Proteasome Implication in Phorbol Ester- and GnRH- Induced Selective Down-Regulation of PKC (, , )i n T3-1 and LT2 Gonadotrope Cell Lines

lysosomal proteases (Calpeptin, E64d, Calpain inhibitor II, and PD150606), were ineffective. As shown in T3-1 subcellular frac- tions, proteasome abrogation did not affect membrane translo- cation of TPA- and GnRH- target isoforms (, ) but, preventing their degradation, favored enzyme accumulation to the mem- brane compartment. Proteolysis processing of PKCs may be de- pendent upon their phosphorylated state and/or catalytic activ- ity. Inhibition of PKC catalytic activity (GF109203X, Go ¨ 6976), selectively prevented the TPA-evoked PKC depletion in both mixed pituitary cells and T3-1 gonadotropes; in T3-1 subcel- lular fractions, PKC inactivation overcame the TPA-evoked isoenzyme degradation by inducing a pronounced membrane accumulation of the isoform without affecting its membrane relocalization. Thus, the proteasome system by adjusting PKC cellular levels, may represent a regulatory proteolytic pathway implicated in the adaptive mechanisms of the time dependent cell responses. (Endocrinology 143: 1386-1403, 2002)