Oxysterols Are Novel Activators of the Hedgehog Signaling Pathway in Pluripotent Mesenchymal Cells

Trafficking, development and hedgehog

Embryogenesis is mediated by a relatively small number of developmental signaling pathways, and the morphogens, receptors and transcription factors integral to these cascades are considered the master regulators of development. However, superimposed on this is an additional layer of control by complex intracellular trafficking networks. The importance of trafficking in controlling the processes of morphogenesis and development is highlighted by recent data regarding the transport and localisation of the morphogen sonic hedgehog (Shh) and the machinery that leads to its secretion, modification, cellular internalisation and signal transduction. Here we review the regulation of hedgehog signaling by intracellular trafficking, including the role of the primary cilium and lipids in mediating pathway activity.

Hedgehog signaling in development and cancer

The Hedgehog (Hh) family of secreted proteins governs a wide variety of processes during embryonic development and adult tissue homeostasis. Here we review the current understanding of the molecular and cellular basis of Hh morphogen gradient formation and signal transduction, and the multifaceted roles of Hh signaling in development and tumorigenesis. We discuss how the Hh pathway has diverged during evolution and how it integrates with other signaling pathways to control cell growth and patterning.

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.

The Hedgehog protein family

The Hedgehog (Hh) pathway is one of the fundamental signal transduction pathways in animal development and is also involved in stem-cell maintenance and carcinogenesis. The hedgehog (hh) gene was first discovered in Drosophila, and members of the family have since been found in most metazoa. Hh proteins are composed of two domains, an amino-terminal domain HhN, which has the biological signal activity, and a carboxy-terminal autocatalytic domain HhC, which cleaves Hh into two parts in an intramolecular reaction and adds a cholesterol moiety to HhN. HhC has sequence similarity to the self-splicing inteins, and the shared region is termed Hint. New classes of proteins containing the Hint domain have been discovered recently in bacteria and eukaryotes, and the Hog class, of which Hh proteins comprise one family, is widespread throughout eukaryotes. The non-Hh Hog proteins have carboxy-terminal domains (the Hog domain) highly similar to HhC, although they lack the HhN domain, and instead have other amino-terminal domains. Hog proteins are found in many protists, but the Hh family emerged only in early metazoan evolution. HhN is modified by cholesterol at its carboxyl terminus and by palmitate at its amino terminus in both flies and mammals. The modified HhN is released from the cell and travels through the extracellular space. On binding its receptor Patched, it relieves the inhibition that Patched exerts on Smoothened, a G-protein-coupled receptor. The resulting signaling cascade converges on the transcription factor Cubitus interruptus (Ci), or its mammalian counterparts, the Gli proteins, which activate or repress target genes.

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

Neuronal subtype specification in the vertebrate neural tube is one of the best-studied examples of embryonic pattern formation. Distinct neuronal subtypes are generated in a precise spatial order from progenitor cells according to their location along the anterior-posterior and dorsal-ventral axes. Underpinning this organization is a complex network of multiple extrinsic and intrinsic factors. This review focuses on the molecular mechanisms and general strategies at play in ventral regions of the forming spinal cord, where sonic hedgehog-based morphogen signaling is a key determinant. We discuss recent advances in our understanding of these events and highlight unresolved questions.

Two patched protein subtypes and a conserved domain of group I proteins that regulates turnover

Patched (Ptc) is a 12-cross membrane protein that binds the secreted Hedgehog protein. Its regulation of the Hedgehog signaling pathway is critical to normal development and to a number of human diseases. This report analyzes features of sequence similarity and divergence in the Ptc protein family and identifies two subtypes distinguished by novel conserved domains. We used these results to propose a rational basis for classification. We show that one of the conserved sequence regions in the C-terminal domain of Ptch1 is responsible, at least in part, for rapid turnover. This sequence is absent in the stable Ptch2 protein.

New Functions for Oxysterols and Their Cellular Receptors

Oxysterols are naturally occurring oxidized derivatives of cholesterol, or by-products of cholesterol biosynthesis, with multiple biologic functions. These compounds display cytotoxic, pro-apoptotic, and pro-inflammatory activities and may play a role in the pathology of atherosclerosis. Their functions as intermediates in the synthesis of bile acids and steroid hormones, and as readily transportable forms of sterol are well established. During the past decade, however, novel physiologic activities of oxysterols have emerged. They are now thought to act as endogenous regulators of gene expression in lipid metabolism. Recently, new intracellular oxysterol receptors have been identified and novel functions of oxysterols in cell signaling discovered, evoking novel interest in these compounds in several branches of biomedical research.

Multiple roles for lipids in the Hedgehog signalling pathway

The identification of endogenous sterol derivatives that modulate the Hedgehog (Hh) signalling pathway has begun to suggest testable hypotheses for the cellular biological functions of Patched, and for the lipoprotein association of Hh. Progress in the field of intracellular sterol trafficking has emphasized how tightly the distribution of intracellular sterol is controlled, and suggests that the synthesis of sterol derivatives can be influenced by specific sterol-delivery pathways. The combination of this field with Hh studies will rapidly give us a more sophisticated understanding of both the Hh signal-transduction pathway and the cell biology of sterol metabolism.

Autonomous Hedgehog signalling is undetectable in PC-3 prostate cancer cells

The Hedgehog signalling pathway has been implicated in the development of prostate cancer, although this area remains controversial. Some but not all studies have noted relatively high Hedgehog pathway activity in commonly used prostate cancer cell lines. We aimed to evaluate the widely used PC-3 cell line as a model to investigate Hedgehog signalling in a prostate cancer setting. Using a sensitive Hedgehog inducible luciferase reporter assay, we found no evidence of autonomous Hedgehog signalling in PC-3 cells, irrespective of passage number. In addition, manipulations that should either increase (an oxysterol) or decrease (cyclopamine) Hedgehog pathway activity had no effect on reporter activity, and cyclopamine treatment did not affect PC-3 cell viability. Therefore, our findings contradict some earlier reports and caution against the use of PC-3 cells to investigate the Hedgehog pathway in a prostate cancer setting.

Greased hedgehogs: new links between hedgehog signaling and cholesterol metabolism

The close link between signaling by the developmental regulators of the Hedgehog family and cholesterol biochemistry has been known for some time. The morphogen is covalently attached to cholesterol in a peculiar autocatalytic reaction and embryonal disruption of cholesterol synthesis leads to malformations that mimic Hh signaling defects. Recently, it was furthermore shown that secreted Hh could hitchhike on lipoprotein particles to establish its morphogenic gradient in the developing embryo. Additionally, there is new evidence that the Hh-receptor Patched transmits the Hh signal by modulating the secretion of an inhibitory sterol molecule from the receiving cells. Here we present some of the most recent discoveries on the Hh-sterol link and discuss their implications from a systems design perspective. We predict that a robust functioning of the Hh pathway will require the involvement of more sterol metabolites, and these should be the subject of future research.

Oxysterols: novel biologic roles for the 21st century

A major focus for the 21st century are the sterol intermediates in cholesterol synthesis and their metabolites. No longer considered inactive way stations in their transformation to cholesterol, both physiologic and pathophysiologic studies, though early in their development, indicate novel biologic roles for these sterols, and their oxysterol metabolites that bypass cholesterol, the expected end product. A major impetus for further inquiry is the recognition that in genetically determined errors in cholesterol synthesis such as Smith-Lemil-Opitz syndrome, the phenotypic effects on the developing fetus are not solely attributable to the lack of cholesterol but the accumulation of 7-dehydrocholesterol and its 27-hydroxy metabolite. This view is now supported by a new mouse model, the double knockout Insig1 & 2 (insulin-induced genes 1 & 2) in which lack of the protein product results in a greater production of lanosterol compared to cholesterol during fetal life with severe dysmorphic consequences. Further support can be derived from in vitro studies of the Sonic hedgehog signaling pathway, essential for normal morphogenesis in the central nervous system and perhaps other organs, which may require the local presence of oxysterols for full expression. Future studies that can delineate the specific role of a sterol intermediate or its metabolite require a paradigm shift away from the generic use of oxysterols as a class of compounds to a focus on specific sterols that can be expected in tissues and techniques for mimicking the local environment. Another class of oxysterols are those arising by photoxidation, now considered to be an expected event generated by the photons of visible blue light and therefore pari passu with normal vision. The sequence of events from peroxides of cholesterol to hydroxy and keto derivatives is the signature of singlet oxygen as opposed to free radicals and other mechanisms for generating reactive oxygen species. Perhaps surprisingly, the retina expresses CYP 27A1 and CYP 46A1, enzymes with broad substrate specificity for ring-modified sterols, implying that, in addition to a rich blood supply for disposing of potentially toxic oxysterols, they can be detoxified locally. Recognition that the retina has nuclear receptors similar to those found in other tissues raises the possibility that the sterols that are generated may function in their traditional role as ligands for modulating gene expression but other, nonligand, activities can be expected since other proteins such as the oxysterol-binding proteins exist and are considered to have biologic activities. To critically evaluate these potentially new biologic roles for oxysterols a need exists for the synthesis and utilization of the expected naturally occurring metabolites rather than available surrogates that may not be truly representative of their tissue effects and to utilize analytical techniques that can identify their existence at the expected concentrations in tissues.

Sonic hedgehog is an autocrine viability factor for myofibroblastic hepatic stellate cells

BACKGROUND/AIMS

Factors released during liver injury, such as platelet derived growth factor-BB (PDGF), promote accumulation of myofibroblastic hepatic stellate cells (MFB) that drive the pathogenesis of cirrhosis. The hedgehog (Hh) pathway regulates remodeling of other injured tissues. This study evaluates the hypothesis that autocrine production of Sonic hedgehog (Shh) promotes MFB growth.

METHODS

Primary rat hepatic stellate cells (HSC) were treated without or with PDGF, a pharmacologic inhibitor of PDGF-regulated kinases, adenovirus expressing activated or dominant negative AKT, or Hh signaling inhibitors. Shh production, expression of Hh inhibitors and target genes, and HSC growth were assessed.

RESULTS

HSC expressed Shh, Hh pathway components, and the Hh inhibitor, Hip. During culture Hip expression fell, Shh production increased, and Hh target gene expression was induced. Neutralizing Shh antibodies promoted apoptosis. Adding PDGF increased Shh expression and MFB growth. Both processes followed activation of AKT and were abrogated by AKT inhibitors. Adenoviral delivery of activated AKT up-regulated Shh expression, demonstrating a direct role for AKT in regulating Shh expression. Shh-neutralizing antibodies and other Hh pathway inhibitors blocked the mitogenic effects of PDGF.

CONCLUSIONS

These results identify Shh as an autocrine growth factor for MFB and suggest a role for Hh signaling in the pathogenesis of cirrhosis.

The primary cilium at the crossroads of mammalian hedgehog signaling

Cilia function as critical sensors of extracellular information, and ciliary dysfunction underlies diverse human disorders including situs inversus, polycystic kidney disease, retinal degeneration, and Bardet-Biedl syndrome. Importantly, mammalian primary cilia have recently been shown to mediate transduction of Hedgehog (Hh) signals, which are involved in a variety of developmental processes. Mutations in several ciliary components disrupt the patterning of the neural tube and limb bud, tissues that rely on precisely coordinated gradients of Hh signal transduction. Numerous components of the Hh pathway, including Patched, Smoothened, and the Gli transcription factors, are present within primary cilia, indicating that key steps of Hh signaling may occur within the cilium. Because dysregulated Hh signaling promotes the development of a variety of human tumors, cilia may also have roles in cancer. Together, these findings have shed light on one mechanism by which primary cilia transduce signals critical for both development and disease.

Patching the gaps in Hedgehog signalling

The Hedgehog (Hh) pathway plays central roles in animal development and stem-cell function. Defects in Hh signalling lead to birth defects and cancer in humans. The first and often genetically damaged step in this pathway is the interaction between two membrane proteins - Patched (Ptc), encoded by a tumour suppressor gene, and Smoothened (Smo), encoded by a proto-oncogene. Recent work linking Hh signalling to sterol metabolites and protein-trafficking events at the primary cilium promises to shed light on the biochemical basis of how Patched inhibits Smoothened, and to provide new avenues for cancer treatment.

Sonic Hedgehog promotes the development of multipotent neural crest progenitors endowed with both mesenchymal and neural potentials

In the vertebrate embryo, the cephalic neural crest cells (CNCCs) produce cells belonging to two main lineages: the neural [including neurons, glial cells of the peripheral nervous system (PNS), and melanocytes] and the mesenchymal (chondrocytes, osteoblasts, smooth muscle cells, and connective tissue cells), whereas the trunk NCCs (TNCCs) in amniotes yield only neural derivatives. Although multipotent cells have previously been evidenced by in vitro clonal analysis, the issue as to whether all of the mesenchymal and neural phenotypes can be derived from a unique NC stem cell has remained elusive. In the present work, we devised culture conditions that led us to identify a highly multipotent NCC endowed with both neural and mesenchymal potentials, which lies upstream of all the other NC progenitors known so far. We found that addition of recombinant Sonic Hedgehog (Shh) increased the number of CNCC progenitors yielding both mesenchymal and neural lineages and promoted the development of such precursors from the TNCC. Shh decreased the neural-restricted precursors without affecting the overall CNCC survival and proliferation. By showing a differential positive effect of Shh on the expression of mesenchymal phenotypes (i.e., chondrocytes and smooth muscle cells) by multipotent CNCCs, these results shed insights on the in vivo requirement of Shh for craniofacial morphogenesis. Together with evolutionary considerations, these data also suggest that the mesenchymal-neural precursor represents the ancestral form of the NC stem cell, which in extinct forms of vertebrates (the ostracoderms) was able to yield both the PNS and superficial skeleton.

The primary cilia, a 'Rab-id' transit system for hedgehog signaling

Intense focus has been centered around how the primary cilia transduces the hedgehog (Hh) signal from smoothened (Smo) to the Gli transcription factors. New data indicate that ligand and signaling lipids help regulate small GTPase-dependent accumulation and activity of signaling components.

Hepatic accumulation of Hedgehog-reactive progenitors increases with severity of fatty liver damage in mice

Progenitors regenerate fatty livers but the mechanisms involved are uncertain. The Hedgehog pathway regulates mesendodermal progenitors and modulates mesenchymal-epithelial interactions during tissue remodeling. To determine if Hedgehog signaling increases in liver progenitors during fatty liver injury, we compared expression of Hedgehog ligands and target genes across a spectrum of injury. Leptin-deficient ob/ob mice with fatty livers and their healthy lean littermates were studied before and after exposure to the hepatotoxin, ethionine. At baseline, ob/ob mice had greater liver damage than controls. Ethionine induced liver injury in both ob/ob and lean mice, with greater injury occurring in ob/ob mice. After ethionine, the ob/ob mice developed liver atrophy and fibrosis. Liver injury increased hepatic accumulation of progenitors, including ductular cells that produced and responded to Hedgehog ligands. A dose-response relationship was demonstrated between liver injury and expansion of Hedgehog-responsive progenitors. In severely damaged, atrophic livers, nuclei in mature-appearing hepatocytes accumulated the Hedgehog-regulated mesenchymal transcription factor, Gli2 and lost expression of the liver epithelial transcription factor, hepatocyte nuclear factor 6 (HNF-6). Hepatic levels of collagen mRNA and pericellular collagen fibrils increased concomitantly. Hence, fatty liver injury increases Hedgehog activity in liver progenitors, and this might promote epithelial-mesenchymal transitions that result in liver fibrosis.

Hedgehog signaling pathway is inactive in colorectal cancer cell lines

The Hedgehog (Hh) signaling pathway plays an important role in human development. Abnormal activation of this pathway has been observed in several types of human cancers, such as the upper gastro-intestinal tract cancers. However, activation of the Hh pathway in colorectal cancers is controversial. We analyzed the expression of the main key members of the Hh pathway in 7 colon cancer cell lines in order to discover whether the pathway is constitutively active in these cells. We estimated the expression of SHH, IHH, PTCH, SMO, GLI1, GLI2, GLI3, SUFU and HHIP genes by RT-PCR. Moreover, Hh ligand, Gli3 and Sufu protein levels were quantified by western blotting. None of the cell lines expressed the complete set of Hh pathway members. The ligands were absent from Colo320 and HCT116 cells, Smo from Colo205, HT29 and WiDr. GLI1 gene was not expressed in SW480 cells nor were GLI2/GLI3 in Colo205 or Caco-2 cells. Furthermore the repressive form of Gli3, characteristic of an inactive pathway, was detected in SW480 and Colo320 cells. Finally treatment of colon cancer cells with cyclopamine, a specific inhibitor of the Hh pathway, did not downregulate PTCH and GLI1 genes expression in the colorectal cells, whereas it did so in PANC1 control cells. Taken together, these results indicate that the aberrant activation of the Hh signaling pathway is not common in colorectal cancer cell lines.

Lipoproteins and their receptors in embryonic development: more than cholesterol clearance

Previously, the relevance of lipoproteins and their receptors has mainly been discussed in terms of cholesterol clearance in the adult organism. Now, findings from nematodes to fruit flies to mammals all point towards novel and unexpected roles for lipoprotein metabolism in the control of key regulatory pathways in the developing embryo, including signaling through steroid hormones and throughout the hedgehog and Wnt signaling pathways. Here, we discuss the emerging view of how lipoproteins and their receptors regulate embryogenesis.

20(S)-hydroxycholesterol inhibits PPARgamma expression and adipogenic differentiation of bone marrow stromal cells through a hedgehog-dependent mechanism

Specific oxysterols have been shown to be pro-osteogenic and anti-adipogenic. However, the molecular mechanism(s) by which oxysterols inhibit adipogenic differentiation is unknown. We show that the anti-adipogenic effects of osteogenic oxysterol, 20(S)-hydroxycholesterol, are mediated through a hedgehog-dependent mechanism(s) and are associated with inhibition of PPARgamma expression.

INTRODUCTION

Multipotent bone marrow stromal cells (MSCs) are common progenitors of osteoblasts and adipocytes. A reciprocal relationship between osteogenic and adipogenic differentiation may explain the increased adipocyte and decreased osteoblast formation in aging and osteoporosis. We have previously reported that specific oxysterols stimulate osteogenic differentiation of MSCs while inhibiting their adipogenic differentiation.

MATERIALS AND METHODS

The M2-10B4 (M2) murine pluripotent bone MSC line was used to assess the inhibitory effects of 20(S)-hydroxycholesterol (20S) and sonic hedgehog (Shh) on peroxisome proliferator-activated receptor gamma (PPARgamma) and adipogenic differentiation. All results were analyzed for statistical significance using ANOVA.

RESULTS AND CONCLUSIONS

Treatment of M2 cells with the osteogenic oxysterol 20S completely inhibited adipocyte formation induced by troglitazone after 10 days. PPARgamma mRNA expression assessed by RT-qPCR was significantly induced by Tro after 48 (5-fold) and 96 h (130-fold), and this induction was completely inhibited by 20S. In contrast, 20S did not inhibit PPARgamma transcriptional activity in M2 cells overexpressing PPARgamma and retinoid X receptor (RXR). To elucidate the molecular mechanism(s) by which 20S inhibits PPARgamma expression and adipogenic differentiation, we focused on the hedgehog signaling pathway, which we previously showed to be the mediator of osteogenic responses to oxysterols. The hedgehog signaling inhibitor, cyclopamine, reversed the inhibitory effects of 20S and Shh on troglitazone-induced adipocyte formation in 10-day cultures of M2 cells by 70% and 100%, respectively, and the inhibitory effect of 20S and Shh on troglitazone-induced PPARgamma expression was fully reversed at 48 h by cyclopamine. Furthermore, 20S and Shh greatly inhibited PPARgamma2 promoter activity induced by CCAAT/enhancer-binding protein alpha overexpression. These studies show that, similar to the induction of osteogenesis, the inhibition of adipogenesis in murine MSCs by the osteogenic oxysterol, 20S, is mediated through a hedgehog-dependent mechanism(s).

Aortic Msx2-Wnt calcification cascade is regulated by TNF-alpha-dependent signals in diabetic Ldlr-/- mice

OBJECTIVE

Aortic calcification is prevalent in type II diabetes (T2DM), enhancing morbidity and tracking metabolic syndrome parameters. Ldlr(-/-) mice fed high-fat "Westernized" diets (HFD) accumulate aortic calcium primarily in the tunica media, mediated via osteogenic morphogens and transcriptional programs that induce aortic alkaline phosphatase (ALP). Because elevated TNF-alpha is characteristic of obesity with T2DM, we examined contributions of this inflammatory cytokine.

METHODS AND RESULTS

HFD promoted obesity, hyperglycemia, and hyperlipidemia, and upregulated serum TNF-alpha in Ldlr(-/-) mice. Serum haptoglobin (inflammatory marker) was increased along with aortic expression of BMP2, Msx2, Wnt3a, and Wnt7a. Dosing with the TNF-alpha neutralizing antibody infliximab did not reduce obesity, hypercholesterolemia, or hyperglycemia; however, haptoglobin, aortic BMP2, Msx2, Wnt3a, and Wnt7a and aortic calcium accumulation were downregulated by infliximab. Mice with vascular TNF-alpha augmented by a transgene (SM22-TNFalphaTg) driven from the SM22 promoter upregulated aortic Msx2, Wnt3a, and Wnt7a. Furthermore, SM22-TNFalphaTg;TOPGAL mice exhibited greater aortic beta-galactosidase reporter staining versus TOPGAL sibs, indicating enhanced mural Wnt signaling. In aortic myofibroblast cultures, TNF-alpha upregulated Msx2, Wnt3a, Wnt7a, and ALP. ALP induction was inhibited by Dkk1, an antagonist of paracrine Wnt actions.

CONCLUSIONS

TNF-alpha promote aortic Msx2-Wnt programs that contribute to aortic calcium accumulation in T2DM.

Patched1 regulates hedgehog signaling at the primary cilium

Primary cilia are essential for transduction of the Hedgehog (Hh) signal in mammals. We investigated the role of primary cilia in regulation of Patched1 (Ptc1), the receptor for Sonic Hedgehog (Shh). Ptc1 localized to cilia and inhibited Smoothened (Smo) by preventing its accumulation within cilia. When Shh bound to Ptc1, Ptc1 left the cilia, leading to accumulation of Smo and activation of signaling. Thus, primary cilia sense Shh and transduce signals that play critical roles in development, carcinogenesis, and stem cell function.