Regulation of mammalian S-adenosylmethionine decarboxylase as studied in a transient expression system

Mammalian S-adenosylmethionine decarboxylase (AdoMetDC), which catalyzes a key step in the biosynthesis of polyamines, is regulated by a multitude of mechanisms. The polyamines exert a strong feedback control of the enzyme. In the present study we have used a transient expression system to study the regulation of mammalian AdoMetDC. COS cells were transfected with a SV 40-based expression vector containing a 5'- and 3'-truncated human AdoMetDC cDNA (pSDC:16). The cells were shown to contain high levels of AdoMetDC activity 2 days after expression. This was partly due to an increase in the synthesis of the enzyme. However a marked stabilization of the enzyme against degradation did also contribute to the high AdoMetDC activity seen in the COS cells after the transfection with pSDC:16. The high expression of AdoMetDC was reflected in a marked change in intracellular polyamine levels. The cells were almost depleted of their putrescine, and their spermidine content was decreased to about 35% of that found in the mock-transfected cells. The spermine content, on the other hand, was increased. This change in polyamine levels was most likely attributable to the pSDC:16-induced increase in decarboxylated S-adenosylmethinine, which favors the accumulation of spermine at the expenses of putrescine and spermidine. The effects on the expression of AdoMetDC of polyamine synthesis inhibitors varied dependent on whether the COS cells were transfected with control vector or pSDC:16, in spite of similar effects on cellular polyamine levels, indicating a difference in feedback regulation of 'native' and recombinant AdoMetDC. The construct used in the present study gave rise to an AdoMetDC mRNA without a 5' and devoid of most of the 3' untranslated regions. However, whether these parts of the mRNA are involved in the polyamine-mediated translational control of the enzyme remains to be confirmed.