Abstract

Cultured murine hepatoma 1c1c7 cells were treated with either the actin filament-disrupting drug cytochalasin D or the microtubule inhibitors colchicine and nocadazole (NOC) to assess the role of the cytoskeleton in the process of cytochrome P450 Cyp1a-1 induction. Indirect fluorescence analyses demonstrated that microtubule or actin networks were disrupted within 1 hr of treatment and remained altered as long as cultures were maintained in the presence of the drugs. Treatment of cultures with cytochalasin D, colchicine, or NOC for 1 hr before the addition of dibenz[a,c]anthracene had no effect of Cyp1a-1 induction, as monitored by measurements of CYP1A1 mRNA. Pretreatment with NOC for > or = 18 hr produced populations of cells that had either a flat or rounded morphology. Both populations, when isolated 20-24 hr after NOC treatment, were arrested in the G2/M phase of the cell cycle (83-98% in G2/M versus approximately 7-10% in nontreated or solvent-treated cultures). Cyp1a-1 induction was suppressed in both of these populations, as monitored by measurement of CYP1A1 mRNA content (reductions of > 68%), 7-ethoxyresorufin O-deethylase activity (reductions of > 80%), or microsomal CYP1A1 protein content (reductions of > 80%). In contrast, overall [3H]leucine incorporation into protein was not affected. Cytosol prepared from these NOC-treated cultures bound approximately 39% of the radiolabeled 2,3,7,8-tetrachlorodibenzo-p-dioxin bound by cytosol isolated from solvent-treated cultures. Nuclear extracts prepared from cultures treated with NOC for 20-24 hr before in vivo exposure to inducer and cytoplasmic extracts isolated from similarly NOC-treated cultures that were exposed to inducer in vitro demonstrated reductions of > or = 54% and > or = 55%, respectively, in their abilities to bind to DNA, when analyzed by gel retardation analyses using an oligonucleotide corresponding to dioxin-responsive element D of the Cyp1a-1 gene. These studies suggest that ligand-dependent induction of Cyp1a-1 transcription is unaffected by short term disruption of the microfilament or microtubule network. However, long term exposure to microtubule inhibitors causes cells to pause in the G2/M stage of the cell cycle and modulates processes involved in the induction of Cyp1a-1 in these cells.