Abstract 1086: Vitamin D3 increases the response to cisplatin in bladder cancer through VDR and TAp73 pathway crosstalk

Abstract

Abstract Neoadjuvant cisplatin-based chemotherapy is used to treat muscle invasive bladder cancer (MIBC); however the relative 5-year survival of advanced disease is only 15%. The two major pitfalls of neoadjuvant chemotherapy are delaying cystectomy in patients with poor response to cisplatin, and the lack of available biomarkers to identify these patients. Previous studies in our lab showed that pretreatment of preclinical bladder cancer models with 1,25 dihydroxyvitamin D3 (1,25D3), the active metabolite of vitamin D3, enhances the apoptotic response to cisplatin. Mechanistic understanding can provide insight into potential markers for response to 1,25D3 and cisplatin combination therapy in patients. Greater than 50% of MIBCs harbor p53 mutations; however p53 status has not been linked to clinical response to cisplatin. We previously determined that TAp73, a pro-apoptotic p53 family member protein induced after DNA damage, is required for the response to 1,25D3 and cisplatin. In two bladder cancer cell lines with mutant p53, T24 and RT-112, we identified signaling crosstalk between the vitamin D receptor (VDR) and TAp73. Treatment with 1,25D3 and cisplatin, compared to either alone, results in greatest increase of TAp73 mRNA and protein, 2-fold (T24) and 15-fold (RT-112), as well as TAp73 transcriptional target BAX, 3-fold (T24), and 4-fold (RT-112). TAp73 and BAX induction is lost in cells transfected with VDR siRNA, indicating a requirement for VDR. Cells treated with 1,25D3 and cisplatin, compared to either alone, also show the greatest increase of VDR mRNA and protein, 3-fold (T24) and 4-fold (RT-112). CYP24A1, a VDR target gene, has the greatest induction after the combination treatment as well, with an increase of approx. 60-fold (T24), and 100-fold (RT-112). CYP24A1 induction requires TAp73, determined using TAp73 shRNA. These data demonstrate that VDR signaling is enhanced by cisplatin treatment, and TAp73 signaling is enhanced by 1,25D3. We expanded these findings in vivo by treating mice on a vitamin D3 sufficient diet (1,000 IU) or a vitamin D3 deficient diet (25 IU) with 5 mg/kg of cisplatin (IP, q.wk for 3 weeks). Initially, mice on both diets respond to cisplatin treatment, as evidence by a 30% reduction in tumor volume. After the second cycle of treatment, tumors in the deficient mice stopped responding and returned to their initial tumor volume within 24 days. In the sufficient mice, tumors continued to respond to cisplatin with a maximum reduction of 50%. This was maintained for approximately 40 days after treatment. Further study will determine the molecular response to confirm our in vitro mechanism. Our work suggests that vitamin D3 is important in and can increase the response to cisplatin, it also provides rationale for investigating TAp73 protein levels and serum vitamin D3 levels as potential markers to determine patients’ likelihood of responding to cisplatin. Citation Format: Brittany L. Bunch, Donald Trump, Candace S. Johnson. Vitamin D3 increases the response to cisplatin in bladder cancer through VDR and TAp73 pathway crosstalk [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1086. doi:10.1158/1538-7445.AM2017-1086