INDUCTION OF ORNITHINE DECARBOXYLASE ACTIVITY IN MOUSE B-LYMPHOCYTES

We have examined the role in B cell activation of ornithine decarboxyl ase (ODC), the labile rate-limiting enzyme in the synthesis of polyami nes thought to be required for S phase entry in all cells. When small resting mouse splenic a cells were stimulated with the mitogenic agent s phorbol myristate acetate (PMA) plus ionomycin (lo), LPS or the a ce ll specific agent F(ab')(2) anti-lg, ODC activity was greatly increase d. ODC activity in small dense B cells remained near baseline levels f or the first 6 h after treatment with LPS, but then increased similar to 150-fold in the next 18 h. When purified a cells were not separated by cell density, ODC activity was 30-fold greater at baseline and ros e earlier after LPS stimulation, reaching a level about three times th at of LPS-stimulated small, dense B cells at 24 h, implying that large (preactivated) B cells have much greater ODC responses than small, de nse a cells. ODC activity, like S phase entry, could also be induced i n small, dense B cells by PMA and lo but failed to respond to either a gent alone. ODC levels rose transiently by similar to 40-fold between 2 and 6 h following stimulation of small B cells with F(ab')(2) anti-l g, then declined to baseline. Whole anti-lg did not stimulate ODC acti vity and also blocked the F(ab')(2) anti-lg mediated increase in ODC a ctivity, just as it produced the expected inhibition of thymidine inco rporation and cellular progression into S phase. Although IL-4 provide d the expected restoration of thymidine incorporation and S phase prog ression in response to whole anti-lg, it failed to restore the ODC res ponse, providing a striking example of cell cycle entry without increa sed ODC activity. ODC mRNA levels were not affected in proportion to O DC activity. Except for the 2-fold increase in ODC mRNA seen with PMA and lo, changes in steady-state ODC mRNA levels were remarkably absent . Inhibition of ODC by difluoromethylornithine had little effect on st eady-state mRNA levels for transferrin receptor, IL-2 receptor or, cla ss II MHC (la), CD23 or ODC itself, with or without PMA and lo. Thus, in the a cell, a large ODC activity increase was a common but not inva riable harbinger of cell cycle entry, changes in steady-state ODC mRNA levels were small or absent, ODC induction showed PMA-lo. synergy and ODC did not appear to regulate the mRNA levels of other activation ma rkers.