Novel mechanisms of microRNA-mediated regulation of key TGFbeta effector CTGF in glioblastoma multiforme

The TGFbeta signaling pathway is known to directly induce transcription of connective tissue growth factor (CTGF, CCN2) with profound and complex consequences in human cancers. While TGFbeta and CTGF have varying and often conflicting roles in tumorigenesis, depending on cell context, both are thought to enhance malignancy of the brain cancer glioblastoma multiforme (GBM). We and others have previously reported that the effects of TGFbeta pathway activation can be abrogated via regulation of multiple components by the classically oncogenic microRNA cluster miR-17∼92. Described herein, we have discovered two novel mechanisms, both dependent on the TGFbeta pathway, whereby microRNAs modulate CTGF in GBM. In the first study, we demonstrate that miR-18a, a member of the miR-17~92 cluster, inhibits CTGF both directly and indirectly. The latter is achieved via targeting of Smad3, a vital component of the TGFbeta pathway. The result is profound downregulation of CTGF at the mRNA and protein levels. Additionally, we discovered an anti-correlation between expression of miR-18 and TGFbeta target genes in primary GBM, which has significant implications in patient survival. This surprising result suggests the classical "oncomiR", miR-17~92, instead functions as a tumor suppressor in GBM. In the second study, we demonstrate that miR-133 regulates CTGF directly, but is only expressed in the absence of TGFbeta signaling. Thus, miR-133 augments the effects of TGFbeta on CTGF expression by inhibiting CTGF only when TGFbeta signaling is inactive. In contrast, miR-133-mediated repression of CTGF is alleviated when TGFbeta signaling is active. The effective result is TGFbeta-dependent post-transcriptional modification of CTGF expression and sustained induction of CTGF. While we discovered these novel mechanisms of microRNA-mediated regulation in the context of a human GBM cell line, similar axes of regulation may be important in other cell types with alternative microRNAs, pathways, and gene targets. Overall, the data presented herein highlight the complexity of microRNA function and importance of cell context.