Osteolysis at distant skeletal sites in mice bearing intramuscular CHO/MIP-1α-secreting tumors. (A) Human MIP-1α was determined in media harvested from exponentially growing empty vector- and MIP-1α-expressing CHO cells (CHO/MIP-1α) by a specific ELISA. (B) Stimulation of osteoclastic bone resorption in ⁴⁵Ca-labeled fetal rat long bone organ cultures by media conditioned by CHO/MIP-1α cells after 5 days. Data are expressed as a ratio of the basal level of stimulation induced by an equivalent concentration of empty vector-transfected (CHO/EV) cells. (C) Tumors were induced by inoculating tumor cells into left thigh muscles of BALB/c nu/nu mice (n = 5 per group), and radiographs were obtained at baseline and thereafter weekly for 5 weeks. Osteolytic lesions were evident in the contralateral hind limbs of CHO/MIP-1α tumor-bearing mice from week 3 and increased progressively thereafter. Lesions were discernable in the contralateral hind limb of only one CHO/EV tumor-bearing mouse, and this was after 5 weeks. □ indicates week 3; ▦, week 4; and ▪, week 5. See “Statistical analysis.”