INTERLEUKIN-1 STIMULATES DEOXYRIBONUCLEIC-ACID SYNTHESIS IN IMMATURE RAT LEYDIG-CELLS INVITRO

The number of interstitial macrophages in the testis fluctuates according to age, increasing gradually during prepubertal development to reach 15-20% in the interstitial compartment in the adult rat. These macrophages are in close morphological association with Leydig cells. Macrophage products, interleukin-1 (IL-1) and tumor necrosis factor a stimulate and/or inhibit steroid production in cultures of Leydig cells. We have studied the effects of macrophage products on DNA synthesis in rat Leydig cells to investigate a possible paracrine role of testicular macrophage products on the proliferation of Leydig cells. Leydig cells isolated from 10-, 20-, and 70-day-old rats were cultured for 48 h in serum-free medium, washed, and treated with different cytokines for 18 h. The medium was then removed, fresh medium containing 0.5 muCi [H-3]thymidine was added, and cells were incubated for 4 h prior to determining the incorporation of [H-3]thymidine into DNA. Human recombinant IL-1beta caused a dose-dependent stimulation in the incorporation of [H-3]thymidine into DNA in the Leydig cells from 10- and 20-day-old rats but had no effect on DNA synthesis in interstitial cells from adult rats. Maximum stimulation of DNA synthesis in immature Leydig cells was observed with 1-2 ng/ml IL-1beta. Autoradiography after incubation with [H-3]thymidine showed a dramatic increase in the number of labeled Leydig cells after treatment with IL-1beta (19.27 +/- 3.77% vs. 1.44 +/- 0.52% in control cultures) indicating that IL-1beta recruited more cells to enter the cell cycle and initiate DNA synthesis. Human recombinant IL-1alpha and tumor necrosis factor a also caused significant stimulation of DNA synthesis in Leydig cells but these cytokines were much less potent (1-10%) than IL-1beta. IL-1beta enhanced the effects of maximally effective concentrations of growth-promoting agents previously known to stimulate DNA synthesis in immature rat Leydig cells, i.e. human CG, steroidogenesis-inducing protein, and transforming growth factor a plus insulin. On the basis of these results it is concluded that IL-1 may play an important role in the proliferation of Leydig cells during prepubertal development in immature rats.