Embryonic Stem Cells & Hedgehog

The Hh pathway helps to cause cell proliferation and morphogenesis of Embryonic Stem Cells (ESC). Stems cells have unique properties. These properties include self-renewal and they can become specialized cells such as nerves, muscles, bone...ect. Hh target genes cause cell division and with different degrees of pathway activities can produce stem cells to become specialized cells. Abnormalities cause severe birth defects that are often fatal. The hedgehog pathway appears to be essential for development of most major organs and most noticeably neuronal tissue. The Hh protein can cause different cellular responses with varying concentrations. This is why the Hh protein is called a morphogen. At highest doses of a Hh protein or Hh agonist researchers can cause a embryonic stem cell to differentiate into a motor neuron. This will probably be curis’s main focus (and could be the main reason for so many partnerships) of research and is their specialty.

Adult Stem Cells & Hedgehog

In the adult, the hedgehog pathway is normally quiescent. However, the Hh has been shown to be active in a few cells that are normally dividing at a high rate. (see the forum under antagonist and then possible side effects for a list of cells). Your body has adult stem cells that are used for repairing or renewal of old or damaged cells. For example if you get hurt or damage your cells have to be repaired! The Hh pathway appears to be one of the major pathways that are responsible for this. When tissue is damage the Hh pathway is turned on and causes adult cells to divide and turn into the correct linage. The Hh is most likely not the only pathway that can do this but appears to be a master regulator that turns on other pathways that are essential for the repair and regeneration process. This is similar to how the Hh operates on embryonic stem cells, but because the adult body is not growing and is fully developed it does not need the hedgehog pathway turned on all the time.

For more info about the different aspects of adult stem cells and embryonic stems can be found at http://stemcells.nih.gov/info/basics

Cancer and the Hedgehog

Cancer is caused by a genetic mutation that occurs by a carcinogen. Carcinogens are often chemicals such as cigarette smoke or other environmental factors. Carcinogens cause the genetic code of a cell(s) to change. This change leads to a genetic lesion and sometimes allows a cell to be unregulated and proliferate. A very simplified view is an adult stem cell gets stuck in continual repair which leads to cancer. Cells that are repeatedly subjected to carcinogens often have a higher chance of turning cancerous. For example, skin cancer (uv radiation) & lung cancer (smokers). While the Hedgehog pathway appears to play a key role in post embryonic patterning, it is not a magic bullet for cancer. Hh antagonist are specific thus they may lead less side effects compared to current cytotoxic approaches, but being specific also means that the cancer needs to be dependent on the Hh pathway for it to be effective. After you read the article snippets below you should see that cancer and Hedgehog pathway activation are sometimes related to adult stem cells.

This is probably one of the best articles that I have read regarding Hh and cancer. I will copy some little bits and post below. Please note all work was done by the authors. If you want the complete article please download it or ask me and I can provide a link. If you don’t understand something ask and somebody will most likely to help you out or point you in the right direction.


NATURE|VOL 432 | 18 NOVEMBER 2004|www.nature.com/nature

Tissue repair and stem cell renewal in carcinogenesis
Philip A. Beachy1,4, Sunil S. Karhadkar1,2 & David M. Berman2,3,4
1Department of Molecular Biology and Genetics, The Howard Hughes Medical Institute, 2Department of Pathology, 3Department of Urology and
4Department of Oncology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA

Cancer is increasingly being viewed as a stem cell disease, both in its propagation by a minority of cells with stem-cell-like properties and in its possible derivation from normal tissue stem cells. But stem cell activity is tightly controlled, raising the question of how normal regulation might be subverted in carcinogenesis. The long-known association between cancer and chronic tissue injury, and the more recently appreciated roles of Hedgehog and Wnt signalling pathways in tissue regeneration, stem cell renewal and cancer growth together suggest that carcinogenesis proceeds by misappropriating homeostatic mechanisms that govern tissue repair and stem cell self-renewal.

Hh and Wnt signalling in cancer
The roles of Hh and Wnt pathways in stem cell renewal are particularly interesting given the genetically implied connection between activity of these pathways and the initiation and growth of a substantial fraction of lethal cancers (Table 2). Familial mutations that facilitate Hh and Wnt pathway activation have been associated with increased incidence of specific brain, skin, skeletal muscle, liver and colon cancers in humans and mice, and of bladder cancer in mice. Additional studies in which pathway activities are antagonized by treatment with pharmacological agents, with antibodies that bind and block ligand action, or by overexpression of negatively acting pathway components further demonstrate an ongoing requirement for pathway activity in the growth of additional cancer types which include small-cell lung cancer and carcinomas of the oesophagus, stomach, pancreas, biliary tract and prostate. The range of organs from which Hh- and Wnt-pathway dependent cancers originate is therefore similar to the range of organs in which these pathways have a role in stem cell renewal. In terms of medical significance, about one-third of total cancer deaths are caused by the cancer types in which current evidence implicates Hh or Wnt pathway activity in most cases30.

The Hh pathway in cancer
The link between Hh pathway activity and cancer was initially established by the identification of heterozygous mutations affecting Patched (PTCH), a negatively acting component of the Hh receptor, as the cause of Gorlin’s syndrome. This syndrome is associated with an increased incidence of basal cell carcinoma, medulloblastoma, and rhabdomyosarcoma, and PTCH is also mutated in sporadic forms of are common mechanisms for Hh protein processing and lipid modification and a dedicated mechanism for the release of lipid-modified Hh protein from producing cells. (reviewed in refs 32, 33). In addition the Gli proteins, like their Drosophila counterpart Ci, can be regulated by interactions with the Suppressor of fused (Su(fu)) protein and can exist in activating forms (primarily Gli and Gli2) as well as in proteolytically processed repressing forms (primarily Gli3). The human SU(FU) gene has also been implicated as a tumour suppressor, with mutations found in familial and sporadic medulloblastoma and in sporadic basal cell carcinoma (see Table 2).

Despite extensive similarities between Drosophila and mammalian pathways, however, significant differences may exist, particularly in the transductory machinery between Smo and Gli. Thus, although recent genetic and biochemical studies in Drosophila have demonstrated that pathway activation is transmitted through association of Smo with a complex of cytoplasmic proteins that includes Ci and a kinesin-like protein, Cos2, a functional mammalian homologue of Cos2 has not been identified (reviewed in ref. 32). Because of its role in maintenance of pathway quiescence in Drosophila, a functional mammalian Cos2 homologue would be of interest as a potential tumour suppressor. In addition, several apparent pathway components identified in mammals either have no counterparts or do not function in the Drosophila Hh pathway (see ref. 34). These include components such as RAB23 (ref. 35) or FKBP8 (ref. 34), which have unknown function, but are of interest as potential tumour suppressors because of their action downstream of Smo as negative regulators of pathway activity (see Fig. 1).

Some tumours of the type associated with Gorlin’s syndrome are not associated with known pathway-activating mutations, despite clear evidence for pathway activity36,37. This suggests that activation of the Hh pathway may occur through mechanisms other than by mutation of pathway components, and raises the possibility that such mechanisms may also have a role in pathway activation in other cancers not typically associated with Gorlin’s syndrome. Consistent with this possibility, recent studies using cyclopamine, a specific Hh pathway antagonist38–40, indeed have demonstrated an ongoing requirement for pathway activity in the growth of a series of lethal cancers arising in organs of endodermal origin, and not typically associated with Gorlin’s syndrome. These cancers include small-cell lung cancer and carcinomas of the oesophagus, stomach, pancreas, biliary tract, and
prostate41–43. Pathway activity in these cancers requires ligand activation, as demonstrated with the use of Hh-blocking antibodies, and contrary to ligand-independent activation arising in tumours associated with Gorlin’s syndrome. Curiously, the limiting factor in pathway activation in these non-Gorlin’s tumours seems not to be
ligand expression, but rather the acquisition of responsiveness to ligand. Thus, whereas the Hh family members Shh (Sonic hedgehog) and Ihh (Indian hedgehog) are expressed in normal endodermal tissues, high-level activation of Hh pathway targets occurs only in cancer cells. In the prostate, the limiting factor for ligand responsiveness is SMO, which is not expressed in normal prostate tissue29. Furthermore, isolated prostate stem/progenitor cells acquire Hh responsiveness simply by introduction of Smo expression constructs, and these cells are oncogenically transformed upon pathway activation. The genetic or epigenetic changes that trigger Smo expression are not identified, although they may be linked to epithelial regeneration (see section ‘Cancer and persistent states of repair’ below).

Hh and Wnt pathways in regeneration and tissue repair
If cancer stem cells arise from tissue stem cells, and if Hh and Wnt pathway activities are critical for the renewal of at least some of these stem cell types, then continuous Hh and Wnt pathway activities may promote cancer growth by continuously recapitulating their roles in promoting normal stem cell renewal. But stem cell renewal must be tightly regulated (otherwise tumours might arise), raising the critical question of how and under what circumstances normal regulation can be circumvented in cancer. Some insight into the regulation of stem cell renewal activity may be gained from a consideration of the role of Hh and Wnt pathways in regenerative responses (Table 3). Wnt pathway activation in the radially symmetric coelenterate Hydra is closely associated with the growth and patterning of new individuals. This may result either from normal asexual budding or from experimental manipulations, such as cell dissociation and re-aggregation49. Hydra tissue thus seems to exist in a constant state of growth and replacement. Amphibia, particularly urodeles (newts and salamanders), are also
capable of mounting impressive regenerative responses to limb amputation or to extirpation of certain organs. The typical sequence of events involves de-differentiation of cells near the site of injury, followed by extensive proliferation and patterning of the regenerating tissues. In the cases of urodele limb and lens regeneration, Hh family members are expressed in the de-differentiated cells following injury, and regeneration
can be blocked by treatment with cyclopamine50–52. Fin regeneration in fish also entails expression of Hh genes and targets, and is disrupted by cyclopamine inhibition53,54. The regenerating structures in these examples encompass several tissue types that are arranged in complex patterns; in this respect pathway roles resemble those in embryonic pattern formation. But Hh and Wnt pathway activity also have a role in regenerative responses that are restricted to single tissue types within organs. For example, transient Hh pathway activity is required for androgentriggered regeneration of prostate epithelium in male mouse castrates29, and Wnt pathway activities similarly are required for muscle regeneration in response to cardiotoxin-induced muscle injury23. Increased Hh pathway activity in Ptch_/_ mice also contributes to an increase in photoreceptor-cell progenitor number and retinal repair in a model of retinal degeneration. Furthermore, mammary progenitors are enriched in mice with Wnt pathway activation caused by increased Wnt ligand levels or by a _-catenin altered to increase its stability55. In addition to these demonstrations of functional Hh and Wnt pathway activity in tissue repair, correlative data suggest a possible role for Wnt pathway activity in response to biliary tract56, liver57 and kidney tubule injury58, and for Hh pathway activity in repair of bone fractures59, bile duct56 and airway injury41.

Cancer and persistent states of repair
We have reviewed evidence highlighting the role of Hh and Wnt pathway activity in cancer growth on the one hand, and in stem cell renewal and tissue regeneration on the other. But is there a link between tissue repair and cancer? A connection is strongly suggested by the known association between chronic tissue injury and cancer60,61, including cancers associated with Hh and/or Wnt pathway activity. Increased cancer risks are associated with exposure to toxins, such as alcohol, cigarette smoke and organic chemicals62–64, with chronic infection of Helicobacter pylori and other pathogens65, and with inflammatory conditions, such as sclerosing cholangitis and inflammatory bowel disease66,67 — all of which entail chronic tissue damage. As discussed above, acute injury is accompanied by the expansion of stem cell pools and by the transient activation of the Hh and Wnt signalling pathways41. Under conditions of chronic injury, pathway activation and presumed expansion of stem cell pools would persist so long as repeated injury prevents full regeneration. This state of continuous pathway and progenitor-cell activation resembles the continuous pathway activity and cell division seen in cancer. These observations suggest that cancer growth may represent the continuous operation of an unregulated state of tissue repair and that continuous Hh/Wnt pathway activities in carcinogenesis may represent a deviation from the return to quiescence that normally follows regeneration (Fig. 2A, a, b). The simplest model for the emergence of this state is that genetic or epigenetic events prevent the return to quiescence of an activated stem or progenitor cell on completion of regeneration, thus initiating a tumour by trapping the cell in an activated state of continuous renewal. Consistent with this model, the Bmi-1 gene required for HSC renewal is also required for the propagation of leukaemias in transfer experiments68,69. The expression of Bmi-1 and nestin, which are both associated with stem cell renewal68–70, is dependent on Hh pathway activity in Hh-dependent tumours29,37,41,71. Of course, multiple genetic or epigenetic changes might be required to trap the activated stem cell initially, and
numerous other events may contribute to rapid proliferation or to other aspects of the phenotype. Conversion of an activated stem cell into a clinically threatening cancer stem cell may involve changes that lock the cell in an active state of renewal and allow the cell to acquire independence from niche signals that are normally required to maintain stem cell identity.
The observed increase in cancer incidence associated with chronic injury strongly supports this model of cancer as a continuous state of repair. If, as hypothesized, the oncogenic event results in trapping activated stem cells in a continual state of renewal, then any condition that increases the pool size of activated stem cells should increase the probability of an oncogenic event by making the cellular substrates for such an event more numerous. The effect of repeated injury over time would be exactly this — to increase the pool size of stem cells in an active state of renewal (Fig. 3). Tissues that normally undergo rapid renewal might also be expected to experience increased cancer incidence, as a high turnover rate might require a sizeable pool of activated stem cells. Indeed, organs such as the skin, the lungs, and the gastrointestinal tract, which are continuously exposed to environmental insults, and consequently in a constant state of renewal, are the tissues of origin for a high proportion of cancers.

Perspectives and implications
If cancer stem cells responsible for driving the growth of cancer types associated with Hh and Wnt pathway activation indeed come from stem cells trapped in a state of active renewal by pathway activities, then a logical therapeutic approach for these cancers would be to impose a state of pathway blockade (see introduction in this issue by (Sawyers, page 294). Potential problems associated with such approaches might include cognitive or affective disturbances, as both Hh and Wnt pathways are active in the mature brain. In addition, the roles of pathway activities in normal stem cell renewal suggest that pathway blockade might cause a complete or partial loss of stem cell pools. Latent symptoms caused by such a loss might include an increased susceptibility to degenerative disorders, and appear only after passage of a significant fraction of lifespan. On the other hand, Hh or Wnt pathway activities might be restricted solely to the stimulation of stem cell self-renewal and not affect signals required for the maintenance of stem cell identity. In this case, endogenous stem cells may remain quiescent during pathway blockade but regain renewal capacity once therapy is completed and the blockade lifted. Stem cell niches might also persist and permit the regeneration of stem cells that are temporarily lost during a period of pathway blockade. Consistent with such a possibility, the well-characterized germline stem
cell niches in the Drosophila ovary and testis have been reported to persist and supply continuous niche signals for a significant fraction of the adult lifespan, even after they are emptied of stem cells13,79. The potential success of such therapeutic approaches is suggested by the achievement of growth inhibition or regression, complete in
some cases with systemic treatment by the Hh pathway antagonist cyclopamine in murine models of several Hh-pathway-associated tumour types29,37,42,43,80. In addition, a recent report demonstrates growth inhibition of spontaneous medulloblastomas arising in Ptch_/_p53_/_mice on systemic treatment with a synthetic Hh pathway antagonist81. Cancer growth in these tumour types apparently requires an active state of renewal, without which cancer stem cells are depleted by differentiation or apoptosis. The lack of toxic effects in mice during periods of systemic cyclopamine treatment extending as long as seven weeks and during follow-up observation periods of nearly half a year also augurs well for this approach. More recently, cyclopamine-induced regression of human basal-cell carcinomas was reported82, suggesting the potential effectiveness of Hh pathway blockade in humans. As cyclopamine application in these human cases was topical, cognitive or affective disturbances that might be caused by systemic pathway blockade cannot be ruled out. Such effects, if they materialize, might be reduced or eliminated by the development of pathway-blocking agents that do not cross the blood/brain barrier. The feasibility of such an approach is suggested by the identification of several structurally distinct classes of Hh pathway antagonists40,83. Some recent success has also been reported in the identification of Wnt pathway antagonists48, suggesting that the therapeutic effects of blocking this pathway may be tested in the near future. Systemic pathway blockade in humans may require consideration of other factors. For example, the inability of prostate epithelium or muscle to regenerate under conditions of Hh or Wnt pathway
blockade may be typical of many tissues, and the loss of stem cell renewal divisions could result in increased sensitivity to injury or other transient demands being placed on stem cell pools. It may therefore be important for patients to avoid even mild sources of trauma while undergoing pathway blockade therapy. This consideration also raises doubts about combining any form of cytotoxic chemotherapy with pathway blockade, as indiscriminate injury imposed by such therapy might affect some of the tissues containing stem cells whose renewal depends on pathway activities. Despite these concerns, preliminary studies in vitro and in mice suggest that blockade of these pathways may offer a new and efficacious therapeutic approach to a large group of highly lethal cancers. Other strategies of potential use in cancer prevention and therapy might also arise from an improved understanding of the response to tissue injury, in particular of the signals that regulate the activation of tissue stem cells.

--------------------------------------------------------------------------------------------------------
A few more snippets from another article:
| DECEMBER 2003 | VOLUME 3 www.nature.com/reviews/cancer
HEDGEHOG SIGNALLING IN CANCER FORMATION AND MAINTENANCE
Marina Pasca di Maglianoand Matthias Hebrok
The Hedgehog signalling pathway is essential for numerous processes during embryonic development.
Members of this family of secreted proteins control cell proliferation, differentiation and tissue patterning in a dose-dependent manner. Although the overall activity of the pathway is diminished after embryogenesis, recent reports show that the pathway remains active in some adult tissues, including adult stem cells in the brain and skin. There is also evidence that uncontrolled activation of the pathway results in specific types of cancer.

Future directions
Adult stem cells, Hh signalling and cancer. One of the most important unresolved questions in cancer biology
concerns the identity of cells that become tumorigenic. Striking similarities between cancer and stem cells have been previously reported, as both cell types have the potential for unlimited self-renewal. Hh signalling is active in and required to maintain stem-cell or precursor populations in several organs, and deregulation is known to result in tumorigenesis (BOX 2). Increasing evidence also indicates that, at least in some organs, uncontrolled Hh signalling results in unregulated self-renewal of progenitor cells. In skin, Hh signalling is required for hair morphogenesis
during embryonic development. In the mature tissue, the multipotent skin and hair stem cells transiently express Ptch during the proliferation phase65. Multipotent cells then give rise to two progenitor populations — epithelial progenitors (which do not express Ptch and give rise to the stratified epithelium) and hair progenitors (which continue to express Ptch while they proliferate and then differentiate into the different cell populations of the hair follicle). The level of Hh signalling, which is mainly mediated by Gli2 (REF. 25), seems to be crucial — loss of Hh signalling prevents proliferation, whereas increased Hh signalling results in formation of BCCs (BOX 2).
Within the adult lung, Hh expression is limited to small patches of epithelial cells43. Expression becomes transiently activated during acute airway epithelial regeneration after tissue injury, indicating that the pathway might mark neuroendocrine progenitor cells within the lung epithelium.SCLCs possess many primitive neuroendocrine features, and some SCLCs require Hh signalling for tumour maintenance. The similarities beween Hh signalling during neuroendocrine-cell regeneration and SCLC formation indicate that deregulation of the pathway in epithelial precursors is involved in tumour formation. Similarly, the duct structures that are believed to harbour adult pancreatic progenitor cells express Ptch1 (REF. 20).Although conclusive evidence is lacking, cells within or attached to pancreatic ducts are thought to give rise to endocrine and exocrine cells during regeneration66, and endocrine cells that are located in epithelial structures known as islets of Langerhans continue to express Ptch1.Although the issue is still controversial, pancreatic adenocarcinomas are thought to arise from duct cells67,68, indicating that Hh expression could mark pancreatic progenitor cells and control their proliferative potential. Identification of these cells would be important for both generating more differentiated -cells to treat diabetics, as well as to better understand the molecular and cellular
principles that result in adenocarcinoma formation. HH signalling is essential for numerous processes during organ development and maintenance of organ function. However, its ability to regulate cell differentiation and renewal in a dose-dependent manner also means that deregulation of this pathway can result in uncontrolled cell proliferation. Fortunately, specific inhibitors of the pathway are available for basic research, and those with therapeutic potential are being developed. However, it should be noted that detailed molecular analysis of tumour types is required to determine which patients will respond to anti-HH therapy. Although all tested pancreatic adenocarcinoma cell lines seem to express HH signalling components, only five out of ten SCLC tumours express both SHH and GLI1 (REF. 43). So, analysis of tumour genexpression profiles69 might be useful in determining which tumour types have activated HH signalling and therefore be useful in predicting the outcome of potential treatments with HH inhibitors.

Discovering the dual role of this pathway — on one hand its requirement for normal organ development and function, and on the other hand its association with tumorigenesis — has proven that the study of signalling pathways in the developing embryo can lead to important insights into disease progression and treatment. In the case of Hh signalling, this knowledge could lead to new therapeutic approaches to treat tumours that have poor prognoses.

Finally, Hh signalling interacts with other embryonic signalling pathways that are known to be involved in cancer formation. Analysis of these connections should provide important insights into the molecular causes of cancer formation and growth.

---

Here is another very good pdf article to down load

PDF Article

---

A few more articles to read

Nature. 2001 Nov 1;414(6859):105-11.
Stem cells, cancer, and cancer stem cells.

Biotechniques. 2003 Dec;35(6):1240-7.
The elements of stem cell self-renewal: a genetic perspective.

Nature. 2005 Apr 14;434(7035):843-50.
Wnt signalling in stem cells and cancer.

Recent Prog Horm Res. 2003;58:283-95.
Regulation of hematopoietic stem cell self-renewal.