MODULATION OF ADRENAL-CELL FUNCTIONS BY CADMIUM SALTS .2. SITES AFFECTED BY CDCL2 DURING UNSTIMULATED STEROID-SYNTHESIS

In previous studies cadmium chloride (CdCl2) nonlethally inhibited Y-1 adrenal mouse adrenal tumour cell 20-dihydroxyprogesterone (20DHP) se cretion, affecting unstimulated and stimulated steroidogenic pathway s ites differently. We studied CdCl2 effects on unstimulated steroidogen esis using Y-1 cells incubated 0.5 h in medium with or without cadmium (using the concentration that inhibited ACTH-stimulated steroid secre tion by 50%). Exogenously added 20-hydroxycholesterol (20OHC), 22(R)-h ydroxycholesterol (22OHC), 25-hydroxycholesterol (25OHC), pregnenolone (PREG), or progesterone (FROG) were used to bypass any rate-limited s teroidogenic pathway sites that CdCl2 might inhibit. 25OHC is a biolog ically active nonpathway steroid, while 20OHC, 22OHC, PREG, and FROG a re pathway steroids; each increased unstimulated 20DHF secretion nearl y 10-fold. Although CdCl2 could not reduce dibutyryl cyclic AMP- (dbcA MF)-stimulated 20DHP secretion significantly, it did significantly red uce basal and 25OHC-induced 20DHF secretion 25% below untreated levels . When 20OHC, 22OHC, PREG, or FROG were incubated with unstimulated Y- 1 cells, their synthesis into 20DHF was unaffected by cadmium. dbcAMP bypasses the plasma membrane enzyme complex that synthesizes intracell ular cAMP during exogenous ACTH stimulation; dbcAMP was not inhibited by CdCl2. The rate-limited step accelerated by cAMP involves plasma me mbrane and/or cytoplasmic cholesterol transport to and through outer a nd inner mitochondrial membranes before the cholesterol is synthesized into pregnenolone by side-chain cleavage enzymes on the inner membran e matrix face. Little is known regarding the mechanisms controlling un stimulated steroidogenesis. Under unstimulated conditions the 25-, 20- and 22(R)-monohydroxyls of cholesterol facilitate plasma membrane, cy toplasm and inner and outer mitochondrial solubility, diffusion and/or transport to bypass rate-limited steps and augment unstimulated stero id synthesis. Since conversion of endogenous mitochondrial cholesterol and 25OHC, but not dbcAMP-mobilized cytoplasmic cholesterol, 20OHC or 22OHC conversion, to 20DHP is inhibited by CdCl2, this suggests that (a) control of mitochondrial cholesterol supplies is independent of th e cAMP-regulated mitochondrial steps in the 20DHP steroid synthetic pa thway, (b) CdCl2 specifically inhibited endogenous mitochondrial chole sterol and 25OHC utilization, (c) CdCl2 toxicity may affect adrenal, t esticular, ovarian, and placental basal steroidogenic functions, and ( d) 25OHC may be a useful compound to examine unstimulated steroid synt hesis