Heparanase deglycanation of syndecan-1 is required for binding of the epithelial-restricted prosecretory mitogen lacritin

Abstract

Cell surface heparan sulfate (HS) proteoglycans are carbohydrate-rich regulators of cell migratory, mitogenic, secretory, and inflammatory activity that bind and present soluble heparin-binding growth factors (e.g., fibroblast growth factor, Wnt, Hh, transforming growth factor β, amphiregulin, and hepatocyte growth factor) to their respective signaling receptors. We demonstrate that the deglycanated core protein of syndecan-1 (SDC1) and not HS chains nor SDC2 or -4, appears to target the epithelial selective prosecretory mitogen lacritin. An important and novel step in this mechanism is that binding necessitates prior partial or complete removal of HS chains by endogenous heparanase. This limits lacritin activity to sites where heparanase appears to predominate, such as sites of exocrine cell migration, secretion, renewal, and inflammation. Binding is mutually specified by lacritin's C-terminal mitogenic domain and SDC1's N terminus. Heparanase modification of the latter transforms a widely expressed HS proteoglycan into a highly selective surface-binding protein. This novel example of cell specification through extracellular modification of an HS proteoglycan has broad implications in development, homeostasis, and disease.

Heparanase Stimulation of Protease Expression Implicates It as a Master Regulator of the Aggressive Tumor Phenotype in Myeloma

“…We know this clipping occurs in the myeloma cells expressing high levels of heparanase because the syndecan-1 has shorter heparan sulfate chains than those from cells expressing low levels of heparanase (19). Such clipping could "activate" the syndecan-1 by exposing cryptic epitopes within the heparan sulfate chains or the ectodomain core protein to facilitate interactions with ligands (48).…”

Targeting of Heparanase-modified Syndecan-1 by Prosecretory Mitogen Lacritin Requires Conserved Core GAGAL plus Heparan and Chondroitin Sulfate as a Novel Hybrid Binding Site That Enhances Selectivity

“…HS chains cleaved with NaBH 4 from lacritin-bound SDC1 were ϳ4 -5 kDa, versus ϳ40 kDa for SDC1 purified on FGF2 (18). Short HS chains were non-existent in cells subjected to heparanase depletion by siRNA, and depleted cells failed to proliferate in the presence of lacritin but could be rescued by exogenous heparanase or heparitinase (18). Similarly, siRNA depletion of SDC1, but not SDC2, abrogated lacritin-dependent proliferation in a dose-dependent manner (18).…”

“…Short HS chains were non-existent in cells subjected to heparanase depletion by siRNA, and depleted cells failed to proliferate in the presence of lacritin but could be rescued by exogenous heparanase or heparitinase (18). Similarly, siRNA depletion of SDC1, but not SDC2, abrogated lacritin-dependent proliferation in a dose-dependent manner (18). No lacritin binding was observed to SDC2 or -4, and SDC1 bound to lacritin was resolved in the pellet after digestion with heparitinase I and chondroitin ABC lyase, suggesting that (i) short HS chains were necessary (or long chains obscured the binding site) and (ii) binding probably involved the SDC1 core protein (18).…”

“…The 1 M NaCl eluant from lacritin columns was dominated by a discrete higher molecular weight band that was identified as SDC1 by mass spectrometry (18). HS chains cleaved with NaBH 4 from lacritin-bound SDC1 were ϳ4 -5 kDa, versus ϳ40 kDa for SDC1 purified on FGF2 (18).…”

“…The 1 M NaCl eluant from lacritin columns was dominated by a discrete higher molecular weight band that was identified as SDC1 by mass spectrometry (18). HS chains cleaved with NaBH 4 from lacritin-bound SDC1 were ϳ4 -5 kDa, versus ϳ40 kDa for SDC1 purified on FGF2 (18). Short HS chains were non-existent in cells subjected to heparanase depletion by siRNA, and depleted cells failed to proliferate in the presence of lacritin but could be rescued by exogenous heparanase or heparitinase (18).…”