Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines, is a labile protein that is regulated by interacting with antizymes (AZs), a family of polyamine-induced proteins. Recently, a novel human gene highly homologous to ODC, termed ODC-like or ODC-paralogue (ODCp), was cloned, but the studies aimed to determine its function rendered contradictory results. We have cloned the mouse orthologue of human ODCp and studied its expression and possible function. mRNA of mouse Odcp was found in the brain and testes, showing a conserved expression pattern with regard to the human gene. Transfection of mouse Odcp in HEK 293T cells elicited an increase in ODC activity, but no signs of arginine decarboxylase activity were evident. On the other hand, whereas the ODCp protein was mainly localized in the mitochondrial/membrane fraction, ODC activity was found in the cytosolic fraction and was markedly decreased by small interfering RNA against human ODC. Co-transfection experiments with combinations of Odc, Az1, Az2, Az3, antizyme inhibitor (Azi), and Odcp genes showed that ODCp mimics the action of AZI, rescuing ODC from the effects of AZs and prevented ODC degradation by the proteasome. A direct interaction between ODCp and AZs was detected by immunoprecipitation experiments. We conclude that mouse ODCp has no intrinsic decarboxylase activity, but it acts as a novel antizyme inhibitory protein (AZI2).The polyamines spermidine and spermine and their precursor putrescine are ubiquitous polycations implicated in the growth, differentiation, and death of eukaryotic cells (1-4). Intracellular levels of polyamines are tightly regulated through multiple mechanisms affecting their biosynthesis, catabolism, and transport (5-9). In mammalian cells, putrescine synthesis, the first step in the polyamine biosynthetic pathway, is mediated by ornithine decarboxylase (ODC) 2 (EC 4.1.1.17) through the decarboxylation of L-ornithine. This enzyme is subject to a complex regulation by transcriptional, translational, and posttranslational mechanisms (10 -16). At the post-translational level, ODC is finely regulated by a family of inhibitory proteins called antizymes (AZ) (15,17,18). AZ1, the first described member of the family, binds to ODC monomers preventing the formation of active ODC homodimers and promoting the degradation of ODC through the 26 S proteasome in a ubiquitinindependent manner (19 -21). Synthesis of AZ is influenced by polyamines through the stimulation of ribosomal frameshifting (22,23). Moreover, the action of AZ on ODC function is also mediated by a protein called antizyme inhibitor (AZI). This protein, having a sequence highly similar to that of ODC, is devoid of ornithine decarboxylating activity; however, it can activate ODC by competing for AZ, because AZI binds to AZ with high affinity preventing or decreasing the formation of the ODC-AZ complex (24, 25). In addition, AZ1 and AZ2 not only decrease polyamine biosynthesis but also prevent the accumulation of excess polyamines by inhibiting or suppre...

The aminoacyl-imidazole dipeptides carnosine (beta-alanyl-L-histidine) and anserine (beta-alanyl-1-methyl-histidine) are present in relatively high concentrations in excitable tissues, such as muscle and nervous tissue. In the present study we describe the existence of a marked sexual dimorphism of carnosine and anserine in skeletal muscles of CD1 mice. In adult animals the concentrations of anserine were higher than those of carnosine in all skeletal muscles studied, and the content of aminoacyl-imidazole dipeptides was remarkably higher in males than in females. Postnatal ontogenic studies and hormonal manipulations indicated that carnosine synthesis was up-regulated by testosterone whereas anserine synthesis increased with age. Regional variations in the concentrations of the dipeptides were observed in both sexes, skeletal muscles from hind legs having higher amounts of carnosine and anserine than those present in fore legs or in the pectoral region. The concentration of L-lysine in skeletal muscles also showed regional variations and a sexual dimorphic pattern with females having higher levels than males in all muscles studied. The results suggest that these differences may be related with the anabolic action of androgens on skeletal muscle.

One of the processes that regulate intracellular levels of polyamines in mammalian cells is polyamine uptake. We have measured polyamine uptake in COS7 cells for putrescine, spermidine, and spermine, obtaining K m values of 4.5, 1.0, and 0.8 M, respectively. Treatment of nonconfluent cells with cycloheximide stimulated polyamine uptake and prevented the inhibitory effect found in cells preloaded with polyamines, suggesting the existence of a feedback repression mechanism mediated by antizymes. Transient transfected cells with mutated antizyme forms of AZ1, AZ2, and AZ3, which do not require frameshifting, showed a total blockade of polyamine uptake. Transfection of COS7 cells with mouse or human AZIN2, a novel member of the antizyme inhibitor family, recently characterized by our group, markedly stimulated polyamine uptake and counteracted the action of any of the three antizymes in co-transfected cells. The stimulatory effect of AZIN2 on polyamine uptake was abrogated when the putative antizyme binding sequence, formed by residues 117-140 in AZIN2, was deleted. Real time reverse transcription-PCR analysis of antizyme inhibitor transcripts revealed that in brain and testes AZIN2 is more expressed than AZIN1, especially in the testes where the relative expression was about 25-fold higher. Collectively, our results clearly indicate that AZIN2 affects polyamine homeostasis not only by increasing ornithine decarboxylase activity but also by stimulating polyamine uptake, through negating the inhibitory effect of the antizymes. This finding may have physiological relevance, mostly in testes where AZ3 and AZIN2 are mainly expressed.

LH plays a relevant role in folliculogenesis, ovulation, and luteinization. Although ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is a target of LH in the ovary, the functional significance of ODC induction has remained elusive. Our study reveals that the blockade of the induction of ovarian ODC by means of the specific inhibitor alpha-difluoromethylornithine (DFMO) affects folliculogenesis and luteinization. In immature female mice, DFMO was found to inhibit ovarian growth, the formation of Graafian follicles, and the secretion of progesterone and estradiol. In adult cycling females, the administration of DFMO on the evening/night of proestrus markedly decreased plasma progesterone levels at diestrus, which was associated to the decrease in the expression of steroidogenic factor 1, cytochrome cholesterol side chain cleavage enzyme, and steroidogenic acute regulatory protein in the ovary and to a reduced vascularization of the corpora lutea. These effects were not reverted by the administration of gonadotropins or prolactin. ODC immunoreactivity was also stimulated by LH in theca and granulosa cells of antral follicles but not in preantral follicles. Overall, these experiments demonstrate that elevated ODC values found in the ovary of immature and adult mice play a relevant function in ovarian physiology and that ODC/polyamines must be considered as important mediators of some of the effects of LH on follicular development and luteinization.

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