Initiation of DNA synthesis in murine B cells is independent of early changes in the cytosolic free calcium

Abstract

The relationship between changes in the intracellular free Ca 2+ concentration, [Ca 2+] i, and the initiation of proliferation of murine B cells after the addition of mitogens and activators was studied. The effects of lipopolysaccharide (LPS), 12- O-tetradecanoyl phorbol-13-acetate (TPA), rabbit IgG antimouse Fab (IgG RAM Fab), and its F(ab′) 2 fragment (F(ab′) 2 anti-Fab) on the [Ca 2+] i were measured using the fluorescent calcium indicator Fura-2. In parallel experiments, DNA and/or RNA synthesis were measured by assaying [ 3H]thymidine and/or [ 3H]uridine uptake. LPS stimulated a 20–120 × increase in the [ 3H]thymidine uptake, and a 3–7 × increase in [ 3H]uridine uptake without inducing any change in the [Ca 2+] i. TPA induced a marginal increase in [ 3H]thymidine and [ 3H]uridine uptake, without effecting any change in the [Ca 2+] i. In contrast, low doses of IgG RAM Fab produced a triphasic change in the [Ca 2+] i, but had no effect on the [ 3H]thymidine or [ 3H]uridine uptake, even at much higher concentrations. Similarly, low doses of the F(ab′) 2 fragment induced sizable increases in the [Ca 2+] i without affecting the [ 3H]nucleoside uptake. However, higher concentrations of F(ab′) 2 anti Fab increased the [ 3H]thymidine uptake and [ 3H]uridine uptake, while also increasing the [Ca 2+] i. Significantly, pretreating the cells with TPA for 3 min virtually abolished the [Ca 2+] i increase induced by IgG RAM Fab while simultaneously potentiating an increase in the IgG RAM Fab-induced [ 3H]thymidine uptake 85-fold. In the presence of TPA, IgG RAM Fab also induced a 2- to 30-fold increase in [ 3H]uridine uptake. Similarly, TPA virtually abolished the [Ca 2+] i increase induced by the F(ab′) 2 anti-Fab fragment, yet it stimulated a F(ab′) 2 anti-Fab-induced uptake of [ 3H]thymidine and [ 3H]uridine by 120 and 10 times, respectively.