All-trans-4-oxo-retinoic acid: A potent inducer of in vivo proliferation of growth-arrested A spermatogonia in the vitamin A-deficient mouse testis

Vitamin A deficiency leads to an arrest of spermatogenesis and a loss of advanced germ cells in male mice. In the present study, the effects of several retinoids and carotenoids on these mouse testis were investigated. First, the proliferative activity of the growth-arrested A spermatogonia in vitamin A-deficient (VAD) mice testis was determined, 20, 24, or 28 h after administration of 0.5 mg all-trans-retinoic acid (RA). The bromodeoxy-uridine (BrdU) labeling index of A spermatogonia in control VAD testis was 5 +/- 1% (n = 4, mean +/- SD). When RA was injected (ip), the highest labeling index was found 24 h after RA administration: 49 +/- 5%. When various concentrations of RA, all-trans-4-oxo-retinoic acid (4-oxo-RA) or all-trans-retinol acetate (ROAc), ranging from 0.13-1 mg, were injected, the labeling index of A spermatogonia always increased in comparison with the VAD situation. A maximum index at 24 h was found when 0.5 mg 4-oxo-RA was injected: 56 +/- 3%. This labeling index was even higher than those after injection of RA or ROAc, 49 +/- 5% and 34 +/- 6% respectively. The increase of the BrdU labeling index was dose dependent. After an initial increase of the labeling indices with increasing retinoid doses, the labeling indices decreased at a higher concentration. This decrease is likely due to a concentration dependent timeshift of the optimum of BrdU labeling to shorter time intervals after retinoid administration because a labeling index of 66 +/- 1% was found 20 h after injection of 1 mg RA. At 24 h, this labeling index was halved: 33 +/- 2%. These indices show that the degree of synchronization of spermatogenesis is also dependent on the retinoid dose. When the dimers of RA and 4-oxo-RA, respectively beta-carotene (PC) and canthaxanthin, were given, 24 h after administration BrdU-labeling indices comparable with the VAD value were found. Repeated injection of beta C twice a week did induce a reinitiation of spermatogenesis, but compared with RA, the activity of beta C was lower and delayed. It is concluded that 4-oxo-RA is active in adult mammals in vivo. It is at least as potent as RA in the induction of the differentiation and subsequent proliferation of growth-arrested A spermatogonia in VAD mice testis. Furthermore, the degree of synchronization of spermatogenesis is influenced by the retinoid dose. Finally, carotenoids were shown to act in the induction of spermatogonial cell proliferation too but with a lower and delayed activity.