Activating mutations in NOTCH1 are present in over 50% of T-cell acute lymphoblastic leukemias (T-ALL). Consequently, inhibition of NOTCH1 signaling with small molecule γ-secretase inhibitors (GSIs) has been proposed as a targeted therapy for this disease. However, GSIs fail to induce robust apoptosis in T-ALL cells and their clinical application has been limited by the development of severe gastrointestinal toxicity (mucous diarrhea and goblet cell metaplasia) resulting from systemic inhibition of NOTCH signaling. Here we show that combination therapy with GSIs plus glucocorticoids can reverse both glucocorticoid resistance in T-ALL and the occurrence of GSI-induced gut toxicity in vivo. The development of glucocorticoid resistance in T-ALL has been associated with the loss of a positive auto-regulatory transcriptional feedback loop required for glucocorticoid induced apoptosis. Using gene expression profiling and ChIP-on-chip analysis we demonstrated that HES1, a transcriptional repressor controlled by NOTCH1, inhibits specific regulatory elements in the glucocorticoid receptor promoter required for effective glucocorticoid receptor autoupregulation and glucocorticoid induced cell death. Consequently, downregulation of HES1 via inhibition of NOTCH1 signaling with GSIs or HES1 inactivation via shRNA knockdown restores glucocorticoid receptor autoupregulation and reverses glucocorticoid resistance in T-ALL cell lines and primary patient samples. Consistent with these results, combination therapy with dexamethasone and GSIs was highly effective in a mouse xenograft model of glucocorticoid resistant T-ALL. Surprisingly, these studies demonstrated not only improved antileukemic responses in mice treated with dexamethasone plus a GSI compared with dexamethasone or placebo treated controls (P<0.05), but also decreased mortality associated with GSI-induced gut toxicity in this model (P<0.01). These results suggested that glucocorticoids may have an enteroprotective effect against GSI-induced gut toxicity. Detailed histological analysis demonstrated a block of proliferation and increased goblet cell numbers in the intestine of GSI treated animals and a moderate increase in proliferation accompanied by increased Paneth cell numbers in the dexamethasone treated group. Notably, cotreatment with dexamethasone plus a GSI completely reversed the induction of goblet cell metaplasia typically induced by systemic inhibition of NOTCH signaling with GSIs. Likewise, dexamethasone also protected RBPJ-K conditional knockout mice, a genetic model of loss of NOTCH signaling, from developing goblet cell metaplasia in the gut. Interestingly, gene expression profiling analysis showed a marked upregulation of cyclin D2 expression in the gut of glucocorticoid treated mice. Moreover, dexamethasone treatment failed to protect Ccnd2 knockout animals from developing GSI-induced metaplasia, demonstrating a critical role of Ccnd2 upregulation in the protective effects of dexamethasone against GSI-induced gut toxicity. Overall, these results uncover a NOTCH1-HES1-glucocorticoid receptor regulatory axis in glucocorticoid resistant T-ALL and demonstrate an unanticipated protective effect of glucocorticoids against GSI-induced gut toxicity. Our studies warrant the clinical testing of glucocorticoids plus GSIs in the treatment of glucocorticoid-resistant T-ALL.

Disclosures: No relevant conflicts of interest to declare.

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