Taken together, leptin mediated crosstalk between adipocytes and breast cancer cells, which further activated STAT3 not only directly to facilitate PAI-1 expression but also to indirectly affect PAI-1 by repressing miR-34a, thus promoting PAI-1-related EMT to strengthen breast cancer metastasis (Figure 8I, Figure S4H). This evidence concerns the gene SERPINE1 and breast cancer.