OGF inhibits growth of human anaplastic thyroid cancer cells in a dose-dependent, receptor-mediated, and reversible manner. (A) Growth of KAT-18 cells subjected to various concentrations of opioid growth factor (OGF) of sterile water (Co) for 72 h. (B) Growth of KAT-18 cells treated with OGF (10-6 M) or an equivalent volume of sterile water (Co) over a 96-h period of time. (C) Opioid receptor mediation of the growth inhibitory effects of OGF in KAT-18 cells. Cell cultures were subjected to OGF (10-6 M), the opioid antagonist naloxone (Nal; 10-6 M), OGF and Nal, or sterile water (Co) for 72 h. (D) Reversibility of the growth inhibitory effects on KAT-18 cells treated with 10-6 M OGF or sterile water (Co). At 72 h, one-half of the culture plates continued to receive OGF for an additional 48 h, and one-half of the plates were treated with sterile water for 48 h. Control cultures received sterile water throughout the 120 h. For all experiments, data represent mean SE for at least 2 aliquots/well from 2 wells/group. Two to five experiments were conducted for each measure. Significantly different from respective controls at p < 0.05 (*), p < 0.01 (**), p < 0.001 (***).