FIGURE 3.

Cross-talk between stromal and cancer cells via the PTN/MK–ALK pathway. PTN and MK are heparin-binding proteins released from cancer or stromal cells. They can bind at nanomolar affinity to glycosaminoglycans (GAGs) such as the heparan sulfate side chains of proteoglycan (HSPG) as well as chondroitin sulfate (CS; Deepa et al., 2002). CS is proposed as a co-receptor for MK (Muramatsu, 2010) as are other GAGs (Li et al., 2010). Also N-syndecan could function as such a co-receptor (Raulo et al., 1994). Both ligands and receptor can be found up-regulated in the stroma of cancer as well as in cancer epithelia (see text).