Platelet-derived growth factor D (PDGF-D), also known as Iris-expressed growth factor, is a member of the PDGF/vascular endothelial growth factor family. The four members of this family are mitogenic factors for cells of mesenchymal origin and are characterized by a core motif of eight cysteines, seven of which are found in this factor. PDGF-D/PDGFD only forms homodimers and, therefore, does not dimerize with the other three family members. It differs from alpha and beta members of this family in having an unusual N-terminal domain, the CUB domain. The expression of PDGF-D/PDGFD in the eye is tissue-specific. In the anterior segment, it is localized to iris and ciliary body, whereas in the retina, PDGF-D/PDGFD is restricted to the outer plexiform layer. PDGF-D/PDGFD is present in aqueous humor but is not detectable in mature lens or in mouse lens-derived alphaTN4-1 cells. PDGF-D/PDGFD is highly expressed in human breast cancer and facilitates tumor growth and lymph node metastasis, making it a potential target in breast cancer. PDGF-D/PDGFD increases drug delivery and hence improves the efficacy of chemotherapy through vessel normalization. Intervention in the PDGF-D pathway in the eye, perhaps by antibody or blocking peptide, could be useful in the treatment of certain cataracts, including post-operative secondary cataract.
