In the present study, we report the cloning of a gene that is differen
tially expressed in normal adult rat Leydig cells and whose expression
is inhibited by hCG but is induced by interferon-gamma (IFN gamma). D
NA sequence analysis identified this gene as rat IFN gamma-inducible p
rotein 10 (TP 10), a member of the -C-X-C- chemokine superfamily of pr
oinflammatory cytokines. High levels of IP-10 messenger RNA (mRNA) wer
e constitutively expressed in freshly isolated and primary cultured Le
ydig cells. hCG inhibited this expression in a dose-dependent manner.
The addition of 1 ng/ml hCG inhibited IP-10 mRNA levels more than 80%.
Conversely, IP-10 mRNA levels were markedly increased in response to
murine interleukin-1 alpha, murine tumor necrosis factor-alpha, and mu
rine IFN gamma by 3.3-, 10-, and 26-fold, respectively. Concomitant ad
dition of murine interleukin-la, murine tumor necrosis factor-alpha, a
nd murine IFN gamma synergistically increased IP-10 mRNA levels by 58-
fold. Furthermore, in addition to one previously described rat IP-10 m
RNA transcript (1.5 kb), another larger transcript (2.7 kb) was identi
fied by Northern blot in rat Leydig cells. After screening a rat testi
s complementary DNA library, we obtained a partial structural gene and
an intron sequence, which possibly originated from the larger transcr
ipt of rat IP-10 mRNA. Histochemical and immunocytochemical staining r
evealed that purified cells were positive for 3 beta-hydroxysteroid de
hydrogenase and IP-10, confirming that IP-10 is indeed present in Leyd
ig cells. IP-10 antisense oligonucleotides enhanced basal and hCG-indu
ced testosterone formation. This suggests that endogenous IP-10 has an
inhibitory effect on Leydig cell steroidogenesis. In conclusion, IP-1
0 is expressed in rat Leydig cells and may have paracrine and autocrin
e effects on testicular function.