| Perlecan, encoded by HSPG2, is a large modular heparan sulfate proteoglycan built from domains homologous to laminin, the LDL receptor, and neural cell adhesion molecules, reflecting its evolutionary assembly from diverse matrix-binding motifs into a single multifunctional scaffold. This modular architecture allows perlecan to serve simultaneously as a structural component of basement membranes and as a reservoir that sequesters growth factors through its heparan sulfate side chains, controlling their local availability at the cell surface. Perlecan directly binds vascular endothelial growth factor and promotes activation of VEGF receptor 2 on endothelial cells, and this receptor engagement is required for vascular invasion into cartilage during endochondral bone formation, with perlecan supplied from the cartilage matrix itself rather than from the endothelial cells being the critical source. Perlecan signaling converges on the master metabolic regulators AMPK and mTOR through this same VEGFR2 interaction, linking matrix-anchored growth factor presentation to intracellular autophagy control. Proteolytic release of the C-terminal fragment endorepellin generates a distinct signaling entity that engages overlapping receptor and downstream pathway components to restrain angiogenesis, in part by promoting autophagy in endothelial cells, creating a built-in counterbalance to the pro-angiogenic activity of the intact proteoglycan. HSPG2 transcription is responsive to TGF-beta signaling through direct nuclear factor-1 binding at its promoter, is suppressed by interferon-gamma, and is activated by NF-κB in desmoplastic tumor stroma, positioning perlecan as an early-response gene whose expression shifts rapidly with the surrounding signaling environment. Loss-of-function mutations in HSPG2 disrupt basement membrane stability, cartilage matrix organization, and vascular integrity, and are associated with skeletal dysplasias including Schwartz-Jampel syndrome, while altered perlecan or endorepellin balance in tumor stroma influences angiogenesis and tumor progression. |
