Plasma protein kinase C (PKC)α as a biomarker for the diagnosis of cancers

Protein kinase C (PKC)alpha plays a key role in the differentiation, proliferation and apoptosis of cancer cells, and its activity is higher in cancer cells than in normal cells. In the present study, we investigated the existence of activated PKC alpha in plasma and its possibility for cancer diagnosis. Plasma samples were prepared from xenograft mouse models of cancer and from normal mice. Phosphorylation ratios for a PKC alpha-specific peptide substrate (Alphatomega) were analyzed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry and activated PKC alpha was identified by western blot analysis. Increased levels of activated PKC alpha were found in the plasma of cancer-bearing mice (U87, A549, A431, HuH-7 and B16 melanoma) compared with the levels found in control mice. Phosphorylation ratios for peptide substrate increased with an increase in tumor size. Moreover, the addition of Ro-31-7549, a highly specific inhibitor of PKC alpha, produced a concentration-dependent reduction of phosphorylation ratios, whereas the non-PKC alpha inhibitors, rottlerin and H-89, did not significantly effect phosphorylation ratios. In addition, the level of activated PKC alpha decreased after cancer resection but increased if the cancer recurred. From these results, we suggest that (i) activated PKC alpha in plasma can be a useful biomarker for the diagnosis of cancers and (ii) the level of activated PKC alpha can be monitored to assess the recurrence of cancer after surgical removal. To our knowledge, this is the first report demonstrating the existence of activated PKC alpha in plasma and its possibility for cancer diagnosis.