Protein synthesis-dependent and -independent mechanisms for the regulation of GnRH RNA transcript levels in GT1 cells.

The cellular mechanism for the suppression of GnRH gene expression by the phorbol ester PMA was investigated in GT1 cells. The protein synthesis inhibitor cycloheximide decreased GnRH primary transcript levels, indicating a protein synthesis requirement for basal GnRH transcription. PMA decreased GnRH primary transcript levels even in the presence of cycloheximide, indicating that the PMA suppression of GnRH gene transcription is protein synthesis-independent. In contrast, the PMA-inhibitory effect on GnRH cytoplasmic mRNA levels was significantly reduced or inhibited in the presence of cycloheximide or RNA synthesis inhibitors given within 4 h of PMA, suggesting a protein/RNA synthesis-dependent mechanism for the regulation of GnRH mRNA levels by PMA. Thus, the mechanism for the PMA inhibition of GnRH primary transcript is mediated through a protein and RNA synthesis-independent mechanism, while the decrease in GnRH mRNA levels occurs through a mechanism that involves the induction of new RNA and protein synthesis that happens within 4 h of PMA administration.