MLLT6/ATF2 axis restrains breast cancer progression by driving DDIT3/4 expression.

Epigenetic deregulation is strongly associated with tumour progression. The identification of natural tumour suppressors to overcome cancer metastasis is urgent for cancer therapy. We investigate whether myeloid/lymphoid or mixed-lineage leukaemia translocated (MLLT) family members contribute to breast cancer progression and found that high MLLT6 expression predicted a better prognosis and that gradually decreased MLLT6 expression was accompanied by breast cancer malignancy. MLLT6 was downregulated by hypoxia-induced enrichment of DNMT1 at the MLLT6 promoter. The results of in vitro functional experiments indicated that MLLT6 depletion promoted colony formation and cell migration, probably by hampering apoptosis. RNA profiling revealed that the apoptotic pathway was downregulated following stable knockdown of MLLT6. DNA damage-inducible transcript 3/4 (DDIT3/4) were among the top 10 downregulated genes and may have expression patterns similar to that of MLLT6. Restoring DDIT3/4 expression in cells with MLLT6 depletion blocked colony formation and cell migration and attenuated the successful colonization of breast cancer cells in vivo. We also determined that the transcription factor activating transcription factor 2 (ATF2) is a binding partner of MLLT6 and participates in the MLLT6/ATF2 axis, which was reinforced by inhibition of AKT signalling, in turn inducing DDIT3/4 expression by establishing an active chromatin structure at the DDIT3/4 gene promoters. Because MLLT6 promotes breast cancer cell apoptosis by inducing DDIT3/4 expression during metastasis, it could be a novel tumour suppressor. Implications: Control of MLLT6 expression via inhibition of PI3K/AKT kinase activity is a potential therapeutic approach for the management of metastatic breast cancer.