Roles of protein kinase C and α-tocopherol in regulation of signal transduction for GATA-4 phosphorylation in HL-1 cardiac muscle cells

Our previous study demonstrated that endothelin-1 induced a phosphorylation of GATA-4 transcription factor, which plays important roles in cardiac hypertrophy and failure [1]. The goal of the present study was to determine whether protein kinase C (PKC) is involved in the signaling pathway, and, if so, whether α-tocopherol inhibits the GATA-4 phosphorylation. Treatment of HL-1 adult mouse cardiac muscle cells with PMA, a known activator of PKC, induced a transient phosphorylation of GATA-4. PMA also phosphorylated MEK and ERK, and PMA-induced GATA-4 phosphorylation was blocked by an MEK inhibitor, PD98059, suggesting that PMA phosphorylates GATA-4 via the MEK-ERK pathway. Treatment of HL-1 cells with 1 μM PMA for 24 h resulted in a downregulation of PKC. In PKC-downregulated cells, PMA- or ET-1-induced GATA-4 phosphorylation was suppressed, suggesting the role of PKC in GATA-4 phosphorylation. However, α-tocopherol (5–100 μM) did not inhibit the phosphorylation of GATA-4 or ERK in HL-1 cells. In contrast, α-tocopherol potently inhibited the PMA-induced ERK activation in smooth muscle cells. Our studies in HL-1 cells showed that PKC inhibitors, such as calphostin C and chelerythrin, failed to inhibit the PMA signaling. Furthermore, HL-1 cells appear to possess a unique PKC-signaling mechanism as PKC is constitutively phosphorylated and PMA did not cause further phosphorylation. Thus, in HL-1 cardiac muscle cells, PMA activates the MEK-ERK-GATA-4 pathway, apparently via a PKC-independent mechanism.