Gallium chloride (GaCI3), an antitumor agent with antagonistic action on iron, magnesium and calcium, was tested for its ability to alter the polymerization of purified tubulin (2.2 mg/ ml) in a cell-free system in vitro. GaCI3 (250 μM) does not mimic the effect of 10 μM paclitaxel and, therefore, is not a microtubule (MT)-stabilizing agent that can promote tubulin polymerization in the absence of glycerol and block MT disassembly. In contrast, GaCI3 mimics the effect of 1 μM vincristine (VCR) and inhibits glycerol-induced tubulin polymerization in a concentration-dependent manner (IC50: 125 μM), indicating that GaCI3 is a MT de-stabilizing agent that prevents MT assembly. However, 150 μM GaCI3 must be used to match or surpass the inhibitions of tubulin polymerization caused by 0.25 μM of known MT de-stabilizing agents, such as colchicine (CLC), nocodazole, podophyllotoxin, tubulozole-C and VCR. The inhibitory effect of 250 μM GaCI3 persists in the presence of up to 9 mM MgCI2, suggesting that the exogenous Mg2+ cations absolutely required for the binding of GTP to tubulin and MT assembly cannot overcome the antitubulin action of Ga3+ ions of a higher valence. The binding of [3H]vinblastine (VBL) to tubulin (0.5 mg/ml) is inhibited by unlabeled VBL but enhanced by concentrations of GaCI3 > 200 μM. However, increasing concentrations of GaCI3 mimic the ability of cold CLC to reduce the amount of [3H]CLC bound to tubulin, suggesting that GaCI3 may interact with the CLC binding site to inhibit tubulin polymerization. The binding of [3H]GTP to tubulin is decreased by unlabeled GTP but markedly enhanced by GaCI3, especially when concentrations of this metal salt of 32 μM or higher are added to the reaction mixture before rather than after the radiolabeled nucleotide. These data suggest that changes in protein conformation following GaCI3 binding might increase the interactions of tubulin with nucleotides and Vinca alkaloids. After a 24 h delay, the viability of GaCI3-treated L1210 leukemic cells is reduced in a concentration-dependent manner at days 2 (IC50:175 μM), 3 (IC50: 35 μM)and 4 (IC50:16 μM). Since GaCI3 (100–625 μM) increases the percentage of mitotic cells at 2–4 days, it might arrest tumor cell progression in M phase, but its antimitotic activity is much weaker than that of 0.25 μM VCR. Because the concentrations of GaCI3 that inhibit tubulin polymerization also increase the mitotic index and decrease the viability of L1210 cells in vitro, the antitubulin and antimitotic effects of GaCI3 might contribute, at least in part, to its antitumor activity.
