Abstract
Annexin V belongs to a large family of calcium‐binding and phospholipid‐binding proteins and may act as an endogenous regulator of the protein kinase C (PKC) activity. This study examines the effect of annexin V on the in vitro PKC activity in cultured mesangial cells using histone H1, the peptide [Ser25]PKC‐(19–31), or endogenous proteins as substrates. The SDS/PAGE pattern of 32P‐labeled mesangial proteins showed that the calcium‐independent PKC [(n+a)PKC] phosphorylated several proteins from 70 kDa to 40 kDa and 22 kDa to 15 kDa. Three additional proteins from 34 kDa to 29 kDa, including annexin I and its proteolytic forms, were detected after activation of calcium‐dependent PKC (cPKC). Increasing concentrations of annexin V did not alter the phosphorylation of (n+a)PKC substrates. By contrast, specific phosphorylation of proteins and annexin I by cPKC, was reduced in a dose‐dependent manner. Addition of high concentration of calcium and phosphatidylserine did not reverse the inhibitory effect of annexin V. Annexin V also inhibited the phosphorylation of histone H1 or peptide [Ser25]PKC‐(19–31) by cPKC. Moreover, removal of annexin V from cytosols increased the annexin I phosphorylation by these isoforms. From these results, we propose that annexin V may regulate the signal‐transduction pathway involving the activation of cPKC, as they act in vitro as an inhibitor of these kinases.
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