GLI1 Inhibitor SRI-38832 Attenuates Chemotherapeutic Resistance by Downregulating NBS1 Transcription in BRAFV600E Colorectal Cancer

Abstract

Resistance to radiation and chemotherapy in colorectal cancer (CRC) patients contribute significantly to refractory disease and disease progression. Herein, we provide mechanistic rationale for acquired or inherent chemotherapeutic resistance to the anti-tumor effects of 5-fluorouracil (5-FU) that is linked to oncogenic GLI1 transcription activity and NBS1 overexpression. Patients with high levels of GLI1 also expressed high levels of NBS1. Non-canonical activation of GLI1 is driven through oncogenic pathways in CRC, like the BRAF V600E mutation. GLI1 was identified as a novel regulator of NBS1 and discovered that by knocking down GLI1 levels in vitro, diminished NBS1 expression, increased DNA damage/apoptosis, and re-sensitization of 5-FU resistant cancer to treatment was observed. Furthermore, a novel GLI1 inhibitor, SRI-38832, which exhibited pharmacokinetic properties suitable for in vivo testing, was identified. GLI1 inhibition in a murine BRAF V600E variant xenograft model of CRC resulted in the same down-regulation of NBS1 observed in vitro as well as significant reduction of tumor growth/burden. GLI1 inhibition could therefore be a therapeutic option for 5-FU resistant and BRAF V600E variant CRC patients.

GLI1: A Therapeutic Target for Cancer

“…For instance, Li discovered that SMO expression was not statistically correlated with CRC-specific or overall survival; the same results were reported by Stefanius, where no correlation between Hh and colorectal serrated adenocarcinomas was observed (62,63). Our lab, like many others, observed a positive correlation between GLI1 expression and disease severity (64). We also demonstrated that both GLI1 and one of its transcriptional targets, NBS1, negatively correlate with CRC patient 5-year survival, driving chemotherapeutic resistance by overcoming FOLFOX induced DNA damage (standard of care treatment).…”

“…Overexpression of any NBS1 vector rescued ~25% of cells from apoptosis mediated by GLI inhibition. Surprisingly, the overexpression of S343E, S343A, or total NBS1 was not statistically different from one another, indicating that total levels of NBS1, elevated by GLI1 transcription, rather than the phosphorylation status, were responsible for protection from GLI inhibition-induced apoptosis ( 64 ). Since GLI1 is not typically expressed by differentiated cells, targeting oncogenic expression of GLI1 would result in fewer off-target effects and provide a specific therapeutic strategy.…”

“…Using GANT61 as an initial scaffold, Southern Research has recently developed a novel GLI1 inhibitor (SRI-38832) that has better PK properties and has shown efficacy in vivo ( 64 ). Additionally, there are several promising compounds showing the biological activity of GLI inhibition ( 180 ), arsenic trioxide (ATO), originally approved by the FDA for the treatment of acute promyelocytic leulemia, has been shown to inhibit GLI proteins by binding to GLI proteins and enhancing degradation ( 181 ).…”

The Role of Smoothened-Dependent and -Independent Hedgehog Signaling Pathway in Tumorigenesis

“…Ongoing efforts are being made to develop more effective and stable GLI inhibitors for their use in clinical settings. Recently, a novel GLI inhibitor, SRI-38832, has shown not only an improved efficacy but also an enhanced bioavailability in a murine xenograft model, representing the first step toward the development of a clinically viable GLI inhibitor [ 241 ]. The improved efficacy and stable pharmacokinetic/bioavailability profiles of SRI-38832 make it a novel scaffold for hit-to-lead optimization, which, if successful, sets a new stage for treating Hh-dependent cancers in future clinical settings.…”

“…Additionally, vismodegib plus arsenic trioxide, in association with temozolomide, resulted in the marked inhibition of glioblastoma tumor growth in mice, while single-agent therapy yielded minimal efficacy [ 253 ]. Cotreatment of HT29 cells with GANT61 and 5-fluorouracil produced a synergistic and marked inhibitory effect compared to treatment with any of the agents alone [ 241 ].…”