Åsa Bergström

Inhibition of GLI-mediated transcription and tumor cell growth by small-molecule antagonists.

The developmentally important Hedgehog (Hh) signaling pathway has recently been implicated in several forms of solid cancer. Current drug development programs focus on targeting the protooncogene Smoothened, a key transmembrane pathway member. These drug candidates, albeit promising, do not address the scenario in which pathway activation occurs downstream of Smoothened, as observed in cases of medulloblastoma, glioma, pericytoma, breast cancer, and prostate cancer. A cellular screen for small-molecule antagonists of GLI-mediated transcription, which constitutes the final step in the Hh pathway, revealed two molecules that are able to selectively inhibit GLI-mediated gene transactivation. We provide genetic evidence of downstream pathway blockade by these compounds and demonstrate the ineffectiveness of upstream antagonists such as cyclopamine in such situations. Mechanistically, both inhibitors act in the nucleus to block GLI function, and one of them interferes with GLI1 DNA binding in living cells. Importantly, the discovered compounds efficiently inhibited in vitro tumor cell proliferation in a GLI-dependent manner and successfully blocked cell growth in an in vivo xenograft model using human prostate cancer cells harboring downstream activation of the Hh pathway.

Wounding enhances epidermal tumorigenesis by recruiting hair follicle keratinocytes

Chronic wounds and acute trauma constitute well-established risk factors for development of epithelial-derived skin tumors, although the underlying mechanisms are largely unknown. Basal cell carcinomas (BCCs) are the most common skin cancers displaying a number of features reminiscent of hair follicle (HF)-derived cells and are dependent on deregulated Hedgehog (Hh)/GLI signaling. Here we show, in a mouse model conditionally expressing GLI1 and in a model with homozygous inactivation of Ptch1, mimicking the situation in human BCCs, that the wound environment accelerates the initiation frequency and growth of BCC-like lesions. Lineage tracing reveals that both oncogene activation and wounding induce emigration of keratinocytes residing in the lower bulge and the nonpermanent part of the HFs toward the interfollicular epidermis (IFE). However, only oncogene activation in combination with a wound environment enables the participation of such cells in the initiation of BCC-like lesions at the HF openings and in the IFE. We conclude that, in addition to the direct enhancement of BCC growth, the tumor-promoting effect of the wound environment is due to recruitment of tumor-initiating cells originating from the neighboring HFs, establishing a link between epidermal wounds and skin cancer risk.

DYRK1B-dependent autocrine-to-paracrine shift of Hedgehog signaling by mutant RAS

Synergism between the RAS and Hedgehog (HH) pathways has been suggested for carcinogenesis in the pancreas, lung and colon. We investigated the molecular cross-talk between RAS and HH signaling and found that, although mutant RAS induces or enhances SHH expression and favors paracrine HH signaling, it antagonizes autocrine HH signal transduction. Activated RAS can be found in primary cilia, the central organelle of HH signal transduction, but functions in a cilium-independent manner and interferes with Gli2 function and Gli3 processing. In addition, the cell-autonomous negative regulation of HH signal transduction involves the RAS effector molecule dual specificity tyrosine phosphorylated and regulated kinase 1B (DYRK1B). In line with a redirection of autocrine toward paracrine HH signaling by a KRAS-DYRK1B network, we find high levels of GLI1 expression restricted to the stromal compartment and not to SHH-expressing tumor cells in human pancreatic adenocarcinoma.

Development of Mammary Tumors by Conditional Expression of GLI1

A diverse set of cellular defects, presumably elicited by multiple genetic alterations, underlies cancer development. Aberrant Hedgehog (Hh) signaling has recently been implicated in the development and maintenance of breast cancer. However, evidence conclusively showing that activated Hh signaling can induce mammary tumors is lacking. We now show that transgenic expression of the Hh effector protein GLI1 under the regulation of the mouse mammary tumor virus promoter, expressed in the mouse mammary gland, is associated with the appearance of hyperplastic lesions, defective terminal end buds, and tumor development. The GLI1-induced tumors are histologically heterogeneous and involve the expansion of a population of epithelial cells expressing the progenitor cell markers keratin 6 and Bmi-1. Moreover, tumor cells express genes involved in proliferation, cell survival, and metastasis. GLI1-induced tumors do not fully regress following transgene deinduction, indicating that some tumors develop and are maintained autonomously, independent of sustained transgenic GLI1 expression. The data strongly support a role of Hh/GLI signaling in breast cancer development and suggest that inhibition of this signaling pathway represents a new therapeutic opportunity for limiting tumorigenesis and early tumorigenic progression.

Real-time assessment of tissue hypoxia in vivo with combined photoacoustics and high-frequency ultrasound.

Purpose: In preclinical cancer studies, non-invasive functional imaging has become an important tool to assess tumor development and therapeutic effects. Tumor hypoxia is closely associated with tumor aggressiveness and is therefore a key parameter to be monitored. Recently, photoacoustic (PA) imaging with inherently co-registered high-frequency ultrasound (US) has reached preclinical applicability, allowing parallel collection of anatomical and functional information. Dual-wavelength PA imaging can be used to quantify tissue oxygen saturation based on the absorbance spectrum differences between hemoglobin and deoxyhemoglobin. Experimental Design: A new bi-modal PA/US system for small animal imaging was employed to test feasibility and reliability of dual-wavelength PA for measuring relative tissue oxygenation. Murine models of pancreatic and colon cancer were imaged, and differences in tissue oxygenation were compared to immunohistochemistry for hypoxia in the corresponding tissue regions. Results: Functional studies proved feasibility and reliability of oxygenation detection in murine tissue in vivo. Tumor models exhibited different levels of hypoxia in localized regions, which positively correlated with immunohistochemical staining for hypoxia. Contrast-enhanced imaging yielded complementary information on tissue perfusion using the same system. Conclusion: Bimodal PA/US imaging can be utilized to reliably detect hypoxic tumor regions in murine tumor models, thus providing the possibility to collect anatomical and functional information on tumor growth and treatment response live in longitudinal preclinical studies.

Epidermal and craniofacial defects in mice overexpressing Klf5 in the basal layer of the epidermis

Krüppel-like factor5 (Klf5) is a zinc-finger transcription factor normally expressed in the skin. Here, we show that overexpression of Klf5 in the basal layer of the epidermis during embryogenesis affects epidermal development and disrupts epithelial-mesenchymal interactions necessary for skin adnexae formation as well as craniofacial morphogenesis. The transgenic mice exhibited exencephaly, craniofacial defects, persistent abdominal herniation and ectodermal dysplasia. Moreover, the epidermis was hypoplastic and underwent abnormal differentiation with expression of keratin8, a marker for single-layered epithelia, in the stratified epidermis. Correspondingly, we observed a downregulation of ΔNp63 expression in the skin. Overexpression of Klf5 in adult mice led to hyperkeratosis, follicle occlusion and epidermal erosions. Further, we observed decrease and even loss of the stem cell population of bulge keratinocytes, as characterized by the expression pattern of α6 integrin and CD34 markers. Our data suggest a new role of Klf5 as a modulator of p63 expression and the differentiation program of epidermal cells relevant for regenerative potential of the epidermis and epithelial-mesenchymal interactions.

Antipsychotic Drugs Regulate Hedgehog Signaling by Modulation of 7-Dehydrocholesterol Reductase Levels

Recently we identified GANT61, a small-molecule antagonist of Gli transcription factors, which are the final effectors of the mammalian Hedgehog (HH) signaling pathway. Here we describe a diamine substructure of GANT61 that carries the biological activity and show that this part of the molecule is structurally related to trans-1,4-bis(2-chlorobenzaminomethyl)cyclohexane dihydrochloride (AY9944), an inhibitor of the enzymatic activity and transcriptional inducer of 7-dehydrocholesterol-reductase (Dhcr7, EC 1.3.1.21). Treatment of cells with the GANT61 diamine, AY9944, or overexpression of DHCR7 results in the attenuation of Smoothened-dependent and -independent HH signaling. Whereas GANT61 function is independent of Dhcr7, AY9944 does require up-regulation of endogenous Dhcr7. In line with these findings, Dhcr7-modulating antipsychotic (clozapine, chlorpromazine, haloperidol) and antidepressant (imipramine) drugs regulate HH signaling in vitro and in vivo. Modulation of HH signaling may represent a hitherto undiscovered biological (side) effect of therapeutics used to treat schizophrenia and depression.

Targeted Expression of GLI1 in the Mammary Gland Disrupts Pregnancy-induced Maturation and Causes Lactation Failure

The Hedgehog signaling pathway regulates the development and function of numerous tissues and when mis-regulated causes tumorigenesis. To assess the role of a deregulated Hedgehog signaling pathway in the mammary gland we targeted the expression of the Hedgehog effector protein, GLI1, to mammary epithelial cells using a bigenic inducible system. A constitutively active Hedgehog signaling pathway resulted with 100% penetrance in an undifferentiated mammary lobuloalveolar network during pregnancy. GLI1-expressing transgenic females were unable to lactate and milk protein gene expression was essentially absent. The inability to lactate was permanent and independent of continued GLI1 transgene expression. An increased expression of the GLI1 response gene Snail coupled to reduced expression of E-cadherin and STAT5 in the transgenic mammary gland provides a likely molecular explanation, underlying the observed phenotypic changes. In addition, remodeling of the mammary gland after parturition was impaired and expression of GLI1 was associated with accumulation of cellular debris in the mammary ducts during involution, indicating a defect in the clearance of dead cells. Areas with highly proliferative epithelial cells were observed in mammary glands with induced expression of GLI1. Within such areas an increased frequency of cells expressing nuclear Cyclin D1 was observed. Taken together the data support the notion that correct regulation of Hedgehog signaling within the epithelial cell compartment is critical for pregnancy-induced mammary gland development and remodeling.

Phorbol esters inhibit the Hedgehog signalling pathway downstream of Suppressor of Fused, but upstream of Gli

The developmentally important Hedgehog (Hh) signal transduction pathway, which has recently been implicated in several forms of cancer, is subject to regulation by several protein kinases. Here, we address the role of protein kinase Cδ in pathway inhibition and show that cellular depletion or pharmacological inhibition of this kinase isoform results in a blockade of signalling between Suppressor of Fused and the Gli transcription factors. We further provide evidence that the observed pathway inhibition is independent of primary cilia and the mitogen-activated protein kinase kinase (Mek1) kinase. These findings allowed for the rapid dissection of downstream Hh pathway activation mechanisms in human tumour cells and demonstrate a surprising variation in how cells can activate signalling in a ligand- and receptor-independent manner.

Stromal Hedgehog signalling is downregulated in colon cancer and its restoration restrains tumour growth

A role for Hedgehog (Hh) signalling in the development of colorectal cancer (CRC) has been proposed. In CRC and other solid tumours, Hh ligands are upregulated; however, a specific Hh antagonist provided no benefit in a clinical trial. Here we use Hh reporter mice to show that downstream Hh activity is unexpectedly diminished in a mouse model of colitis-associated colon cancer, and that downstream Hh signalling is restricted to the stroma. Functionally, stroma-specific Hh activation in mice markedly reduces the tumour load and blocks progression of advanced neoplasms, partly via the modulation of BMP signalling and restriction of the colonic stem cell signature. By contrast, attenuated Hh signalling accelerates colonic tumourigenesis. In human CRC, downstream Hh activity is similarly reduced and canonical Hh signalling remains predominantly paracrine. Our results suggest that diminished downstream Hh signalling enhances CRC development, and that stromal Hh activation can act as a colonic tumour suppressor.