As part of the Reproducibility Project: Cancer Biology, we published a Registered Report (Li et al., 2015), that described how we intended to replicate selected experiments from the paper 'The microRNA miR-34a inhibits prostate cancer stem cells and metastasis by directly repressing CD44' (Liu et al., 2011). Here we report the results. We found the microRNA, miR-34a, was expressed at twice the level in CD44+ prostate cancer cells purified from xenograft tumors (LAPC4 cells) compared to CD44- LAPC4 cells, whereas the original study reported miR-34a was underexpressed in CD44+ LAPC4 cells (Figure 1B; Liu et al., 2011). When LAPC4 cells engineered to express miR-34a were injected into mice, we did not observe changes in tumor growth or CD44 expression; however, unexpectedly miR-34a expression was lost in vivo. In the original study, LAPC4 cells expressing miR-34a had a statistically significant reduction in tumor regeneration and reduced CD44 expression compared to control (Figure 4A and Supplemental Figures 4A,B and 5C; Liu et al., 2011). Furthermore, when we tested if miR-34a regulated CD44 through binding sites in the 3'UTR we did not find a statistically significant difference, whereas the original study reported miR-34a decreased CD44 expression that was partially abrogated by mutation of the binding sites in the CD44 3'UTR (Figure 4D; Liu et al., 2011). Finally, where possible, we report meta-analyses for each result.

The Reproducibility Project: Cancer Biology seeks to address growing concerns about reproducibility in scientific research by conducting replications of selected experiments from a number of high-profile papers in the field of cancer biology. The papers, which were published between 2010 and 2012, were selected on the basis of citations and Altimetric scores (Errington et al., 2014). This Registered report describes the proposed replication plan of key experiments from 'The microRNA miR-34a inhibits prostate cancer stem cells and metastasis by directly repressing CD44' by Liu and colleagues published in Nature Medicine in 2011 (Liu et al., 2011). Liu and colleagues first demonstrated that miR-34a levels were reduced in CD44+ prostate cancer cells (Figure 1B). They then showed that xenograft tumors from cells expressing exogenous miR-34a were smaller in size than control tumors (Supplemental Figure 5C). Tumors with exogenous miR-34a showed reduced levels of CD44 expression (Figure 4A), and mutation of two putative miR-34a binding sites in the CD33 3' UTR partially abrogated signal repression in a luciferase assay (Figure 4D). The Reproducibility Project: Cancer Biology is a collaboration between the Center for Open Science and Science Exchange, and the results of the replications will be published by eLife.