Possible involvement of phospholipase C and protein kinase C in stimulatory actions of L-leucine and its keto acid, alpha-ketoisocaproic acid, on protein synthesis in RLC-16 hepatocytes.
Effects of leucine and related compounds on protein synthesis were studied in RLC-16 hepatocytes. The incorporation of [(3)H] tyrosine into cellular protein was measured as an indexof protein synthesis. In leucine-depleted RLC-16 cells, L-leucineand its keto acid, alpha-ketoisocaproic acid (KIC), stimulated protein synthesis, while D-leucine did not. Mepacrine, an inhibitor of both phospholipase A(2) and C canceled stimulatory actions of L-leucine and KIC on protein synthesis, suggesting a possible involvement of either arachidonic acid metabolism by phospholipase A(2), cyclooxygenase or lipoxygenase, or phosphatidylinositol degradation by phospholipase C in the stimulatory actions of L-leucine and KIC.Neither indomethacin, an inhibitor of cyclooxygenase, nor caffeic acid, an inhibitor of lipoxygenase, diminished their stimulatory actions, suggesting no involvement of arachidonic acid metabolism. Conversely, 1-O-hexadecyl-2-O-methylglycerol, an inhibitor of protein kinase C, significantly canceled the stimulatory actions of L-leucine and KIC on protein synthesis, suggesting an involvement of phosphatidylinositol degradation and activation of protein kinase C. These results strongly suggest that both L-leucine and KIC stimulate protein synthesis in RLC-16 cells via activation of phospholipase C and production of diacylglycerol and inositol triphosphate from phosphatidylinositol, which in turn activate protein kinase C.
Yagasaki
Kazumi
K
Department of Applied Biological Science, Tokyo Noko University, Saiwaicho 3-5-8, Fuchu, Tokyo, 183-8509, Japan, yagasaki@cc.tuat.ac.jp.
Morisaki-Tsuji
Naoko
N
Miura
Atsuhito
A
Funabiki
Ryuhei
R