PDE5A and glioblastoma: The finding that sildenafil alone increased MMP-2 secretion by T98G cells after an overnight incubation is in line with its effects on glioblastoma cell migration and is similar to the effects induced by PDE5 silencing, suggesting that continuous PDE5 inhibition might stimulate GC activity in tumor cells, as it was shown by the increase of cGMP levels.