The Roles And Mechanisms Of MiR100 And Its Downstream Target NMT1 In Regulating Breast Cancer Progression

MicroRNAs(MiRNAs)play important roles in cellular self-renewal,differentiation and oncogenesis.MircroRNA100(MiR100)was first decribed in 2002 and is localized on chromosome 11q24.1(Intron 2)in a cluster with let7a-2.MiR100 was shown to inhibit cell proliferation in many cancer types.We have previously demonstrated that breast cancer cell lines and normal breast epithelial cells contain a subpopulation with stem cell properties that can be isolated by virtue of their expression of Aldehyde dehydrogenase(ALDH)as assessed by the Aldefluor assay.We demonstrated,among microRNAs differently expressed in ALDH-positive and ALDH-negative populations of breast cancer cell lines we identified micrRNA100(MiR100).We found that MiR100 is significantly increased in the ALDH-negative population compared to the ALDH-positive population.Utilizing a tetracycline inducible lentivirus driving MiR100 expression,we found that induction of MiR100 expression decreased the ALDH-positive population in vitro as well as in mouse xenografts where this reduction was associated with decreased tumor growth and metastasis.The mechanism was due to that overexpression of MiR100 downregulated BMPR2 and multiple stem cell regulatory and chromatin remodeling genes including SMARCA5,SMARCD1,BAZ2A,FGFR and IGFR.MiR100 inhibition resulted in promoting tumor development in mammary fat pads in mouse xenografts.These studies demonstrate that MiR100 can regulate cancer stem cell self-renewal and development of breast cancer,the existences of which have important biological and clinical implications.In addition,we found that N-myristoyltransferase NMT1 might be one of MiR100 downstream targets.Myristoylation is one of key translational and post-translational modifications that involved in signal transduction,cellular transformation and tumorigenesis.Increasing evidence demonstrates that targeting myristoylation might provide a new strategy for eliminating cancers.However,the underlying mechanisms are still unclear.In this study,we demonstrated that genetic inhibition of NMT1 suppressed initiation,proliferation and invasion of breast cancer cells either in vitro or in vivo.We identified ROS could negatively regulate NMT1 expression and NMT1 knockdown conversely promoted oxidative stress,which formed a feedback loop.Furthermore,inhibition of NMT1 caused the accumulation of degraded proteins and ER stress,which cross-talked with mitochondria to produce more ROS.And both of oxidative stress and ER stress could activate JNK pathway,leading to autophagy which abrogated breast cancer progression especially triple-negative breast cancer(TNBC).These studies provide a preclinical proof of novel concept for targeting NMT1 as a strategy to treat breast cancer.