Thrombin and histamine rapidly stimulate the phosphorylation of the myristoylated alanine‐rich C‐kinase substrate in human umbilical vein endothelial cells: Evidence for distinct patterns of protein kinase activation

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Abstract
Human α‐thrombin and histamine each stimulates protein phosphorylation in human umbilical vein endothelial cells (HUVEC). We have identified the most prominent of these phosphoproteins by immunoprecipitation as the human homolog of the widely distributed myristoylated alanine‐rich C‐kinase substrate (MARCKS). Stimulation by 0.1–10 U/ml of α‐thrombin produces a time‐dependent, sustained (plateau 3–5 min) level of MARCKS phosphorylation. MARCKS phosphorylation requires thrombin catalytic activity but not receptor binding and is also seen in response to stimulation by a peptide, TR (42–55), that duplicates a portion of the thrombin receptor tethered ligand created by thrombin proteolytic activity. One micromolar histamine, like α‐thrombin, produces sustained phosphorylation of MARCKS (plateau 3–5 min). In contrast, 100 μM histamine results in rapid but transient MARCKS phosphorylation (peak 1–3 min). HUVEC treated with 100 μM histamine for 5 min can be restimulated by α‐thrombin but not fresh histamine, suggesting that the histamine receptor was desensitized. MARCKS phosphorylation can also be induced by several exogenous protein kinase C (PKC) activators and both α‐thrombin‐ and histamine‐induced MARCKS phosphorylation are inhibited by the PKC antagonist staurosporine. However, while prolonged PMA pretreatment ablates histamine‐induced MARCKS phosphorylation, the ability of thrombin to induce MARCKS phosphorylation is retained. These findings provide evidence for agonist‐specific pathways of protein kinase activation in response to thrombin and histamine in HUVEC.

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