β-Catenin is a signaling molecule that promotes cell proliferation by the induction of gene transcription through the activation of T-cell factor (TCF)/lymphoid enhancer factor (LEF) transcription factors. The canonical mechanism of the regulation of β-catenin involves its phosphorylation by casein kinase 1 at the Ser-45 site and by glycogen synthase kinase 3 (GSK3) at the Thr-41, Ser-37, and Ser-33 sites. This phosphorylation targets β-catenin to ubiquitination and degradation by the proteasome system. Mitogenic factors promote β-catenin signaling through the inhibition of GSK3, resulting in reduced β-catenin phosphorylation, its stabilization, and subsequent accumulation in the nucleus, where it stimulates TCF/LEF-dependent gene transcription. In the present study, we have shown that (i) β-catenin can be phosphorylated by protein kinase A (PKA) in vitro and in intact cells at two novel sites, Ser-552 and Ser-675; (ii) phosphorylation by PKA promotes the transcriptional activity (TCF/LEF transactivation) ofβ-catenin; (iii) mutation of Ser-675 attenuates the promoting effect of PKA; (iv) phosphorylation by PKA does not affect the GSK3-dependent phosphorylation ofβ-catenin, its stability, or intracellular localization; and (v) phosphorylation at the Ser-675 site promotes the binding of β-catenin to its transcriptional coactivator, CREB-binding protein. In conclusion, this study identifies a novel, noncanonical mechanism of modulation of β-catenin signaling through direct phosphorylation of β-catenin by PKA, promoting its interaction with CREB-binding protein.

The growing research interest in nanomedicine for the treatment of cancer and inflammatory-related pathologies is yielding encouraging results. Unfortunately, enthusiasm is tempered by the limited specificity of the enhanced permeability and retention effect. Factors such as lack of cellular specificity, low vascular density, and early release of active agents prior to reaching their target contribute to the limitations of the enhanced permeability and retention effect. However, improved nanomedicine designs are creating opportunities to overcome these problems. In this review, we present examples of the advances made in this field and endeavor to highlight the potential of these emerging technologies to improve targeting of nanomedicine to specific pathological cells and tissues.

Transforming growth factor-β (TGF-β) is a cytokine implicated in wound healing and in the pathogenesis of pulmonary fibrosis. TGF-β stimulates myofibroblast differentiation characterized by expression of contractile smooth muscle (SM)-specific proteins such as SM–α-actin. In the present study, we examined the role of serum response factor (SRF) in the mechanism of TGF-β–induced pulmonary myofibroblast differentiation of human lung fibroblasts (HLF). TGF-β stimulated SM–α-actin expression in HLF, which paralleled with a profound induction of SRF expression and activity. Inhibition of SRF by the pharmacologic SRF inhibitor (CCG-1423), or via adenovirus-mediated transduction of SRF short hairpin RNA (shSRF), blocked the expression of both SRF and SM–α-actin in response to TGF-β without affecting Smad-mediated signaling of TGF-β. However, forced expression of SRF on its own did not promote SM–α-actin expression, whereas expression of the constitutively transactivated SRF fusion protein (SRF-VP16) was sufficient to induce SM–α-actin expression, suggesting that both expression and transactivation of SRF are important. Activation of protein kinase A (PKA) by forskolin or iloprost resulted in a significant inhibition of SM–α-actin expression induced by TGF-β, and this was associated with inhibition of both SRF expression and activity, but not of Smad-mediated gene transcription. In summary, this is the first direct demonstration that TGF-β–induced pulmonary myofibroblast differentiation is mediated by SRF, and that inhibition of myofibroblast differentiation by PKA occurs through down-regulation of SRF expression levels and SRF activity, independent of Smad signaling.

Apoptosis of vascular smooth muscle cells (VSMCs) plays an important role in remodeling of vessel walls, one of the major determinants of long-term blood pressure elevation and an independent risk factor for cardiovascular morbidity and mortality. Recently, we have found that apoptosis in cultured VSMCs can be inhibited by inversion of the intracellular [Na ⁺ ]/[K ⁺ ] ratio after the sustained blockage of the Na ⁺ ,K ⁺ -ATPase by ouabain. To understand the mechanism of ouabain action, we analyzed subsets of hydrophilic and hydrophobic VSMC proteins from control and ouabain-treated cells by 2-dimensional electrophoresis. Ouabain treatment led to overexpression of numerous soluble and hydrophobic cellular proteins. Among proteins that showed the highest level of ouabain-induced expression, we identified mortalin (also known as GRP75 or PBP-74), a member of the heat shock protein 70 (HSP70) superfamily and a marker for cellular mortal and immortal phenotypes. Northern and Western blotting and immunocytochemistry all have confirmed that treatment of VSMCs with ouabain results in potent induction of mortalin expression. Transient transfection of cells with mortalin cDNA led to at least a 6-hour delay in the development of apoptosis after serum deprivation. The expression of tumor suppressor gene, p53, in mortalin-transfected cells was delayed to the same extent, and the expressed protein showed abnormal perinuclear distribution, suggesting that p53 is retained and inactivated by mortalin. Our studies therefore define a new [Na ⁺ ] _i /[K ⁺ ] _i -responsive signaling pathway that may play an important role in the regulation of programmed cell death in VSMCs.