Overexpression of antizyme in the hearts of transgenic mice prevents the isoprenaline-induced increase in cardiac ornithine decarboxylase activity and polyamines, but does not prevent cardiac hypertrophy

Two lines of transgenic mice were produced with constitutive expression of antizyme-1 in the heart, driven from the cardiac alpha-myosin heavy chain p romoter. The use of engineered antizyme cDNA in which nucleotide 205 had be en deleted eliminated the need for polyamine-mediated frameshifting, normal ly necessary for translation of antizyme mRNA, and thus ensured the constit utive expression of antizyme. Antizyme-1 is thought to be a major factor in regulating cellular polyamine content, acting both to inhibit ornithine de carboxylase (ODC) activity and to target it for degradation, as well as pre venting polyamine uptake. The two transgenic lines had substantial, but dif ferent, levels of antizyme in the heart, as detected by Western blotting an d by the ability of heart extracts to inhibit exogenous purified ODC. Despi te the high levels of antizyme, endogenous ODC activity was not completely abolished, with 10-39% remaining, depending on the transgenic line. Additio nally, a relatively small decrease (30-32%) in cardiac spermidine content w as observed, with levels of putrescine and spermine unaffected. Interesting ly, although the two lines of transgenic mice had different antizyme expres sion levels, they had almost identical cardiac polyamine content. When trea ted with a single acute dose of isoprenaline (isoproterenol), cardiac ODC a ctivity and putrescine content were substantially increased (by 14-fold and 4.7-fold respectively) in non-transgenic littermate mice, but these increa ses were completely prevented in the transgenic mice from both founder line s. Prolonged exposure to isoprenaline also caused increases in cardiac ODC activity and polyamine content, as well as an increase in cardiac growth, i n non-transgenic mice. Although the increases in cardiac ODC activity and p olyamine content were prevented in the transgenic mice from both founder li nes, the increase in cardiac growth was unaffected. These transgenic mice t hus provide a valuable model system in which to study the importance of pol yamine levels in cardiac growth and electrophysiology in response to stress .