N-acetylation is required for the lactotrope recruitment activity of alpha-melanocyte-stimulating hormone and beta-endorphin.

Pituitary neurointermediate lobe (NIL) cells obtained from ovariectomized rats and exposed to 17 beta-estradiol in vitro have been shown to release an activity that induces acute recruitment of additional PRL-secreting cells. We have recently reported that alpha MSH, a major secretory product of the NIL, can substitute for this lactotrope-recruiting factor released by NIL cells in response to 17 beta-estradiol. beta-Endorphin (beta END) was not effective in this regard. Inasmuch as the degree of acetylation is critical to the activities of both of these molecules in other systems, we decided to assess its importance to lactotrope-recruiting activity in the present study. Anterior pituitary cells from ovariectomized rats were cultured overnight, exposed to various treatments for 3 h, and then subjected to a reverse hemolytic plaque assay for PRL. Exposure to mono- and diacetylated alpha MSH (N-ac-alpha MSH and di-ac-alpha MSH, respectively) or N-acetylated beta END (N-ac-beta END) caused a significant increase in the fraction of anterior pituitary cells that released PRL. In contrast, the Des-acetylated variants of both molecules had no lactotrope-recruiting activity. In a dose-response study, maximally effective doses of di-ac-alpha MSH and N-ac-beta END were equally effective with respect to their function as lactotrope-recruiting factors. Furthermore, the two peptides acted cooperatively when simultaneously applied in submaximal concentrations. Taken together, these results demonstrate that 1) N-acetylation is an essential requirement for the lactotrope-recruiting activity of alpha MSH and beta END in vitro; and 2) di-ac-alpha MSH and N-ac-beta END can act in a cooperative fashion to recruit additional cells into the PRL-secreting population.