Biotin uptake at specific phases of the cell cycle

The studies described in the previous section were conducted using non-synchronized cultures of PBMCs, i.e. populations of cells from various phases of the cell cycle. Thus, these studies did not allow identification of a specific phase of the cell cycle during which biotin uptake is increased: G1, S, G2 or M phase. This gap in knowledge was addressed in another series of studies (J.S. Stanley et al., 2002): proliferating PBMCs were arrested at designated phases of the cell cycle by incubation with one of the following chemicals:

  • i) wortmannin (100nmoll-1) to cause G1 arrest;
  • ii) aphidicolin (118 |lmol l-1) to cause arrest S arrest; (iii) doxorubicin (1 |lmoll-1) to cause G2 arrest; or (iv) colchicine (5 |lmol l-1) to cause M arrest; quiescent PBMCs (G0 phase) served as controls. Biotin uptake rates into these cells were measured using [3H]biotin.

Biotin uptake rates increased in the G1 phase of the cell cycle (4.5 times the control value in G0 phase) and remained increased during later phases of the cell cycle (S, G2 and M phase; Fig. 5.5). Differences among G1, S, G2, and M phase were not significant. These findings suggest (i) that the number of biotin transporters on the cell surface increases early in the cell cycle (see below); and (ii) that the number oftransporters is increased during the entire course of the cell cycle.

  1. 5.4. Uptake of [3H]biotin into proliferating human PBMCs and quiescent controls. Proliferation was induced by incubation with pokeweed lectin (0.5, 1.0 or 2.0 mg ml-1) at 37°C for 3 days; controls were incubated without pokeweed lectin. Values are means ± 1 sd; n = 6. *P < 0.01 vs. control by ANOVA with Dunnett's procedure (Zempleni and Mock, 1999c).
  2. 5.4. Uptake of [3H]biotin into proliferating human PBMCs and quiescent controls. Proliferation was induced by incubation with pokeweed lectin (0.5, 1.0 or 2.0 mg ml-1) at 37°C for 3 days; controls were incubated without pokeweed lectin. Values are means ± 1 sd; n = 6. *P < 0.01 vs. control by ANOVA with Dunnett's procedure (Zempleni and Mock, 1999c).
Fig. 5.5. Uptake of [3H]biotin into cell cycle-arrested human PBMCs. Values are means ± sd; n ♦Significantly different from controls (P < 0.01 vs. control) (Stanley et al., 2002).
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