A mechanism of monoamine oxidases (MAOs) driven epithelium-to-mesenchymal transition (EMT) is disclosed. Also disclosed are methods for treating cancer by inhibiting or suppressing MAOs in cancer cells. Novel MAOs inhibitors, such as small molecules, siRNA, shRNA, antisense oligonucleotides, aptamers, decoys, and pharmaceutical compositions useful for treating cancer by disrupting the workings of MAOs are provided. In particular, a class of conjugates formed by covalently conjugating near infrared dye 783, IR-780, and MHI-148 to a MAO inhibitor, such as clorgyline, with and without encapsulation it in a nanoparticle is provided. Other aspects of the invention include methods for forming the nano-conjugates, method for monitoring treatment progress in a cancer patient by monitoring the changes in MAO activity, methods for screening patients who are at risk of cancer or differentiating different forms of cancer by assaying the level and location of MAO activity.