SHP2 tyrosine phosphatase converts parafibromin/Cdc73 from a tumor suppressor to an oncogenic driver.

Deregulation of SHP2 is associated with malignant diseases as well as developmental disorders. Although SHP2 is required for full activation of RAS signaling, other potential roles in cell physiology have not been elucidated. Here we show that SHP2 dephosphorylates parafibromin/Cdc73, a core component of the RNA polymerase II-associated factor (PAF) complex. Parafibromin is known to act as a tumor suppressor that inhibits cyclin D1 and c-myc by recruiting SUV39H1 histone methyltransferase. However, parafibromin can also act in the opposing direction by binding ß-catenin, thereby activating promitogenic/oncogenic Wnt signaling. We found that, on tyrosine dephosphorylation by SHP2, parafibromin acquires the ability to stably bind ß-catenin. The parafibromin/ß-catenin interaction overrides parafibromin/SUV39H1-mediated transrepression and induces expression of Wnt target genes, including cyclin D1 and c-myc. Hence, SHP2 governs the opposing functions of parafibromin, deregulation of which may cause the development of tumors or developmental malformations.

Apolipoprotein(a) stimulates nuclear translocation of ß-catenin: a novel pathogenic mechanism for lipoprotein(a).

Lipoprotein(a) (Lp(a)) is associated with cardiovascular disease risk. This may be attributable to the ability of Lp(a) to elicit endothelial dysfunction. We previously reported that apolipoprotein(a) (apo(a); the distinguishing kringle-containing component of Lp(a)) elicits cytoskeletal rearrangements in vascular endothelial cells, resulting in increased cellular permeability. These effects require a strong lysine-binding site (LBS) in apo(a). We now report that apo(a) induces both nuclear ß-catenin-mediated cyclooxygenase-2 (COX-2) expression and prostaglandin E2 secretion, indicating a proinflammatory role for Lp(a). Apo(a) caused the disruption of VE-cadherin/ß-catenin complexes in a Src-dependent manner, decreased ß-catenin phosphorylation, and increased phosphorylation of Akt and glycogen synthase kinase-3ß, ultimately resulting in increased nuclear translocation of ß-catenin; all of these effects are downstream of apo(a) attenuation of phosphatase and tensin homologue deleted on chromosome 10 activity. The ß-catenin-mediated effects of apo(a) on COX-2 expression were absent using a mutant apo(a) lacking the strong LBS. Of interest, the normal and LBS mutant forms of apo(a) bound to human umbilical vein endothelial cells in a similar manner, and the binding of neither was affected by lysine analogues. Taken together, our findings suggest a novel mechanism by which apo(a) can induce proinflammatory and proatherosclerotic effects through modulation of vascular endothelial cell function.

Apolipoprotein(a), through its strong lysine-binding site in KIV(10'), mediates increased endothelial cell contraction and permeability via a Rho/Rho kinase/MYPT1-dependent pathway.

Substantial evidence indicates that endothelial dysfunction plays a critical role in atherogenesis. We previously demonstrated that apolipoprotein(a) (apo(a); the distinguishing protein component of the atherothrombotic risk factor lipoprotein(a)) elicits rearrangement of the actin cytoskeleton in human umbilical vein endothelial cells, characterized by increased myosin light chain (MLC) phosphorylation via a Rho/Rho kinase-dependent signaling pathway. Apo(a) contains kringle (K)IV and KV domains similar to those in plasminogen: apo(a) contains 10 types of plasminogen KIV-like sequences, followed by sequences homologous to the plasminogen KV and protease domains. Several of the apo(a) kringles contain lysine-binding sites (LBS) that have been proposed to contribute to the pathogenicity of Lp(a). Here we demonstrate that apo(a)-induced endothelial barrier dysfunction is mediated via a Rho/Rho kinase-dependent signaling pathway that results in increased MYPT1 phosphorylation and hence decreased MLC phosphatase activity, thus leading to an increase in MLC phosphorylation, stress fiber formation, cell contraction, and permeability. In addition, studies using recombinant apo(a) variants indicated that these effects of apo(a) are dependent on sequences within the C-terminal half of the apo(a) molecule, specifically, the strong LBS in KIV(10). In parallel experiments, the apo(a)-induced effects were completely abolished by treatment of the cells with the lysine analogue epsilon-aminocaproic acid and the Rho kinase inhibitor Y27632. Taken together, our findings indicate that the strong LBS in apo(a) KIV(10) mediates all of our observed effects of apo(a) on human umbilical vein endothelial cell barrier dysfunction. Studies are ongoing to further dissect the molecular basis of these findings.

The apolipoprotein(a) component of lipoprotein(a) stimulates actin stress fiber formation and loss of cell-cell contact in cultured endothelial cells.

Elevated plasma concentrations of lipoprotein(a) (Lp(a)) are a risk factor for a variety of atherosclerotic disorders including coronary heart disease. In the current study, we report that incubation of cultured human umbilical vein or coronary artery endothelial cells with Lp(a) elicits a dramatic rearrangement of the actin cytoskeleton characterized by increased central stress fiber formation and redistribution of focal adhesions. These effects are mediated by the apolipoprotein(a) (apo(a)) component of Lp(a) since incubation of apo(a) with the cells evoked similar cytoskeletal rearrangements, while incubation with low density lipoprotein had no effect. Apo(a) also produced a time-dependent increase in transendothelial permeability. The cytoskeletal rearrangements evoked by apo(a) were abolished by C3 transferase, which inhibits Rho, and by Y-27632, an inhibitor of Rho kinase. In addition to actin cytoskeleton remodeling, apo(a) was found to cause VE-cadherin disruption and focal adhesion molecule reorganization in a Rho- and Rho kinase-dependent manner. Cell-cell contacts were found to be regulated by Rho and Rac but not Cdc42. Apo(a) caused a transient increase in the extent of myosin light chain phosphorylation. Finally apo(a) did not evoke increases in intracellular calcium levels, although the effects of apo(a) on the cytoskeleton were found to be calcium-dependent. We conclude that the apo(a) component of Lp(a) activates a Rho/Rho kinase-dependent intracellular signaling cascade that results in increased myosin light chain phosphorylation with attendant rearrangements of the actin cytoskeleton. We propose that the resultant increase in endothelial permeability caused by Lp(a) may help explain the atherosclerotic risk posed by elevated concentrations of this lipoprotein.