Mast cells (MC) occur normally in the testis with a species-specific distribution, yet their precise role remains unclear. Testicular MC express histidine decarboxylase (HDC), the unique enzyme responsible for histamine (HA) generation. Evidence to date supports a role for HA as a local regulator of steroidogenesis via functional H₁ and H₂ receptor subtypes (HRH1 and HRH2, respectively) present in Leydig cells. Given that HA is a well-known modulator of physiological and pathological proliferation in many different cell types, we aimed in the present study to evaluate whether HA might contribute to the regulation of Leydig cell number as well as to the control of androgen production. Herein, we demonstrate, to our knowledge for the first time, that MA-10 Leydig tumor cells, but not normal immature Leydig cells (ILC), exhibit a proliferative response upon stimulation with HA that involves HRH2 activation, transient elevation of cAMP levels, and increased extracellular signal-regulated kinase (ERK) phosphorylation. Our results also reveal that MA-10 cells show significantly heightened HDC expression compared to normal ILC or whole-testicular lysate and that inhibition of HDC activity decreases MA-10 cell proliferation, suggesting a possible correlation between autocrine overproduction of HA and abnormally increased proliferation in Leydig cells. The facts that germ cells are also both source and target of HA and that multiple testicular cells are susceptible to HA action underline the importance of the present study, which we hope will serve as a first step for further research into regulation of non-MC-related HDC expression within the testis and its significance for testicular function.

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Testicular Leydig cells (LC) are modulated by several pathways, one of them being the histaminergic system. Heme oxygenase-1 (HO-1), whose upregulation comprises the primary response to oxidative noxae, has a central homeostatic role and might dysregulate LC functions when induced. In this report, we aimed to determine how hemin, an HO-1 inducer, affects LC proliferative capacity and whether HO-1 effects on LC functions are reversible. It was also evaluated if HO-1 interacts in any way with histamine, affecting its regulatory action over LC. MA-10 and R2C cell lines and immature rat LC were used as models. Firstly, we show that after a 24-h incubation with 25 µmol/L hemin, LC proliferation is reversibly impaired by cell cycle arrest in G2/M phase, with no evidence of apoptosis induction. Even though steroid production is abrogated after a 48-h exposure to 25 µmol/L hemin, steroidogenesis can be restored to control levels in a time-dependent manner if the inducer is removed from the medium. Regarding HO-1 and histamine interaction, it is shown that hemin abrogates histamine biphasic effect on steroidogenesis and proliferation. Working with histamine receptors agonists, we elucidated that HO-1 induction affects the regulation mediated by receptor types 1, 2 and 4. In summary, HO-1 induction arrests LC functions, inhibiting steroid production and cell cycle progression. Despite their reversibility, HO-1 actions might negatively influence critical phases of LC development and differentiation affecting their function as well as other androgen-dependent organs. What’s more, we have described a hitherto unknown interaction between HO-1 induction and histamine effects.