Role of testicular interstitial macrophages in regulating testosterone release in hyperprolactinemia

Abstract

Hyperprolactinemia-induced hypogonadism has been linked to a dysfunction of the hypothalamus-pituitary-testis axis. The direct inhibitory effects of prolactin on the testicular release of testosterone have also been demonstrated, though their mechanisms remain unclear. Incubation of rat testicular interstitial cells (TICs) with prolactin stimulated the release of testosterone. TICs from rats with anterior pituitary-grafting-induced hyperprolactinemia release lower amounts of testosterone than controls. However, Leydig cells isolated from anterior pituitary-grafted rats release a greater amount of testosterone. These paradoxical observations have remained unexplained. This study examined the roles of testicular interstitial macrophages and of their product, tumor necrosis factor-alpha (TNF-alpha), in regulating Leydig cells under condition of hyperprolactinemia. Hyperprolactinemia was induced by grafting two anterior pituitary glands of rats under the renal capsule. Control animals were grafted with rat cortex tissue. The rats were sacrificed 6 weeks later. TICs and macrophages, and Leydig cells were isolated for in vitro incubation and drugs challenge. Testosterone released by testicular interstitial or Leydig cells was measured by radioimmunoassay. TNF-alpha concentration in the medium of TICs or macrophages was measured by enzyme-linked immunosorbent assay (ELISA). A dose-dependent stimulation of TNF-alpha secretion in the medium of TICs or macrophages by the prolactin challenge was observed. Higher amounts of TNF-alpha were released by TICs in the anterior pituitary-grafted rats than in the control group. In contrast, the release of TNF-alpha by testicular interstitial macrophages isolated from the anterior pituitary- and cortex-grafted groups was quantitatively similar. Challenge with human chorionic gonadotropin did not modify the TNF-alpha release by testicular interstitial macrophages in either group. Challenge of Leydig cells with TNF-alpha inhibited their release of testosterone stimulated by human chorionic gonadotropin, but not their basal testosterone release. These different patterns of testosterone release in TICs versus Leydig cells cultures in anterior pituitary-grafted rats may be due to the influence of testicular interstitial macrophages. These observations correlate with in vivo conditions, where prolactin increases the release of TNF-alpha by testicular interstitial macrophages, which, in turn, decreases the human chorionic gonadotropin-stimulated release of testosterone by Leydig cells. In summary, hyperprolactinemia-induced hypogonadism involves a mechanism of prolactin-originated, macrophage-mediated inhibitory regulation of testosterone release by Leydig cells. TNF-alpha, one of the cytokines secreted by macrophages, may play a key role in this mechanism.

Common and specific effects of the two major forms of prolactin in the rat testis

“…This conclusion is consistent with the work of Huang et al (24). These investigators set out to explain the apparent paradox that PRL decreased testosterone in vivo while increasing its production from isolated Leydig cells in vitro, a paradox they were able to attribute to the intermediary role of interstitial macrophages and TNF-␣ production (24). Thus it is possible that both PRLs exert their common effects on testosterone biosynthesis via the testicular macrophage but their differential effects on apoptosis via PRL receptors on Leydig cells, spermatogonia, and elongated spermatids.…”

“…The negative effect of hyperprolactinemia on testosterone production in adult animals therefore is in part direct and not only a consequence of reduced LH secretion. This conclusion is consistent with the work of Huang et al (24). These investigators set out to explain the apparent paradox that PRL decreased testosterone in vivo while increasing its production from isolated Leydig cells in vitro, a paradox they were able to attribute to the intermediary role of interstitial macrophages and TNF-␣ production (24).…”

Effect of warm‐rearing and heat acclimation on pituitary‐gonadal axis in male rats

“…Huang et al (1999Huang et al ( , 2001 found that hyperprolactinaemia in male rats diminished the response of Leydig cells, incubated with interstitial macrophages, to exogenous LH. This was due to high TNF-a production by macrophages during hyperprolactinaemia (Huang et al, 2003). Lower basal in vitro production of testosterone by WR Leydig cells occurred simultaneously with higher basal E 2 secretion.…”

“…(1999, 2001) found that hyperprolactinaemia in male rats diminished the response of Leydig cells, incubated with interstitial macrophages, to exogenous LH. This was due to high TNF‐α production by macrophages during hyperprolactinaemia (Huang et al. , 2003).…”

Regulatory Cytokine Expression and Interstitial Fluid Formation in the Normal and Inflamed Rat Testis Are Under Leydig Cell Control

“…With a ribonuclease protection assay, expression of TGFβ1, IFNγ, and MIF was clearly demonstrated in the testis, whereas IL‐6, TNFα, and IL‐10 mRNAs were barely detectable using this approach, in spite of the fact that the production of at least IL‐6 and TNF‐α has been described in the adult testis (De et al, 1993; Syed et al, 1993). However, previous studies either used extremely sensitive methods such as reverse transcription—polymerase chain reaction or investigated synthesis and production in isolated cells or a combination of both rather than total testis (De et al, 1993; Xiong and Hales, 1993; Kern et al, 1995; Huang et al, 2003), suggesting that the sensitivity of the assay was insufficient to detect expression of low copy number cytokines in total testes RNA. Conversely, this also suggests that TGFβ1, IFNγ, and MIF are relatively highly expressed cytokines in the testis.…”