Arachidonate 12-lipoxygenase in porcine anterior pituitary cells: its localization and possible function in gonadotrophs

Abstract

Arachidonate 12-lipoxygenase, which oxygenates positions 12 and 13 of arachidonic and linoleic acids, is present in porcine anterior pituitary cells. Colocalization of the 12-lipoxygenase with various pituitary hormones was examined by immunohistochemical double-staining using antibodies against 12-lipoxygenase and various anterior pituitary hormones. Under light microscopy, approximately 7% of the cells producing luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were positive for 12-lipoxygenase, whereas the enzyme was detected in less than 2% of the cells producing thyrotrophin, prolactin, growth hormone (GH), and adrenocorticotrophin. In an attempt to examine the participation of 12-lipoxygenase metabolites in pituitary hormone release, we incubated the primary culture of porcine anterior pituitary cells with 12-hydroperoxy-arachidonic acid or 13-hydroperoxy-linoleic acid. Significant stimulation of LH and FSH release by these hydroperoxides was observed at 10 microM in a time-dependent manner. At doses around 10 microM these compounds produced responses of similar magnitude to 1 nM gonadotrophin-releasing hormone (GnRH), but higher concentrations (30 microM) of the compounds were required for GH release. In contrast, 12-hydroxy-arachidonic and 13-hydroxy-linoleic acids were almost ineffective. Furthermore, the gonadotrophin release by 1 nM GnRH was inhibited by nordihydroguaiaretic acid (a lipoxygenase inhibitor) with an IC50 of about 5 microM. Thus, the hydroperoxy (but not hydroxy) products of 12-lipoxygenase may be involved in the release of pituitary hormones especially LH and FSH.

Dietaryn-3 andn-6 Fatty Acids Alter Avian Pituitary Sensitivity

“…In the tallow fed birds, a diet containing 25.8% w/w of LA (18:2n-6), the rise in circulating plasma GH concentrations fell between the two PUFA groups. Previous studies have shown that fatty acids from the n-6 series, specifically LA (18:2n-6), enhance the release of GHRH (Seflaris et al, 1992;Ikawa et al, 1996;Barb et al, 1997). In addition, Seflaris and co-workers (1992) have also shown an inhibitory effect of LA (18:2n-6) on the GH inhibiting factor, somatostatin, when cultured hypothalamic cells were exposed to oleic acid.…”

“…This finding may simply reflect the pulsatile LH secretory pattern, the lower LH concentrations a result of the sampling regimen particularly in respect to the pre-infusion period where only three basal sampling points were measured. However, it is also plausible that other factors may override the LH stimulatory nature of some fatty acids such as AA (20:4n-6) and oleic acid (Kennedy et al, 1994;Ikawa et al, 1996) and impinge on reproductive status. A possible candidate could be the hormone leptin.…”

“…For example, the addition of AA (20:4n-6) (Ikawa et al, 1996) and oleic acids (Kennedy et al, 1994) to culture media enhances the release of the gonadotrophin-releasing hormones from cultured mammalian anterior pituitary cells. Earlier studies in vitro have demonstrated that eicosanoids derived from AA (20:4n-6) added to culture media modulated corticotrophin-releasing hormone (CRF) secretion from rat hypothalamic explants (Bernardini et al, 1989).…”

Platelet-type Arachidonate 12-Lipoxygenase in Mouse Gastrointestinal Tract.

“…For example, in porcine anterior pituitary cells the 12-lipoxygenase of the leukocyte-type was colocalized not only with gonadotrophin but also with other anterior pituitary hormones [8].…”

“…The biological significance of 12-lipoxygenase has not been fully understood [20] . Stimulatory effects of 12-lipoxygenase metabolites of arachidonic acid have been reported for various endocrine systems; LH-RH [3], melatonin [16], insulin [15], gonadotrophin [8] and aldosterone [11]. In addition, the presence of leukocytetype 12-lipoxygenase was demonstrated in various endocrine cells on biochemical, molecular biological and immunohistochemical bases: porcine anterior pituitary cells [19], rat pancreatic islet fl-cells [17], human adrenal glomerulosa cells [4] and rat pineal cells [5].…”