Figure 2. The masking of functional domains of OPN by syndecan-4. (A) OPN becomes less sensitive to thrombin digestion when bound by syndecan-4. Recombinant OPN (50 μl of 3 μg/ml) was mixed with either 50 μl of human IgG1 (hIgG) or 6 μg/ml Syn4Ig. The mixture was subjected to thrombin digestion (2 U/ml) for 37°C for 30 min. The full-length form of OPN was measured using ELISA. Note that this ELISA is unable to detect thrombin-cleaved OPN. (B) OPN recognition by integrin receptors is inhibited by syndecan-4. The binding of CHO cells to OPN is RGD dependent. The binding of CHO cells to either precoated OPN alone or OPN together with Syn4Ig was examined. (C) The binding of CHO cells expressing α4 integrin (α4/CHO) to OPN or OPN bound by syndecan-4 was examined in the presence or absence of 100 μg/ml GRGDS or 10 μg/ml anti–α4 integrin antibody. Data are presented as means ± SEM. *, P < 0.05; **, P < 0.005.
Figure 2.

The masking of functional domains of OPN by syndecan-4. (A) OPN becomes less sensitive to thrombin digestion when bound by syndecan-4. Recombinant OPN (50 μl of 3 μg/ml) was mixed with either 50 μl of human IgG1 (hIgG) or 6 μg/ml Syn4Ig. The mixture was subjected to thrombin digestion (2 U/ml) for 37°C for 30 min. The full-length form of OPN was measured using ELISA. Note that this ELISA is unable to detect thrombin-cleaved OPN. (B) OPN recognition by integrin receptors is inhibited by syndecan-4. The binding of CHO cells to OPN is RGD dependent. The binding of CHO cells to either precoated OPN alone or OPN together with Syn4Ig was examined. (C) The binding of CHO cells expressing α4 integrin (α4/CHO) to OPN or OPN bound by syndecan-4 was examined in the presence or absence of 100 μg/ml GRGDS or 10 μg/ml anti–α4 integrin antibody. Data are presented as means ± SEM. *, P < 0.05; **, P < 0.005.

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