PGP and gastric cancer: Regarding soluble factors involved in the ERK-induced up-regulation of ABC transporters, it is known that the granulocytic-colony stimulating factor (G-CSF) induces Pgp by activating the RAS/ERK axis in gastric cancer [128], and the extracellular ATP up-regulates MRP2 by increasing MEK/ERK1/2 signaling in transformed colon cells [129].