The Low Density Lipoprotein Receptor-related Protein Functions as an Endocytic Receptor for Decorin

Abstract

Decorin is a small leucine-rich proteoglycan that modulates the activity of transforming growth factor type ␤ and other growth factors and thereby influences the processes of proliferation and differentiation in a wide array of physiological and pathological reactions. Hence, understanding the regulatory mechanisms of decorin activity has broad implications. Here we report that the extracellular levels of decorin are controlled by receptor-mediated catabolism, involving the low density lipoprotein receptor family member, low density lipoprotein receptor-related protein (LRP). We show that decorin is endocytosed and degraded by C2C12 myoblast cells and that both processes are blocked by suppressing LRP expression using short interfering RNA. The same occurs with CHO cells, but not with CHO cells genetically deficient in LRP. Finally, we show that LRP-null CHO cells, transfected to express mini-LRP polypeptides containing either the second or fourth LRP ligand-binding domains, carry out decorin endocytosis and lysosomal degradation. These findings point to LRP-mediated catabolism as a new control pathway for the biological activities of decorin, specifically for its ability to influence extracellular matrix signaling.

A Novel Modulatory Mechanism of Transforming Growth Factor-β Signaling through Decorin and LRP-1

“…Cells were then incubated in Opti-MEM I containing 1 g of each plasmid, 0.02 g of pRL-SV40, 2 l of PLUS reagent, and 1 l of Lipofectamine. After 6 h, FBS was added to the medium, and the cells were cultured for a further 12 h. Medium was changed to fresh growth medium, and the following reagents were added: TGF-␤1 dissolved in 0.5% FBS; GST or GST-RAP in phosphatebuffered saline (23), or inhibitors of p38MAPK activity (SB203580, from Pierce), TGF-␤ receptor I kinase activity FIGURE 1. Dcn null myoblasts are less responsiveness to TGF-␤, whereas re-expression of decorin recovers its sensitivity to TGF-␤.…”

“…A candidate for this function is LRP-1, the endocytic receptor for decorin, as we have recently shown (23). To evaluate whether LRP-1 participates in the regulation that decorin exerts in response to TGF-␤, we determined TGF-␤-dependent p3TP-Lux activity in the presence or absence of RAP, a protein that inhibits the binding and endocytosis of decorin by LRP-1 (23). RAP inhibited about 50% of TGF-␤1-dependent p3TP-Lux activity in wild type myoblasts (Fig.…”

“…One possibility to explain how decorin positively regulates the TGF-␤ response in myoblasts is through its interaction with a specific cell surface receptor. A candidate for this function is LRP-1, the endocytic receptor for decorin, as we have recently shown (23). To evaluate whether LRP-1 participates in the regulation that decorin exerts in response to TGF-␤, we determined TGF-␤-dependent p3TP-Lux activity in the presence or absence of RAP, a protein that inhibits the binding and endocytosis of decorin by LRP-1 (23).…”

“…described (23). Briefly, for transfection, myoblasts were seeded into six-well plates until they reached 70% confluence.…”

“…The folding process of the receptor in the endoplasmic reticulum requires the participation of a 39-kDa receptor chaperone, the receptor-associated protein (RAP) (25). This protein competes efficiently with the binding of decorin to LRP-1 at the cell surface (23). To study the possible role of LRP-1 in the modulation of TGF-␤ signaling by decorin, we decided to study its role in myoblasts since it has been characterized with regard to TGF-␤ response (18,26,27 …”