The testicular gene expression of the retinoic acid receptors, RARα,
-β, and -γ, was studied in normal mice and in vitamin A-deficient
mice after the administration of all-trans-retinoic acid
(ATRA). ...All three types of RARs were expressed in normal and/or vitamin
A-deficient testes. Only the expression of RARβ messenger RNA was
transiently induced within 24 h after ATRA injection. ATRA-induced
RARβ expression was also found in purified Sertoli cells, suggesting
that these cells mediate at least part of the effect of retinoids on
germ cells. When an equimolar amount of retinol was administered
instead of ATRA, no induction of RARβ was seen at the point of
maximal induction by ATRA, suggesting that the effect of retinol was
delayed and probably less.
The related nuclear receptors, RXRα, -β, and, for the first time,γ
, were also shown to be present in the mouse testis. Upon
administration of ATRA, messenger RNA expression of RXRα and -β did
not change significantly. The expression of RXRγ was too low to allow
quantification.
Finally, the effect of the retinoid metabolism inhibitor liarozole on
ATRA-induced proliferation of A spermatogonia was examined. The
labeling index of A spermatogonia, 24 h after the administration
of 0.25 mg ATRA, was significantly lowered by liarozole due to a shift
of the maximal 5-bromo-deoxyuridine incorporation to an earlier point
(20 h). This indicates that liarozole delays retinoid metabolism,
thereby increasing the actual ATRA concentration, and more importantly,
that ATRA by itself is an active retinoid in spermatogenesis.
Apparently, ATRA does not need to be metabolized to 4-oxo-RA, which was
previously shown to be a more potent inducer of spermatogonial
proliferation than ATRA, to be effective.
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