Purpose: Gemcitabine-based chemotherapy regimens are first-line for several advanced cancers. Because of better tolerability, gemcitabine þ cisplatin is a preferred neoadjuvant, adjuvant, and/or palliative chemotherapy regimen for advanced bladder cancer. Nevertheless, predicting treatment failure and overcoming resistance remain unmet clinical needs. We discovered that splice variant (V1) of HYAL-4 is a first-in-class eukaryotic chondroitinase (Chase), and CD44 is its major substrate. V1 is upregulated in bladder cancer and drives a malignant phenotype. In this study, we investigated whether V1 drives chemotherapy resistance. Experimental Design: V1 expression was measured in muscle-invasive bladder cancer (MIBC) specimens by qRT-PCR and IHC. HYAL-4 wild-type (Wt) and V1 were stably expressed or silenced in normal urothelial and three bladder cancer cell lines. Transfectants were analyzed for chemoresistance and associated mechanism in preclinical models. Results: V1 levels in MIBC specimens of patients who developed metastasis, predicted response to gemcitabine þ cisplatin adjuvant/salvage treatment and disease-specific mortality. V1-expressing bladder cells were resistant to gemcitabine but not to cisplatin. V1 expression neither affected gemcitabine influx nor the drug-efflux transporters. Instead, V1 increased gemcitabine metabolism and subsequent efflux of difluorodeoxyuridine, by upregulating cytidine deaminase (CDA) expression through increased CD44–JAK2/STAT3 signaling. CDA inhibitor tetrahydrouridine resensitized V1-expressing cells to gemcitabine. While gemcitabine (25–50 mg/kg) inhibited bladder cancer xenograft growth, V1-expressing tumors were resistant. Low-dose combination of gemcitabine and tetrahydrouridine abrogated the growth of V1 tumors with minimal toxicity. Conclusions: V1/Chase drives gemcitabine resistance and potentially predicts gemcitabine þ cisplatin failure. CDA inhibition resensitizes V1-expressing tumors to gemcitabine. Because several chemotherapy regimens include gemcitabine, our study could have broad significance.
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