Abstract Introduction: MiRNAs are now recognized as major players in cancer. MiR-148a has been shown to be a tumor suppressor in the context of several cancers including prostate, gastric, and lung but a study investigating its role in bladder cancer has never been performed. Here we sought to characterize the expression and role of miR-148a in bladder cancer. Methods: Real time PCR was used to assess the levels of miR-148a across several bladder cancer cell lines. MiR-148a mimics were employed in T24 and UM-UC-3 cells to assess proliferation by CCK-8 proliferation assay, colony formation assay, and flow cytometry. Mimics were also used to assess apoptosis by immunoblot and annexin-V staining. SiRNA was used to knock-down DNMT1 to phenocopy miR-148a treatment. CCK-8 proliferation assays and crystal violet staining were used to assess the effects of miR-148a added to either cisplatin or doxorubicin on T24 and UM-UC-3 cells. Results: All bladder cancer cell lines had significantly reduced expression of miR-148a than that of a control immortalized urothelial cell line. MiR-148a mimic treatment of T24 and UM-UC-3 cells lead to large decreases in viability. Flow cytometry demonstrated an S-phase arrest in miR-148a mimic treated cells versus control oligo treated cells. Immunoblots for cleaved caspase-3 and cleaved PARP along with annexin-V staining showed strong induction of apoptosis in miR-148a mimic treated cells. MiR-148a was shown to target DNMT1 and DNMT1 knock-down was able to phenocopy results from miR-148a mimic treatment suggesting DNMT1 is downstream of miR-148a. Finally, we found that miR-148a can be used with either cisplatin or doxorubicin for an additive effect in inducing apoptosis. Conclusions: Our work demonstrates for the first time that miR-148a plays a tumor suppressive role in bladder cancer and suggests it could be used as a novel therapeutic agent either alone or in conjunction with chemotherapeutics. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B226. Citation Format: Alan P. Lombard, Steve Libertini, Nicole Costanzo, Rebecca Lim, Maria Mudryj. MiR-148a dependent apoptosis of bladder cancer cells is mediated by the epigenetic modifier DNMT1. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B226.