Widespread mRNA 3′ UTR shortening through alternative polyadenylation promotes tumor growth in vivo. A prevailing hypothesis is that it induces proto-oncogene expression in cis through escaping microRNA-mediated repression. Here we report a surprising enrichment of 3′UTR shortening among transcripts that are predicted to act as competing-endogenous RNAs (ceRNAs) for tumor-suppressor genes. Our model-based analysis of the trans effect of 3′ UTR shortening (MAT3UTR) reveals a significant role in altering ceRNA expression. MAT3UTR predicts many trans-targets of 3′ UTR shortening, including PTEN, a crucial tumor-suppressor gene involved in ceRNA crosstalk with nine 3′UTR-shortening genes, including EPS15 and NFIA. Knockdown of NUDT21, a master 3′ UTR-shortening regulator, represses tumor-suppressor genes such as PHF6 and LARP1 in trans in a miRNA-dependent manner. Together, the results of our analysis suggest a major role of 3′ UTR shortening in repressing tumor-suppressor genes in trans by disrupting ceRNA crosstalk, rather than inducing proto-oncogenes in cis.