 repeats in RACK1. RACK1DeltaWD1 expression restored contact inhibition, stress fiber formation and reduced ERK phosphorylation in Ki-Ras transformed NIH 3T3 cells. We demonstrate that truncated RACK1 is involved in complexes consisting of wild-type RACK1 and protein kinase C isoforms alpha, betaI and delta, compromising the transduction of an activated Ras signal to the Raf-MEK-ERK pathway. The cellular localization of RACK1DeltaWD1 differed from wtRACK1, indicating that signaling complexes containing the truncated version of RACK1 are incorrectly localized. Notably, 12-O-tetradecanoyl-13-phorbol acetate (TPA) mediated intracellular translocation of RACK1-interacting PKC alpha and delta was abrogated in RACK1DeltaWD1-expressing cells. Our data support a model where RACK1 acts as a key factor in Ki-Ras-mediated morphological transformation.

Modulation of gingival fibroblast minocycline accumulation by biological mediators

Gingival fibroblasts actively accumulate tetracyclines, thereby enhancing their redistribution from blood to gingiva. Since growth factors and pro-inflammatory cytokines regulate many fibroblast activities, they could potentially enhance fibroblast minocycline accumulation. To test this hypothesis, we treated gingival fibroblast monolayers for 1 or 6 hours with platelet-derived growth factor-BB (PDGF), fibroblast growth factor-2 (FGF), transforming growth factor-beta1 (TGF), or tumor necrosis factor-alpha (TNF). Minocycline uptake was assayed at 37 degrees by a fluorescence method. All 4 factors significantly enhanced minocycline uptake (P < or = 0.008, ANOVA), primarily by increasing the affinity of transport. Treatment for 6 hours with 10 ng/mL FGF, PDGF, TGF, or TNF enhanced fibroblast minocycline uptake by 19% to 25%. Phorbol myristate acetate enhanced fibroblast minocycline uptake by 28%, suggesting that protein kinase C plays a role in up-regulating transport. These effects on transport provide a mechanism by which systemic tetracyclines could be preferentially distributed to gingival wound or inflammatory sites.

Inhibitors of cytochrome P450 4A suppress angiogenic responses

Cytochrome P450 enzymes of the 4A family (CYP4A) convert arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-HETE) in blood vessels of several vascular beds. The present study examined the effects of inhibiting the formation of 20-HETE with N-hydroxy-N'-(4-butyl-2-methylphenol) formamidine (HET0016) on the mitogenic response of vascular endothelial growth factor (VEGF) in human umbilical vein endothelial cells (HUVECs) in vitro, and on growth factor-induced angiogenesis in the cornea of rats in vivo. HET0016 (10 micromol/L and 20 microg, respectively) abolished the mitogenic response to VEGF in HUVECs and the angiogenic response to VEGF, basic fibroblast growth factor, and epidermal growth factor in vivo by 80 to 90% (P < 0.001). Dibromododecenyl methylsulfonimide (DDMS), a structurally and mechanistically different inhibitor of 20-HETE synthesis, also abolished angiogenic responses when tested with VEGF. Additionally, administration of the stable 20-HETE agonist, 20-hydroxyeicosa-6(Z) 15(Z)-dienoic acid (WIT003) induced mitogenesis in HUVECs and angiogenesis in the rat cornea in vivo. We studied the ability of HET0016 to alter the angiogenic response in the rat cornea to human glioblastoma cancer cells (U251). When administered locally into the cornea, HET0016 (20 microg) reduced the angiogenic response to U251 cancer cells by 70%. These results suggest that a product of CYP4A product, possibly 20-HETE, plays a critical role in the regulation of angiogenesis and may provide a useful target for reduction of pathological angiogenesis.

v-Src accelerates spontaneous motility via phosphoinositide 3-kinase, phospholipase C and phospholipase D, but abrogates chemotaxis in Rat-1 and MDCK cells

In Rat-1 fibroblasts, v-Src causes a profound remodelling of cortical actin cytoskeleton. This transformation includes membrane ruffling, a hallmark of the leading edge in migrating cells, and results from activation of phosphoinositide 3-kinase (PI 3-kinase), phospholipase C (PLC) and phospholipase D (PLD). We therefore reexamined whether motility is constitutively triggered by v-Src and studied whether this response is controlled by the same signalling pathway. The study was performed using Rat-1/tsLA29 and MDCK/tsLA31 cells, each harbouring a different thermosensitive v-Src kinase, active at 34 degrees C but inactivated at 40 degrees C. In both cell lines, overnight v-Src activation induced transformation and accelerated spontaneous motility by approximately twofold, as evidenced by wound-healing assay and by single-cell track, time-lapse recording in Dunn chambers. Inhibitors of PI 3-kinase, PLC and PLD selectively abrogated acceleration of motility by v-Src. Since mechanisms that co-ordinate spontaneous, as distinct from oriented, cell migration are separable, we further analysed in Dunn chambers chemotactic response of Rat-1/tsLA29 cells to PDGF and of MDCK/tsLA31 cells to EGF. In both cases, v-Src decreased the steady-state level of growth factor receptors at the cell surface twofold, and abrogated movement directionality at comparable level of occupancy as in non-transformed cells. The burst of pinocytosis in response to growth factors was also abolished by v-Src. Altogether, these results indicate that v-Src triggers motility in a PI 3-kinase-, PLC- and PLD-dependent manner, but abrogates directionality by suppressing polarised signalling downstream of growth factor receptors.

PKCδ inhibits PKCα-mediated activation of phospholipase D1 in a manner independent of its protein kinase activity

The regulation of phospholipase D1 (PLD1) by protein kinase C (PKC) isoforms was analyzed in human melanoma cell lines. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced PLD1 activation was suppressed by the introduction of PKCdelta as well as its kinase-negative mutant in MeWo cells, which contain PKCalpha but lack PKCbeta. PLD activity was not affected by PKCdelta in G361 cells, which have PKCbeta but are deficient in PKCalpha. In MeWo cells introduced by PKCalpha and PLD1, the association of these proteins was observed, which was enhanced by the TPA treatment. In cells overexpressing PKCdelta in addition to PKCalpha and PLD1, TPA treatment increased the association of PKCdelta and PLD1, while it attenuated the association of PKCalpha and PLD1. These results indicate that PKCdelta inhibits TPA-induced PLD1 activation mediated by PKCalpha through the association with PLD1.