miR-99a/100∼125b tricistrons regulate hematopoietic stem and progenitor cell homeostasis by shifting the balance between TGFβ and Wnt signaling

Abstract

Although regulation of stem cell homeostasis by microRNAs (miRNAs) is well studied, it is unclear how individual miRNAs genomically encoded within an organized polycistron can interact to induce an integrated phenotype. miR-99a/100, let-7, and miR-125b paralogs are encoded in two tricistrons on human chromosomes 11 and 21. They are highly expressed in hematopoietic stem cells (HSCs) and acute megakaryoblastic leukemia (AMKL), an aggressive form of leukemia with poor prognosis. Here, we show that miR-99a/100~125b tricistrons are transcribed as a polycistronic message transactivated by the homeobox transcription factor HOXA10. Integrative analysis of global gene expression profiling, miRNA target prediction, and pathway architecture revealed that miR-99a/100, let-7, and miR-125b functionally converge at the combinatorial block of the transforming growth factor b (TGFb) pathway by targeting four receptor subunits and two SMAD signaling transducers. In addition, down-regulation of tumor suppressor genes adenomatous polyposis coli (APC)/APC2 stabilizes active b-catenin and enhances Wnt signaling. By switching the balance between Wnt and TGFb signaling, the concerted action of these tricistronic miRNAs promoted sustained expansion of murine and human HSCs in vitro or in vivo while favoring megakaryocytic differentiation. Hence, our study explains the high phylogenetic conservation of the miR-99a/100~125b tricistrons controlling stem cell homeostasis, the deregulation of which contributes to the development of AMKL.

miR-125b can enhance skin tumor initiation and promote malignant progression by repressing differentiation and prolonging cell survival

“…Hence, it is likely that other miR-125b targets beyond Vps4b and Mapks will contribute to the oncogenic effects that we described here. In this regard, it is interesting that in hematopoietic stem cells (HSCs), miR-125b was recently found to regulate multiple target genes involved in TGF-b signaling (Emmrich et al 2014).TGF-b signaling also functions in skin tumorigenesis, and Smad4, one of the miR-125b targets in HSCs, was also among our miR-125b targets (Supplemental Table S1, sheet 1). Another previously reported miR-125b target is TACSTD2, which functions in MAPK signaling (Nakanishi et al 2014).…”

“…In the mammalian genome, three distinct miRNAsmiR-99a/100, let-7a/c, and miR-125b-form an adjacently positioned and cotranscribed polycistronic miRNA cluster (Emmrich et al 2014) whose alterations are frequently found in cancers (Calin et al 2004). While let-7 family miRs are well-established suppressors (Johnson et al 2005;Mayr et al 2007), the roles of other individual members of this miR cluster in cancer are less clear.…”

“…However, assessing the significance of these tumor progression-related changes in miR-125b expression necessitated functional studies in which miR-125b expression could be regulated independently of its tightly linked and polycistronically transcribed miRNA gene neighbors, which include the tumor suppressor let-7 (Supplemental Fig. S1C; Emmrich et al 2014). …”

Umbilical Cord Blood Hematopoietic Stem and Progenitor Cell Expansion for Therapeutic Use

“…By regulating the balance of these two signaling pathways, the miR-99a/100∼125b tricistronic miRNAs are reported to promote human HSCs expansion and to favor megakaryocytic diferentiation [143]. Additionally, miR-126 regulates HSC proliferation and diferentiation by targeting PI3K/AKT/mTOR signaling [131].…”

“…Wnt and TGF pathways also have opposing roles in regulating the balance between HSC selfrenewal, quiescence, and diferentiation [143], and these are controlled by miRNAs. By regulating the balance of these two signaling pathways, the miR-99a/100∼125b tricistronic miRNAs are reported to promote human HSCs expansion and to favor megakaryocytic diferentiation [143].…”