Mitogen-stimulated, proliferating PBMCs accumulate biotin at a rate five times faster than unstimulated controls (Zempleni and Mock,
1999c). For example, biotin uptake into PBMCs increased in a dose-dependent fashion from 481 to 722% of the control value in quiescent cells if proliferation was induced by incubation with pokeweed mitogen (lectin from Phytolacca americana) for 3 days (Fig. 5.4). A similar pattern of biotin uptake was observed when proliferation was induced by either concanavalin A or phytohaemagglutinin.
Next, PBMCs were incubated with mitogen for up to 4 days, and biotin transport rates were determined at timed intervals. Stimulation of biotin uptake paralleled the rate ofcell proliferation as judged by uptake of the proliferation marker thymidine into PBMCs (Zempleni and Mock, 1999c). Uptake of both biotin and thymidine was maximal 48-72 h after addition of poke-weed mitogen to the culture medium (Zempleni and Mock, 1999c). These data provide evidence that increased biotin transport into mitogen-stimulated PBMCs is caused by increased biotin demand due to proliferation. Moreover, short-term exposure (<15 min) of PBMCs to mitogens did not affect transport rates of biotin. This suggests that mitogens do not increase biotin transport by chemically interacting with biotin or transporters on the cell surface, but that effects of long-term stimulation with mitogens (1-4 days) are mediated by induction of cell proliferation.
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