MYC/miR-17-92 interplay maintains B-lymphoma cell homeostasis

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Cellular homeostasis, the intrinsic property of the cell to maintain the stability of the internal environment in spite of changing external conditions, relies on a complex network of intertwining regulatory circuits. When the internal equilibrium is broken, cells undergo uncontrolled proliferation or programmed cell death.

The oncogene c-MYC (hereafter MYC) is a major gatekeeper of the equilibrium between cell death and proliferation and the deregulation of either its expression or function is observed in approximately 70% human cancers. The tight control of MYC is therefore crucial for cell homeostasis and survival.

MicroRNA (miRNAs) have also emerged as important regulators of cellular homeostasis. At the molecular level these small non-coding RNAs dampen gene expression post-transcriptionally, by either eliciting degradation or inhibiting translation of several target mRNAs in parallel. This molecular activity results in the regulation of key cellular processes, such as growth, proliferation, metabolism and apoptosis. Hence, miRNAs emerge as key players in the regulatory circuits that maintain complex gene expression programs, thus ensuring cellular homeostasis.

The miR-17-92 cluster is paradigmatic in this sense, having been described as a regulatory hub at the crossroad between cellular proliferation and cell death.^1,2 miR-17-92 is transcribed as a polycistron, which is subsequently processed into 7 mature miRNAs: miR-17-3p and -5p, miR-18a, miR-19a, miR-19b, miR-20a and miR-92a. Both, oncogenic and tumor-suppressor roles have been ascribed to this cluster, depending on the different contexts.³ Interestingly, the oncogenic role of the cluster has been correlated to distinct functional interplays with MYC. For instance, during MYC-driven B cell lymphomagenesis miR-17-19b (a truncated version of miR-17-92 cluster) was shown to synergize with MYC to reduce tumor latency and the oncogenic activity was attributed to miR-19-mediated down-regulation of the pro-apoptotic factor Pten. Following the description of this molecular synergy between MYC and miR-19, Olive and collaborators showed that another member of the cluster, miR-92, can inhibit MYC degradation indirectly, thus stimulating uncontrolled cell proliferation, while also increasing programmed cell death.⁴ Therefore, distinct mature miRNAs originated from the same cluster can affect MYC functions through different molecular mechanisms, impinging on the balance between proliferation and apoptosis.

In our recent study, we further expanded the knowledge on the composite functional interplay linking MYC and miR-17-92, emphasizing the importance of their tight equilibrium for the maintenance of MYC-driven, full-blown lymphoma.⁵ At this stage of tumor progression, the transcriptome has been pervasively shaped by high levels of MYC: in particular, we observed that, in addition to different transcriptional programs being active, extensive 3′ UTR (untranslated region) shortening contributes to change the mRNA landscape. This observation, combined with the fact that miRNAs bind to the 3′ UTRs of their target genes, led us to hypothesize that the transcriptome plasticity, occurring in the transition from early- to late- tumor stage, can lead to a substantial modification of the pool of targets for any given miRNA, and thus modify the MYC/miR-17-92 functional interplay.

We explored this hypothesis by carrying out a comprehensive miR-17-19b target deconvolution analysis upon miR-17-19b overexpression in established B cell lymphoma. We employed a systems biology approach that combines in silico miRNA-target prediction with transcriptomics, proteomics and in-depth 3′ UTR analysis. We identified over one hundred miR-17-19b targets, most of which were novel. Surprisingly, Pten - the major target of the cluster during lymphomagenesis- was not down regulated in full blown tumor, as a consequence of its 3′ UTR shortening. However, among the experimental miR-17-19b targets we identified 36 genes generally involved in the regulation of cell cycle and apoptosis. While some were already annotated as targets of different miRNAs from the cluster in distinct cell types and tissues (such as ATM, Stat3 and Cyclin D1), most of them were identified as miR-17-19b targets for the first time. Interestingly, we found Checkpoint kinase 2 (Chek2) as a novel miR-17/20 target. Further in-depth investigation revealed that miR-17/20 negatively regulates MYC translation through the downregulation of Chek2, which alters the HuR/RISC/MYC-mRNA axis. In line with these results, we observed that miR-17-19b induction in established lymphoma causes a block of the cell cycle and increased apoptosis, which, in turn, lead to a less aggressive phenotype, both in vitro and in vivo.

Overall our study sheds new lights on the plasticity of miRNA activity during cancer progression. Specifically, we demonstrated that the functional interplay between miR-17-92 and the oncogene MYC is highly dynamic, following the transcriptional changes between early and late stage B cell lymphoma. While during lymphoma onset the cluster synergizes with MYC by stimulating proliferation and inhibiting apoptosis, it functions as tumor-suppressor in full-blown tumors, where it dampens both MYC level and functions. Yet, by keeping MYC level under control, miR-17-92 cluster maintains the correct equilibrium between apoptosis and proliferation that guarantees cancer cell homeostasis and sustains unlimited tumor growth.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.