PDE5A and breast carcinoma: Soft agar assay showed that colony numbers of MCF-7 breast cancer cells were significantly increased when cells were incubated with CM derived from vehicle-treated PDE5-overexpressing MEFs compared to CM derived from vehicle-treated vector-expressing MEFs, while this induction was no longer evident when cells were co-cultured with CM derived from sildenafil-treated PDE5-overexpressing MEFs (Figure 4I, upper panel).