Abstract-The initiation of atherosclerosis results from complex interactions of circulating factors and various cell types in the vessel wall, including endothelial cells, lymphocytes, monocytes, and smooth muscle cells (SMCs). Recent reviews highlight the role of activated endothelium and inflammatory cell recruitment in the initiation of and progression of early atherosclerosis. Yet, human autopsy studies, in vitro mechanistic studies, and in vivo correlative data suggest an important role for SMCs in the initiation of atherosclerosis. SMCs are the major producers of extracellular matrix within the vessel wall and in response to atherogenic stimuli can modify the type of matrix proteins produced. In turn, the type of matrix present can affect the lipid content of the developing plaque and the proliferative index of the cells that are adherent to it. SMCs are also capable of functions typically attributed to other cell types. Like macrophages, SMCs can express a variety of receptors for lipid uptake and can form foam-like cells, thereby participating in the early accumulation of plaque lipid. Like endothelial cells, SMCs can also express a variety of adhesion molecules such as vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 to which monocytes and lymphocytes can adhere and migrate into the vessel wall. In addition, through these adhesion molecules, SMCs can also stabilize these cells against apoptosis, thus contributing to the early cellularity of the lesion. Like many cells within the developing plaque, SMCs also produce many cytokines such as PDGF, transforming growth factor-␤, IFN␥, and MCP-1, all of which contribute to the initiation and propagation of the inflammatory response to lipid. Recent advances in SMC-specific gene modulation have enhanced our ability to determine the role of SMCs in early atherogenesis.

The proper function of Rho GTPases requires precise spatial and temporal regulation of effector interactions. Integrin-mediated cell adhesion modulates the interaction of GTP-Rac with its effectors by controlling GTP-Rac membrane targeting. Here, we show that the translocation of GTP-Rac to membranes is independent of effector interactions, but instead requires the polybasic sequence near the carboxyl terminus. Cdc42 also requires integrin-mediated adhesion for translocation to membranes. A recently developed fluorescence resonance energy transfer (FRET)-based assay yields the surprising result that, despite its uniform distribution, the interaction of activated V12-Rac with a soluble, cytoplasmic effector domain is enhanced at specific regions near cell edges and is induced locally by integrin stimulation. This enhancement requires Rac membrane targeting. We show that Rho-GDI, which associates with cytoplasmic GTP-Rac, blocks effector binding. Release of Rho-GDI after membrane translocation allows Rac to bind to effectors. Thus, Rho-GDI confers spatially restricted regulation of Rac-effector interactions.

The Rho family of small GTPases are important regulators of multiple cellular activities and, most notably, reorganization of the actin cytoskeleton. Dbl-homology (DH)-domain-containing proteins are the classical guanine nucleotide exchange factors (GEFs) responsible for activation of Rho GTPases. However, members of a newly discovered family can also act as Rho-GEFs. These CZH proteins include: CDM (Ced-5, Dock180 and Myoblast city) proteins, which activate Rac; and zizimin proteins, which activate Cdc42. The family contains 11 mammalian proteins and has members in many other eukaryotes. The GEF activity is carried out by a novel, DH-unrelated domain named the DOCKER, CZH2 or DHR2 domain. CZH proteins have been implicated in cell migration, phagocytosis of apoptotic cells, T-cell activation and neurite outgrowth, and probably arose relatively early in eukaryotic evolution.

The Rho family GTPases Rac, Rho and Cdc42 are critical in regulating the actin-based cytoskeleton, cell migration, growth, survival and gene expression. These GTPases are activated by guanine nucleotide-exchange factors (GEFs). A biochemical search for Cdc42 activators led to the cloning of zizimin1, a new protein whose overexpression induces Cdc42 activation. Sequence comparison combined with mutational analysis identified a new domain, which we named CZH2, that mediates direct interaction with Cdc42. CZH2-containing proteins constitute a new superfamily that includes the so-called 'CDM' proteins that bind to and activate Rac. Together, the results suggest that CZH2 is a new GEF domain for the Rho family of proteins.

Recent studies have documented direct interactions between 14-3-3 proteins and several oncogene and proto-oncogene products involved in signal transduction pathways. Studies on the effects of 14-3-3 proteins on protein kinase C (PKC) activity in vitro have reported conflicting results, and previous attempts to demonstrate a direct association between PKC and 14-3-3 were unsuccessful. Here, we examined potential physical and functional interactions between PKC theta, a Ca(2+)-independent PKC enzyme which is expressed selectively in T lymphocytes, and the 14-3-3 tau isoform in vitro and in intact T cells. PKC theta and 14-3-3 tau coimmunoprecipitated from Jurkat T cells, and recombinant 14-3-3 tau interacted directly with purified PKC theta in vitro. Transient overexpression of 14-3-3 tau suppressed stimulation of the interleukin 2 (IL-2) promoter mediated by cotransfected wild-type or constitutively active PKC theta, as well as by endogenous PKC in ionomycin- and/or phorbol ester-stimulated cells. This did not represent a general inhibition of activation events, since PKC-independent (but Ca(2+)-dependent) activation of an IL-4 promoter element was not inhibited by 14-3-3 tau under similar conditions. Overexpression of wild-type 14-3-3 tau also inhibited phorbol ester-induced PKC theta translocation from the cytosol to the membrane in Jurkat cells, while a membrane-targeted form of 14-3-3 tau caused increased localization of PKC theta in the particulate fraction in unstimulated cells. Membrane-targeted 14-3-3 tau was more effective than wild-type 14-3-3 tau in suppressing PKC theta-dependent IL-2 promoter activity, suggesting that 14-3-3 tau inhibits the function of PKC theta not only by preventing its translocation to the membrane but also by associating with it. The interaction between 14-3-3 and PKC theta may represent an important general mechanism for regulating PKC-dependent signals and, more specifically, PKC theta-mediated functions during T-cell activation.