Tl(I) and Tl(III) alter the expression of EGF-dependent signals and cyclins required for pheochromocytoma (PC12) cell-cycle resumption and progression

Abstract

The effects of thallium [Tl(I) and Tl(III)] on the PC12 cell cycle were evaluated without (EGF−) or with (EGF+) media supplementation with epidermal growth factor (EGF). The following markers of cell-cycle phases were analyzed: cyclin D1 (G1); E2F-1, cyclin E and cytosolic p21 (G1→S transition); nuclear PCNA and cyclin A (S); and cyclin B1 (G2). The amount of cells in each phase and the activation of the signaling cascade triggered by EGF were also analyzed. Tl(I) and Tl(III) (5–100 μM) caused dissimilar effects on PC12 cell proliferation. In EGF− cells, Tl(I) increased the expression of G1→S transition markers and nuclear PCNA, without affecting cyclin A or cyclin B1. In addition to those, cyclin B1 was also increased in EGF+ cells. In EGF− cells, Tl(III) increased the expression of cyclin D1, all the G1→S and S phase markers and cyclin B1. In EGF+ cells, Tl(III) increased cyclin D1 expression and decreased all the markers of G1→S transition and the S phase. Even when these cations did not induce the activation of EGF receptor (EGFR) in EGF− cells, they promoted the phosphorylation of ERK1/2 and Akt. In the presence of EGF, the cations anticipated EGFR phosphorylation without affecting the kinetics of EGF-dependent ERK1/2 and Akt phosphorylation. Altogether, results indicate that Tl(I) promoted cell proliferation in both EGF− and EGF+ cells. In contrast, Tl(III) promoted the proliferation of EGF− cells but delayed it in EGF+ cells, which may be related to the toxic effects of this cation in PC12 cells.