PHYSIOLOGICAL AND PHARMACOLOGICAL INHIBITORS OF LUTEINIZING HORMONE-DEPENDENT STEROIDOGENESIS INDUCE HEAT-SHOCK PROTEIN-70 IN RAT LUTEAL CELLS

Heat shock protein (HSP) synthesis increases in cells with a broad range of stress conditions. We recently showed that induction of HSP-70 is associated with inhibition of hormone-sensitive steroidogenesis, but not hormone-sensitive cAMP accumulation, in rat luteal cells by a mechanism associated with interruption of cholesterol translocation in mitochondria. As HSP induction may be an early mediator of luteal regression, we investigated whether physiological and pharmacological inhibitors of luteal function would induce HSP-70 in rat luteal cells, Both [S-35]methionine labeling and Western blotting with antibodies against the inducible form of HSP-70 revealed HSP induction in rat luteal cells by 1 mu M prostaglandin F-2 alpha (PGF(2 alpha)) coincident with inhibition of progesterone synthesis. In contrast, PGE(2) (1 mu M) failed to increase HSP-70 synthesis. Phorbol 12-myristate 13 acetate (3 mu M), tumor necrosis factor-alpha (100 ng/ml), and ionomycin (1 mu M) also induced HSP synthesis. Induction of HSP-70 was preceded by the rapid activation of heat shock transcription factor, which binds to the heat shock transcriptional control element. Gel retardation assays demonstrated heat shock transcription factor activation within 15 min of PGF(2 alpha) treatment. Northern analysis with an oligonucleotide probe specific for inducible HSP-70 showed induction at the transcriptional level by the above agents within 30 min. As functional luteal regression is known to display elements of a stress response, the finding that a number of factors that inhibit hormone-sensitive progesterone synthesis rapidly activate the heat shock response further implicates HSPs as possible mediators of luteolysis.