REGULATION OF ORNITHINE DECARBOXYLASE BY HYPOXIA IN PULMONARY-ARTERY SMOOTH-MUSCLE CELLS

The polyamines are a family of low-molecular-weight organic cations th at play essential intracellular regulatory roles in cell growth and di fferentiation. Elevations in cellular polyamine contents necessary for most physiological and pathological events in the lung appear to be d riven by increased de novo synthesis. In contrast, increases in lung c ell polyamines required for hypoxic pulmonary vascular disease can be attributed to augmented transmembrane polyamine transport which may, i n turn, be the result of hypoxia-related decreases in the activity of the initial and generally rate-limiting enzyme in de novo polyamine sy nthesis, ornithine decarboxylase (ODC). To begin to define the unusual mechanism whereby hypoxia governs polyamine regulatory pathways, the present study examined the impact of varying severity and durations of hypoxic exposure on ODC activity and mRNA content in cultured bovine main pulmonary artery smooth muscle cells (PASMC). The effect of hypox ia on the activity of another rate-limiting enzyme in polyamine synthe sis, S-adenosylmethionine decarboxylase (AdoMet-DC), also was examined . Hypoxia caused time-dependent decreases in ODC and AdoMet-DC activit ies that were related to the severity of hypoxic exposure. Similarly, ODC mRNA content also was depressed by hypoxic exposure. The relations hip between the decline in ODC activity and mRNA content was roughly l inear. To determine whether hypoxia impairs ODC mRNA stability, two di fferent inhibitors of transcription and Northern analyses were used to follow the decay in ODC mRNA abundance in hypoxic and normoxic PASMC. Densitometric scanning of Northern analysis indicated that ODC mRNA s tability did not differ between hypoxic and normoxic PASMC. These resu lts suggest that the reduction in ODC activity provoked by hypoxia in cultured bovine PASMC can be ascribed in part to a diminished transcri ptional rate rather than to alterations in mRNA stability.