DPP4 and breast cancer: Moreover, we have reported that a DPP-4 inhibitor and shRNA-mediated DPP-4 knockdown both increases the expression of phosphorylated S6K in the primary tumors of 4T1 mammary tumor-bearing mice; metformin significantly repressed such a DPP-4 deficiency-induced mTORC1 pathway in vivo as well [29].