HEDGEHOG/GLI-E2F1 axis modulates iASPP expression and function and regulates melanoma cell growth

Abstract

HEDGEHOG (HH) signaling is a key regulator of tissue development and its aberrant activation is involved in several cancer types, including melanoma. We and others have shown a reciprocal cross talk between HH signaling and p53, whose function is often impaired in melanoma. Here we present evidence that both GLI1 and GLI2, the final effectors of HH signaling, regulate the transcription factor E2F1 in melanoma cells, by binding to a functional non-canonical GLI consensus sequence. Consistently, we find a significant correlation between E2F1 and PATCHED1 (PTCH1), GLI1 and GLI2 expression in human melanomas. Functionally, we find that E2F1 is a crucial mediator of HH signaling and it is required for melanoma cell proliferation and xenograft growth induced by activation of the HH pathway. Interestingly, we present evidence that the HH/GLI-E2F1 axis positively modulates the inhibitor of apoptosis-stimulating protein of p53 (iASPP) at multiple levels. HH activation induces iASPP expression through E2F1, which directly binds to iASPP promoter. HH pathway also contributes to iASPP function, by the induction of Cyclin B1 and by the E2F1-dependent regulation of CDK1, which are both involved in iASPP activation. Our data show that activation of HH signaling enhances proliferation in presence of E2F1 and promotes apoptosis in its absence or upon CDK1 inhibition, suggesting that E2F1/iASPP dictates the outcome of HH signaling in melanoma. Together, these findings identify a novel HH/GLI-E2F1-iASPP axis that regulates melanoma cell growth and survival, providing an additional mechanism through which HH signaling restrains p53 proapoptotic function.

KLF4 is regulated by RAS/RAF/MEK/ERK signaling through E2F1 and promotes melanoma cell growth

“…44 Ablation of E2F1 led to a reduction in KLF4 protein levels in A375 cells (Figure 3d). The involvement of E2F1 in the regulation of KLF4 was further confirmed by transiently overexpression of the former in melanoma cells treated with CI-1040.…”

“…43, 44 This finding led us to hypothesize that the modulation of KLF4 by the RAS/RAF/MEK/ERK signaling might be mediated by E2F1. To begin to investigate this hypothesis, we tested the effect of MEK1/2 inhibition on E2F1 expression.…”

Down-Regulation of SOX2 Underlies the Inhibitory Effects of the Triphenylmethane Gentian Violet on Melanoma Cell Self-Renewal and Survival

“…Western blot was performed as already described (Pandolfi et al, 2015). See Supplementary Materials for antibodies.…”

Comparative oncogenomics identifies tyrosine kinase FES as a tumor suppressor in melanoma

“…Consistent with this possibility, several components of Hh signaling pathway, including the well-established negative regulators Sufu and Kctd11, are among the 30 melanoma TSGs we identified. To date, only few studies have tentatively implicated Hh signaling in melanoma (62)(63)(64). Our work therefore warrants further dissection of the role of Hh signaling in melanomagenesis and leads us to entertain the possibility that Hh inhibitors should be considered in the context of (combination) melanoma targeted therapies.…”