Steroidogenic adrenocortical cells synthesize alpha 2-macroglobulin in vitro, not in vivo

We previously identified alpha 2-macroglobulin as the major protein secreted by primary cultures of adrenocortical cells. We report here that in the adrenal gland, the distribution of alpha 2-macroglobulin in the adrenocortical tissue is restricted to the endothelium of blood vessels and that no immunoreactivity is found in steroidogenic cells. A time course study revealed that freshly dissociated bovine adrenocortical cells were void of alpha 2-macroglobulin immunoreactivity whereas the proportion of alpha 2-macroglobulin-positive cells reached more than two-thirds of the population between day 4 and day 7 of culture. Double immunoenzymatic labeling of 6-day-old cultures revealed a co-localization of alpha 2-macroglobulin and the steroidogenic enzyme P-450SCC. Treatment of 5-day-old cultures (expressing alpha 2-macroglobulin) for 24 h by either ACTH (10(-9)-10(-6) M) or alpha 2-macroglobulin (2.5 mg/ml) resulted in a marked decrease of the expression of alpha 2-macroglobulin. These data indicate that ACTH and plasmatic alpha 2-macroglobulin could physiologically repress alpha 2-macroglobulin expression in the adrenal cortex in vivo.