Inducible Nitric Oxide Synthase in the Anterior Pituitary Gland: Induction by Interferon-γ in a Subpopulation of Folliculostellate Cells and in an Unidentifiable Population of Non-hormone-secreting Cells

Abstract

SUMMARYIn the context of immune-endocrine relationships, we have previously shown that interferon-␥ (IFN-␥) inhibits hormone secretion in anterior pituitary (AP) cell cultures. The non-hormone-secreting folliculostellate (FS) cells were found to mediate this inhibitory action. Because in the immune system IFN-␥ is a strong stimulator of nitric oxide (NO) release through the induction of NO synthase (NOS), we investigated whether the inducible form of NOS (iNOS) is present in (rat) AP cell cultures, and whether its expression is stimulated by IFN-␥. Immunocytochemistry revealed that under basal in vitro conditions only a very few AP cells contained iNOS. Treatment with IFN-␥ caused a sixfold rise in the number of iNOS-positive cells and augmented the intensity of the staining. The increased number of iNOS-expressing cells was paralleled by elevated production of NO. Some of the iNOSpositive cells extended cytoplasmic processes between hormone-secreting cells, which is a characteristic of FS cells. Immunostaining of FS cell-poor and FS cell-enriched populations (obtained by gradient sedimentation) also suggested the presence of iNOS in a subpopulation of FS cells. By double immunofluorescence techniques we found that about 65% of iNOS-expressing cells were positive for S-100, a marker protein for FS cells. However, around 80% of the S-100-positive cells were not labeled for iNOS. On the other hand, the majority of the S-100-negative iNOS-containing cells could not be further identified by antisera against the classical AP hormones, suggesting the presence of iNOS in a still unidentified non-hormone-secreting cell type of the AP gland. This report is the first to demonstrate the expression of the inducible form of NOS in the AP gland. IFN-␥ upregulates this expression, showing that cytokines may use the same signaling mechanisms in both the immune and the endocrine system. In addition, a putative new function of a subpopulation of FS cells in the paracrine regulation of the AP gland is suggested. by the presence of S-100, a cytoplasmic protein which, at least in rat and human AP, is found exclusively in FS cells (Vankelecom et al. 1993). The functional significance of FS cells is poorly characterized despite their discovery as early as 1953 (Rinehart and Farquhar 1953). Our group provided evidence for a paracrine regulatory role in the AP gland (Denef 1994;Allaerts et al. 1990a). FS cells attenuate the secretory responses of hormone-secreting cells to both stimulatory and inhibitory hypothalamic factors, possibly through the release of (a) paracrine molecule(s) (Denef

Folliculostellate cell network: A route for long-distance communication in the anterior pituitary

“…With their long cytoplasmic processes between other cell types, they are in an ideal position to play a role in intercellular communication mechanisms. Although FS cells do not secrete hormones, they are known to release biologically important factors and signaling molecules (e.g., IL-6, nitric oxide, vascular endothelial growth factor, basic fibroblast growth factor, follistatin) (14,29,36) and are responsive to central and peripheral stimuli [e.g., pituitary adenylate cyclaseactivating peptide (PACAP), vasoactive intestinal peptide, estrogens] (37-39) including immune factors (tumor necrosis factor ␣, transforming growth factor ␤3, IFN-␥) (40)(41)(42). Moreover and most remarkably, the FS cell network seems to provide a unique system of large-scale communication that may rapidly adjust cellular activities within the anterior pituitary.…”

History and perspectives of pituitary folliculo-stellate cell research

“…This finding was instrumental in the discovery of NO as one of the mediators of the FS cell inhibitory action. Similar to its activity in macrophages, IFN-g stimulated the expression of inducible NO synthase (iNOS) in FS cells, at least part of them, and the NO produced was shown to mediate the inhibitory effect of IFN-g (19,20). IL-6, on the other hand, was not involved since antibodies to IL-6 were unable to block the effect of IFN-g on stimulated hormone secretion (H Vankelecom, unpublished observations).…”

“…The hypothesis, which remains to be tested, is that electrical coupling of FS cells (28), in separate or in mixed electrically coupled systems with gonadotropes, may cause the long-term effects on GnRHinduced LH secretion (52). Alternatively, NO released from gonadotropes or FS cells (18,19), or from as yet unidentified cells (19), may form a distinct pathway of autocrine or paracrine regulation respectively of gonadotropin secretion (see also legend to Fig. 2).…”

“…Ceccatelli et al (18) further suggested that NO participates in the pulsatile secretion patterns of LH and GH (18). Apart from being a player in the intrapituitary cytokine network, NO production -in particular by FS cells -may also represent a point of contact between the endocrine and the immune system in view of its activation by IFN-g (19).…”

“…In the anterior pituitary, FS cells are an important link in this signaling pathway (19,20), and moreover they may function as a model system to study the role of NO and cytokines in the communication between neurons and glial cells (see e.g. 92).…”