A synthetic combinatorial approach to disabling deviant Hedgehog signaling

Abstract

Mutations in components of the Hedgehog (HH) signal transduction pathway are found in the majority of basal cell carcinoma (BCC) and medulloblastoma incidents. Cancerous cells with intrinsic or acquired resistance to antagonists targeting the seven transmembrane effector Smoothened (SMO) frequently invoke alternative mechanisms for maintaining deviant activity of the GLI DNA binding proteins. Here we introduce a chemical agent that simultaneously achieves inhibition of SMO and GLI activity by direct targeting of the SMO heptahelical domain and the GLI-modifying enzymes belonging to the histone deacetylase (HDAC) family. We demonstrate a small molecule SMO-HDAC antagonist (IHR-SAHA) retains inhibitory activity for GLI transcription induced by SMO-dependent and -independent mechanisms frequently associated with cancer biogenesis. Synthetic combinatorial therapeutic agents such as IHR-SAHA that a priori disable cancer drivers and anticipated mechanisms of drug resistance could extend the duration of disease remission, and provide an alternative clinical development path for realizing combinatorial therapy modalities.Cellular response to the secreted HH proteins is initiated upon their binding to the multi-pass protein Patched 1 (PTCH1), a suppressor of the seven transmembrane receptor Smoothened (SMO) 1 . Activated SMO promotes SUFU disassociation from the GLI DNA binding proteins thus licensing them for gene transcriptional activation 2,3 . Deviant HH pathway activity associated with several cancers including medulloblastoma (MB) and basal cell carcinoma (BCC) is commonly induced by mutations in PTCH1 4,5 . SMO antagonists that are FDA-approved for the management of metastatic BCC (Vismodegib and Sonidegib) are able to restore homeostatic levels of signaling and blunt tumor growth 6 .Despite an impressive initial response in some metastatic BCC patients, durable tumor growth suppression by SMO antagonists has been elusive and few treatment options that are available to patients after progression. Yet, the majority of the tumors that re-emerge are likely to be still dependent upon GLI transcriptional activity as determined by the appearance of mutations in SMO that prevent drug binding 7-11 , kinase-dependent mechanisms promoting sustained GLI activity in the absence of SMO input 12,13 , or GLI2 gene amplification 8,14 . Thus, agents that disrupt GLI activity have broader indications than those targeting SMO in HH-associated cancers particularly in cases of drug resistance.A number of strategies for disrupting GLI activity have been evaluated including those that promote GLI protein turn-over such as arsenic trioxide 15,16 or GANT61 17 , 18 , or have limited mechanistic accounting 19 . The activity of GLI proteins also appear to be blunted by their acetylation thus offering opportunities for disabling GLI activity by blocking GLI deacetylases 20 . This strategy appears to be useful in blocking the growth of medulloblastomas in preclinical models of the disease 21 .We had previously described a symmetric molecule w...

Implication for Cancer Stem Cells in Solid Cancer Chemo-Resistance: Promising Therapeutic Strategies Based on the Use of HDAC Inhibitors

“…Finally, as VPA and TSA, vorinostat modulates SMAD4 localization and Nocth3 expression [64,65], but Fan and colleagues also showed that a small molecule SMO-HDAC antagonist was able to retain inhibitory activity for Gli transcription induced in SMO-dependent and SMO-independent ways [66]. Furthermore, TSA and domatinostat, two specific class I and II HDAC inhibitors, switched off Gli signaling by downregulating a transcriptional factor of FoxM1 [67] in the first case and by inhibition of HDAC1/2/3 in the second case [68].…”

Towards Precision Oncology: The Role of Smoothened and Its Variants in Cancer

“…For example, itraconazole is believed to be a good candidate for second generation therapy against Hh-driven cancers [ 108 ]. Additionally, combinatorial approaches targeting both Smo and downstream signalling molecules have been recognised as promising in cancer treatment [ 109 ]. Targeting regulatory mechanisms of Smo presents another therapeutic approach for acquired resistance.…”

Inhibition of EZH2 by chidamide exerts antileukemia activity and increases chemosensitivity through Smo/Gli-1 pathway in acute myeloid leukemia

“…A recent study shows that the activity of Hedgehog pathway is largely independent of Smo and therefore inherently resistant to Smo inhibitors in AML, and hypomethylating agent improves glasdegib sensitivity by increasing the level of Gli-3 repressor and modulation of Gli-1 [ 71 ]. HDAC inhibitors abrogate Smo-dependent and Smo-independent GLI activation and Hedgehog-targeted gene expression and overcome drug resistance to Smo inhibitors in cancer cells [ 72 , 73 ]. Combinatorial inhibition of HDACs and Hedgehog pathway synergistically suppresses tumor growth and homologous recombination in aerodigestive cancers, and a dual-targeted inhibitor is capable of effectively overcoming the Smo inhibitor resistance conferred by Smo mutations [ 74 , 75 ].…”