Epidermal Growth Factor Receptor Signaling Synergizes with Hedgehog/GLI in Oncogenic Transformation via Activation of the MEK/ERK/JUN Pathway

Department of Molecular Biology, University of Salzburg, Salzburg, Austria.
Cancer Research (Impact Factor: 9.33). 03/2009; 69(4):1284-92. DOI: 10.1158/0008-5472.CAN-08-2331
Source: PubMed


Persistent activation of the Hedgehog (HH)/GLI signaling pathway has been implicated in the development of a number of human cancers. The GLI zinc finger transcription factors act at the end of the HH signaling cascade to control gene expression, and recent studies have shown that the activity of GLI proteins can be additionally modified by integration of distinct signals, such as the MEK/extracellular signal-regulated kinase (ERK) and phosphinositide-3 kinase (PI3K)/AKT pathway. However, little is known about the identity of the upstream activators of these HH/GLI interacting signaling pathways in cancer. Here, we provide evidence that integration of the HH/GLI and epidermal growth factor receptor (EGFR) pathway synergistically induces oncogenic transformation, which depends on EGFR-mediated activation of the RAS/RAF/MEK/ERK but not of the PI3K/AKT pathway. EGFR/MEK/ERK signaling induces JUN/activator protein 1 activation, which is essential for oncogenic transformation, in combination with the GLI activator forms GLI1 and GLI2. Furthermore, pharmacologic inhibition of EGFR and HH/GLI efficiently reduces growth of basal cell carcinoma (BCC) cell lines derived from mice with activated HH/GLI signaling. The results identify the synergistic integration of GLI activator function and EGFR signaling as a critical step in oncogenic transformation and provide a molecular basis for therapeutic opportunities relying on combined inhibition of the HH/GLI and EGFR/MEK/ERK/JUN pathway in BCC.

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    • "Notably, class III genes, also referred to as HH-EGFR target genes or cooperation response genes, contain functional GLI binding sites in their promoters, suggesting that signal integration occurs at the level of HH-EGFR target gene promoters [92]. It is important to note that signal cooperation is a selective process as classical HH-GLI target genes such as PTCH1 or HHIP are not affected by parallel EGF signaling in keratinocytes [90,92,93]. "
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    ABSTRACT: Canonical Hedgehog (HH) signaling leads to the regulation of the GLI code: the sum of all positive and negative functions of all GLI proteins. In humans, the three GLI factors encode context-dependent activities with GLI1 being mostly an activator and GLI3 often a repressor. Modulation of GLI activity occurs at multiple levels, including by co-factors and by direct modification of GLI structure. Surprisingly, the GLI proteins, and thus the GLI code, is also regulated by multiple inputs beyond HH signaling. In normal development and homeostasis these include a multitude of signaling pathways that regulate proto-oncogenes, which boost positive GLI function, as well as tumor suppressors, which restrict positive GLI activity. In cancer, the acquisition of oncogenic mutations and the loss of tumor suppressors - the oncogenic load - regulates the GLI code towards progressively more activating states. The fine and reversible balance of GLI activating GLIA and GLI repressing GLIR states is lost in cancer. Here, the acquisition of GLIA levels above a given threshold is predicted to lead to advanced malignant stages. In this review we highlight the concepts of the GLI code, the oncogenic load, the context-dependency of GLI action, and different modes of signaling integration such as that of HH and EGF. Targeting the GLI code directly or indirectly promises therapeutic benefits beyond the direct blockade of individual pathways.
    Full-text · Article · Sep 2014 · Seminars in Cell and Developmental Biology
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    • "During the last decade, it has become obvious that progression and severity of malignant diseases is often not caused by a single genetic aberration or deregulation of a single signaling pathway, but actually requires the cooperation of oncogenic-signaling pathways in cancer cells. For instance, Hedgehog (HH)/GLI and EGF-driven signaling can synergize and promote events, such as neural stem cell proliferation, as well as tumor initiation and progression [1] [2] [3] [4] [5] [6] [7]. Deregulation of at least one of the two pathways has been implicated in about one-third of all cancers, and frequently, both pathways are found aberrantly activated in the same tumor. "
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    ABSTRACT: Aberrant activation of Hedgehog (HH) signaling has been identified as a key etiologic factor in many human malignancies. Signal strength, target gene specificity, and oncogenic activity of HH signaling depend profoundly on interactions with other pathways, such as epidermal growth factor receptor-mediated signaling, which has been shown to cooperate with HH/GLI in basal cell carcinoma and pancreatic cancer. Our experimental data demonstrated that the Daoy human medulloblastoma cell line possesses a fully inducible endogenous HH pathway. Treatment of Daoy cells with Sonic HH or Smoothened agonist induced expression of GLI1 protein and simultaneously prevented the processing of GLI3 to its repressor form. To study interactions between HH- and EGF-induced signaling in greater detail, time-resolved measurements were carried out and analyzed at the transcriptomic and proteomic levels. The Daoy cells responded to the HH/EGF co-treatment by downregulating GLI1, PTCH, and HHIP at the transcript level; this was also observed when Amphiregulin (AREG) was used instead of EGF. We identified a novel crosstalk mechanism whereby EGFR signaling silences proteins acting as negative regulators of HH signaling, as AKT- and ERK-signaling independent process. EGFR/HH signaling maintained high GLI1 protein levels which contrasted the GLI1 downregulation on the transcript level. Conversely, a high-level synergism was also observed, due to a strong and significant upregulation of numerous canonical EGF-targets with putative tumor-promoting properties such as MMP7, VEGFA, and IL-8. In conclusion, synergistic effects between EGFR and HH signaling can selectively induce a switch from a canonical HH/GLI profile to a modulated specific target gene profile. This suggests that there are more wide-spread, yet context-dependent interactions, between HH/GLI and growth factor receptor signaling in human malignancies.
    Full-text · Article · Jun 2013 · PLoS ONE
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    • "These findings indicate that JNK and c-Jun are important for the oncogenic activity of Hedgehog/Gli proteins in BCC. Moreover, the Hedgehog/Gli signaling pathway is found to act in synergy with the epidermal growth factor receptor signaling pathway to drive oncogenesis of a mouse BCC cell line [65]. In this case, Gli-driven tumorigenesis requires c-Jun activation by MEK/ERK but not JNK. "
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    ABSTRACT: The c-Jun N-terminal Kinases (JNK), along with Erk and p38, constitute the principle members of the mitogen-activated protein kinase (MAPK) family. JNK functions primarily through AP1 family transcription factors to regulate a plethora of cellular processes, including cell proliferation, differentiation, survival and migration. It also cross-talks and integrates with other signaling pathways in a cell context-specific and cell type-specific manner. The current views of JNK function in various skin cancers and the need of developing JNK subunit-specific inhibitors for cancer type-specific applications have been summarized in this review.
    Full-text · Article · Dec 2012 · American Journal of Cancer Research
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