Modulation by staurosporine of phorbol‐ester‐induced effects on growth and protein kinase C localization in a549 human lung‐carcinoma cells

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Abstract
12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) and bryostatin I are activators of protein kinase C (PKC). TPA is a potent inhibitor of the growth of A549 cells, while bryostatin I exerts a weak antiproliferative effect upon this cell line. We tested the hypothesis that the PKC inhibitor staurosporine (STAU) can interfere with the effects of TPA or bryostatin I on A549 cells. STAU alone arrested A549 cell growth effectively with an IC50 of 0.65 nM as determined by cell counting after incubation for 96 hr. It also caused the release of lactate dehydrogenase from cells with an IC50 of 18.4 nM. On incubation with cells for up to 8 hr, STAU (100 nM) alone did not reduce thymidine incorporation into cells. However, it partially abrogated the inhibition of DNA synthesis caused by TPA or bryostatin 1(10 nM). The IC50 for inhibition by STAU of the activity of PKC purified from A549 cells was 6.1 nM. Localization and levels of PKC were studied by Western blot and phorbol ester receptor binding analyses. STAU (100 nM) did not prevent the TPA‐induced rapid redistribution of PKC to the cell membrane, but instead increased it by 25%. The PKC downregulation caused by TPA was not reduced in the presence of STAU. The results suggest that (i) PKC activation is involved in growth inhibition caused by TPA or bryostatin I in A549 cells, and (ii) subcellular localization or levels of PKC can be pharmacologically manipulated even under conditions of inhibited kinase function.