The PI3 kinase-Akt pathway mediates Wnt3a-induced proliferation

Oxysterol-induced osteogenic differentiation of marrow stromal cells is regulated by Dkk-1 inhibitable and PI3-kinase mediated signaling

Osteoporosis and its complications cause morbidity and mortality in the aging population, and result from increased bone resorption by osteoclasts in parallel with decreased bone formation by osteoblasts. A widely accepted strategy for improving bone health is targeting osteoprogenitor cells in order to stimulate their osteogenic differentiation and bone forming properties through the use of osteoinductive/anabolic factors. We previously reported that specific naturally occurring oxysterols have potent osteoinductive properties, mediated in part through activation of hedgehog signaling in osteoprogenitor cells. In the present report, we further demonstrate the molecular mechanism(s) by which oxysterols induce osteogenesis. In addition to activating the hedgehog signaling pathway, oxysterol-induced osteogenic differentiation is mediated through a Wnt signaling-related, Dkk-1-inhibitable mechanism. Bone marrow stromal cells (MSC) treated with oxysterols demonstrated increased expression of osteogenic differentiation markers, along with selective induced expression of Wnt target genes. These oxysterol effects, which occurred in the absence of beta-catenin accumulation or TCF/Lef activation, were inhibited by the hedgehog pathway inhibitor, cyclopamine, and/or by the Wnt pathway inhibitor, Dkk-1. Furthermore, the inhibitors of PI3-Kinase signaling, LY 294002 and wortmanin, inhibited oxysterol-induced osteogenic differentiation and induction of Wnt signaling target genes. Finally, activators of canonical Wnt signaling, Wnt3a and Wnt1, inhibited spontaneous, oxysterol-, and Shh-induced osteogenic differentiation of bone marrow stromal cells, suggesting the involvement of a non-canonical Wnt pathway in pro-osteogenic differentiation events. Osteogenic oxysterols are, therefore, important small molecule modulators of critical signaling pathways in pluripotent mesenchymal cells that regulate numerous developmental and post-developmental processes.

Signaling mechanisms mediating muscarinic enhancement of GABAergic synaptic transmission in the spinal cord

Activation of muscarinic acetylcholine receptors (mAChRs) inhibits spinal nociceptive transmission by potentiation of GABAergic tone through M(2), M(3), and M(4) subtypes. To study the signaling mechanisms involved in this unique mAChR action, GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of lamina II neurons were recorded using whole-cell patch clamp techniques in rat spinal cord slices. The mAChR agonist oxotremorine-M caused a profound increase in the frequency of GABAergic sIPSCs, which was abolished in the Ca(2+)-free solution. Inhibition of voltage-gated Ca(2+) channels with Cd(2+) and Ni(2+) largely reduced the effect of oxotremorine-M on sIPSCs. Blocking nonselective cation channels (NSCCs) with SKF96365 or 2-APB also largely attenuated the effect of oxotremorine-M. However, the KCNQ channel blocker XE991 and the adenylyl cyclase inhibitor MDL12330A had no significant effect on oxotremorine-M-induced increases in sIPSCs. Furthermore, the phosphoinositide-3-kinase (PI3K) inhibitor wortmannin or LY294002 significantly reduced the potentiating effect of oxotremorine-M on sIPSCs. In the spinal cord in which the M(3) subtype was specifically knocked down by intrathecal small interfering RNA (siRNA) treatment, SKF96365 and wortmannin still significantly attenuated the effect of oxotremorine-M. In contrast, SKF96365 and wortmannin both failed to alter the effect of oxotremorine-M on sIPSCs when the M(2)/M(4) mAChRs were blocked. Therefore, our study provides new evidence that activation of mAChRs increases synaptic GABA release through Ca(2+) influx and voltage-gated Ca(2+) channels. The PI3K-NSCC signaling cascade is primarily involved in the excitation of GABAergic interneurons by the M(2)/M(4) mAChRs in the spinal dorsal horn.

Autocrine WNT signaling contributes to breast cancer cell proliferation via the canonical WNT pathway and EGFR transactivation

Background

De-regulation of the wingless and integration site growth factor (WNT) signaling pathway via mutations in APC and Axin, proteins that target β-catenin for destruction, have been linked to various types of human cancer. These genetic alterations rarely, if ever, are observed in breast tumors. However, various lines of evidence suggest that WNT signaling may also be de-regulated in breast cancer. Most breast tumors show hypermethylation of the promoter region of secreted Frizzled-related protein 1 (sFRP1), a negative WNT pathway regulator, leading to downregulation of its expression. As a consequence, WNT signaling is enhanced and may contribute to proliferation of human breast tumor cells. We previously demonstrated that, in addition to the canonical WNT/β-catenin pathway, WNT signaling activates the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in mouse mammary epithelial cells via epidermal growth factor receptor (EGFR) transactivation.

Methods

Using the WNT modulator sFRP1 and short interfering RNA-mediated Dishevelled (DVL) knockdown, we interfered with autocrine WNT signaling at the ligand-receptor level. The impact on proliferation was measured by cell counting, YOPRO, and the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide) assay; β-catenin, EGFR, ERK1/2 activation, and PARP (poly [ADP-ribose]polymerase) cleavages were assessed by Western blotting after treatment of human breast cancer cell lines with conditioned media, purified proteins, small-molecule inhibitors, or blocking antibodies.

Results

Phospho-DVL and stabilized β-catenin are present in many breast tumor cell lines, indicating autocrine WNT signaling activity. Interfering with this loop decreases active β-catenin levels, lowers ERK1/2 activity, blocks proliferation, and induces apoptosis in MDA-MB-231, BT474, SkBr3, JIMT-1, and MCF-7 cells. The effects of WNT signaling are mediated partly by EGFR transactivation in human breast cancer cells in a metalloprotease- and Src-dependent manner. Furthermore, Wnt1 rescues estrogen receptor-positive (ER⁺) breast cancer cells from the anti-proliferative effects of 4-hydroxytamoxifen (4-HT) and this activity can be blocked by an EGFR tyrosine kinase inhibitor.

Conclusion

Our data show that interference with autocrine WNT signaling in human breast cancer reduces proliferation and survival of human breast cancer cells and rescues ER⁺ tumor cells from 4-HT by activation of the canonical WNT pathway and EGFR transactivation. These findings suggest that interference with WNT signaling at the ligand-receptor level in combination with other targeted therapies may improve the efficiency of breast cancer treatments.

Translational embryology: using embryonic principles to generate pancreatic endocrine cells from embryonic stem cells

Diseases that affect endodermally derived organs such as the lungs, liver, and pancreas include cystic fibrosis, chronic hepatitis, and diabetes, respectively. Despite the prevalence of these diseases, cures remain elusive. While several promising transplantation-based therapies exist for some diseases such as Type 1 diabetes, they are currently limited by the availability of donor-derived tissues. Embryonic stem cells are a promising and renewable source of tissue for transplantation; however, directing their differentiation into specific, adult cell lineages remains a significant challenge. In this review, we will focus on one endodermally derived organ, the pancreas, and discuss how studies of embryonic pancreas development have been used as the basis for the directed, step-wise differentiation of mouse and human embryonic stem cells into pancreatic endocrine cells that are capable of rescuing Type 1 diabetes in animal models.

The primary cilium coordinates signaling pathways in cell cycle control and migration during development and tissue repair

Cell cycle control and migration are critical processes during development and maintenance of tissue functions. Recently, primary cilia were shown to take part in coordination of the signaling pathways that control these cellular processes in human health and disease. In this review, we present an overview of the function of primary cilia and the centrosome in the signaling pathways that regulate cell cycle control and migration with focus on ciliary signaling via platelet-derived growth factor receptor alpha (PDGFRalpha). We also consider how the primary cilium and the centrosome interact with the extracellular matrix, coordinate Wnt signaling, and modulate cytoskeletal changes that impinge on both cell cycle control and cell migration.

Gi-coupled P2Y-ADP receptor mediates GSK-3 phosphorylation and beta-catenin nuclear translocation in granule neurons

Glycogen synthase kinase-3 (GSK-3) is a multifaceted enzyme involved in development, neurogenesis, and survival at the CNS. We investigated nucleotides signaling to GSK-3 in cerebellar granule neurons and found that the metabotropic agonist 2-methyl-thio-ADP (2MeSADP) was able to induce GSK-3 phosphorylation and inhibition of its catalytic activity. 2MeSADP could be acting through several P2Y-ADP receptors expressed in granule neurons, as RT-PCR expression was found for P2Y(1), P2Y(12), and P2Y(13) receptors, but the pharmacological data fitted well with a Gi-coupled P2Y(13) receptor: the effect was sensitive to pertussis toxin, was unaffected by specific antagonists of P2Y(1) and P2Y(12) receptors, such as 2'-deoxy-N(6)-methyl-adenosine 3',5'-diphosphate and 2-methyl-thio-AMP, respectively, and the EC(50) values for 2MeSADP and ADP were in the same low nanomolar range. 2MeSADP was able to phosphorylate and activate extracellular signal-regulated kinase (ERK)-1,2 and Akt proteins, but its effect on GSK-3 phosphorylation was primarily dependent on the phosphatidyl inositol-3 kinase (PI3-K)/Akt pathway, as it was abolished by the PI3-K inhibitor wortmannin. GSK-3 inactivation by 2MeSADP in granule neurons resulted in nuclear translocation of its substrate beta-catenin, which functions as a transcriptional regulator, this effect being lost with wortmaninn. The present study first describes the coupling of a Gi-coupled P2Y(13)-like receptor to GSK-3 and beta-catenin through PI3-K/Akt signaling.

Wnt5a promotes adhesion of human dermal fibroblasts by triggering a phosphatidylinositol-3 kinase/Akt signal

Frizzled-3 (Fzd3), highly expressed in both the central nervous system (CNS) and skin, plays essential roles in axonal growth and guidance during the CNS development and may be involved in maintenance of skin integrity, although its ligand remains undetermined. In this study, we demonstrate that Wnt5a specifically binds to Fzd3 in vitro and triggers phosphorylation of Akt mediated by phosphatidylinositol-3 kinase (PI3K), but not that of ERK or protein kinase C, in human primary-cultured dermal fibroblasts. We have further found that such Wnt5a/Fzd3-triggered activation of the PI3K/Akt signal promotes integrin-mediated adhesion of human dermal fibroblasts to collagen I-coated dishes. Based on another finding that Wnt5a/Fzd3-triggered activation of the PI3K/Akt signal was blocked by an excess amount of a recombinant Fzd3-cysteine-rich domain (CRD), but not by that of a recombinant Fzd6-CRD, it is concluded that Wnt5a is a natural ligand of Fzd3 that triggers the PI3K/Akt signal and promotes adhesion of human dermal fibroblasts.

The soluble wnt receptor Frizzled8CRD-hFc inhibits the growth of teratocarcinomas in vivo

Wnt signaling is important for normal cell proliferation and differentiation, and mutations in pathway components are associated with human cancers. Recent studies suggest that altered wnt ligand/receptor interactions might also contribute to human tumorigenesis. Therefore, agents that antagonize wnt signaling at the extracellular level would be attractive therapeutics for these cancers. We have generated a soluble wnt receptor comprising the Frizzled8 cysteine-rich domain (CRD) fused to the human Fc domain (F8CRDhFc) that exhibits favorable pharmacologic properties in vivo. Potent antitumor efficacy was shown using the mouse mammary tumor virus-Wnt1 tumor model under dosing conditions that did not produce detectable toxicity in regenerating tissue compartments. In vitro, F8CRDhFc inhibited autocrine wnt signaling in the teratoma cell lines PA-1, NTera-2, Tera-2, and NCCIT. In vivo, systemic administration of F8CRDhFc significantly retarded the growth of tumor xenografts derived from two of these cell lines, PA-1 and NTera-2. Pharmacodynamic markers of wnt signaling, identified by gene expression analysis of cultured teratoma cells, were also modulated in the tumor xenografts following treatment with F8CRDhFc. Additionally, these markers could be used as indicators of treatment efficacy and might also be useful in identifying patients that would benefit from the therapeutic agent. This is the first report showing the efficacy of a soluble wnt receptor as an antitumor agent and suggests that further development of wnt antagonists will have utility in treating human cancer.

Epiregulin promotes proliferation and migration of renal proximal tubular cells

Epiregulin is an epidermal growth factor (EGF) member that activates ErBB1 and ErBB4 homodimers and all possible heterodimeric ErbB complexes. Because its role in renal cell regeneration has not been investigated, we assessed the effect of exogenous epiregulin on regeneration of renal proximal tubular cells (RPTC) in primary culture. Epiregulin (10 ng/ml) was equivalent to EGF (10 ng/ml) in enhancing RPTC proliferation and migration. Epiregulin induced activation of the EGF receptor (EGFR), Akt, a downstream kinase of phosphoinositide 3-kinase (PI3K), and extracellular signaling-regulated kinase 1/2 (ERK1/2). Treatment with AG1478, a specific EGFR inhibitor, blocked phosphorylation of EGFR, Akt, ERK1/2, proliferation, and migration. Furthermore, inactivation of PI3K with LY-294002 blocked epiregulin-induced RPTC proliferation and, to a lesser extent, migration. However, blockade of ERK1/2 had no such effects. We suggest that epiregulin is a potent mitogen for renal epithelial cells and may contribute to renal regeneration through activation of EGFR and PI3/Akt pathways.

The emerging role of Wnt signaling in the pathogenesis of acute myeloid leukemia

Wnt signaling plays an important role in stem cell self-renewal and proliferation. Aberrant activation of Wnt signaling and its downstream targets are intimately linked with several types of cancer with colon cancer being the best-studied example. However, recent results also suggest an important role of Wnt signaling in normal as well as leukemic hematopoietic stem cells. Aberrant activation of Wnt signaling and downstream effectors has been demonstrated in acute myeloid leukemia. Here, mutant receptor tyrosine kinases, such as Flt3 and chimeric transcription factors such as promyelocytic leukemia-retinoic acid receptor-alpha and acute myeloid leukemia1-ETO, induce downstream Wnt signaling events. These findings suggest that the Wnt signaling pathway is an important target in several leukemogenic pathways and may provide a novel opportunity for targeting leukemic stem cells.