Human ornithine decarboxylase paralogue (ODCp) is an antizyme inhibitor but not an arginine decarboxylase

Abstract

ODC (ornithine decarboxylase), the rate-limiting enzyme in polyamine biosynthesis, is regulated by specific inhibitors, AZs (antizymes), which in turn are inhibited by AZI (AZ inhibitor). We originally identified and cloned the cDNA for a novel human ODC-like protein called ODCp (ODC paralogue). Since ODCp was devoid of ODC catalytic activity, we proposed that ODCp is a novel form of AZI. ODCp has subsequently been suggested to function either as mammalian ADC (arginine decarboxylase) or as AZI in mice. Here, we report that human ODCp is a novel AZI (AZIN2). By using yeast two-hybrid screening and in vitro binding assay, we show that ODCp binds AZ1-3. Measurements of the ODC activity and ODC degradation assay reveal that ODCp inhibits AZ1 function as efficiently as AZI both in vitro and in vivo. We further demonstrate that the degradation of ODCp is ubiquitin-dependent and AZ1-independent similar to the degradation of AZI. We also show that human ODCp has no intrinsic ADC activity.

Antizyme Inhibitor 2 (AZIN2/ODCp) Stimulates Polyamine Uptake in Mammalian Cells

“…These results have later been corroborated for human ODCp by others (30). AZIN2/ODCp is mainly expressed in testes and brain both in mice and human (29,31), and by both functional and co-immunoprecipitation experiments, we demonstrate that this protein is able to interact with the three antizyme isoforms as is also shown for the former antizyme inhibitor, the AZIN1 (32).…”

“…Nevertheless, in COS7 cells transiently co-transfected with AZ1 and AZIN1 or AZIN2, both antizyme inhibitors exerted a similar effect (result not shown). In this regard, both mouse and human AZIN1 and AZIN2 showed similar capacity in counteracting the effects of AZs on ODC activity (29,30).…”

Antizyme and antizyme inhibitor, a regulatory tango

“…ODCp was demonstrated to lack ornithine decarboxylase and arginine decarboxylase activity, but is able to rescue ODC from degradation and to inhibit the antizyme-mediated inhibition of polyamine uptake; therefore, it was termed antizyme inhibitor 2 [109][110][111][112][113]. Like antizyme inhibitor 1, antizyme inhibitor 2 is also rapidly degraded in a ubiquitin-dependent manner [111,113]; it stimulates ODC activity, polyamine uptake and cellular proliferation although less efficiently than antizyme inhibitor 1 [113]. Antizyme inhibitor 2 and antizyme 3 are both expressed in the haploid germinal cells, a location different from that of ODC mRNA, which localizes predominantly to the outer part of the seminiferous tubules.…”

Polyamine-independent Expression of Caenorhabditis elegans Antizyme

“…Because protein sequences of ODC and AZI are usually very similar, we identified the gene F53F10.2, which is annotated as a C. elegans ODC-1 paralogue, to be the most likely candidate for C. elegans AZI. Remarkably, the longest F53F10.2 transcript (isoform a) encodes a protein of 477 amino acids that shows highest sequence similarity with the filarial ODC-like proteins of Loa loa and Brugia malayi (31% identities, data not shown), followed by the human ODC paralogue (29% identities; data not shown), which has recently been described to be an AZI (35). Furthermore, the F53F10.2a sequence lacks several amino acids described to be crucial for ODC function (14).…”

“…Although in mammalia, the antizyme inhibitor binds the antizyme, hence freeing ODC from antizyme suppression, it also does not promote but rather protects it from degradation (3). Here it is important to note that the putative C. elegans AZI shows higher homologies to the mammalian ODC paralogue also termed antizyme inhibitor AZI-two than toward the mammalian AZI-one (35). AZI-two expression has been found in testis and brain and more recently also in secretory tissues and other cell types (44,45).…”