Microenvironment Stimuli HGF and Hypoxia Differently Affected miR-125b and Ets-1 Function with Opposite Effects on the Invasiveness of Bone Metastatic Cells: A Comparison with Breast Carcinoma Cells

We examined the influence of microenvironment stimuli on molecular events relevant to the biological functions of 1833-bone metastatic clone and the parental MDA-MB231 cells. (i) In both the cell lines, hepatocyte growth factor (HGF) and the osteoblasts' biological products down regulated nuclear Ets-1-protein level in concomitance with endogenous miR-125b accumulation. In contrast, under hypoxia nuclear Ets-1 was unchanged, notwithstanding the miR-125b increase. (ii) Also, the 1833-cell invasiveness and the expression of Endothelin-1, the target gene of Ets-1/HIF-1, showed opposite patterns under HGF and hypoxia. We clarified the molecular mechanism(s) reproducing the high miR-125b levels with the mimic in 1833 cells. Under hypoxia, the miR-125b mimic maintained a basal level and functional Ets-1 protein, as testified by the elevated cell invasiveness. However, under HGF ectopic miR-125b downregulated Ets-1 protein and cell motility, likely involving an Ets-1-dominant negative form sensible to serum conditions; Ets-1-activity inhibition by HGF implicated HIF-1α accumulation, which drugged Ets-1 in the complex bound to the Endothelin-1 promoter. Altogether, 1833-cell exposure to HGF would decrease Endothelin-1 transactivation and protein expression, with the possible impairment of Endothelin-1-dependent induction of E-cadherin, and the reversion towards an invasive phenotype: this was favoured by Ets-1 overexpression, which inhibited HIF-1α expression and HIF-1 activity. (iii) In MDA-MB231 cells, HGF strongly and rapidly decreased Ets-1, hampering invasiveness and reducing Ets-1-binding to Endothelin-1 promoter; HIF-1α did not form a complex with Ets-1 and Endothelin-1-luciferase activity was unchanged. Overall, depending on the microenvironment conditions and endogenous miR-125b levels, bone-metastatic cells might switch from Ets-1-dependent motility towards colonization/growth, regulated by the balance between Ets-1 and HIF-1.