Both primary and acquired resistance to the growth-inhibitory effects of anti-estrogens (e.g., tamoxifen) limits the clinical usefulness of these drugs in the treatment of breast cancer. The new, steroidal anti-estrogen ICI 182,780 was tested for its ability to inhibit the proliferation of a tamoxifen-resistant variant of the parental MCF-7 human breast-cancer cell line. Two cell lines cloned from the MCF-7 line were used for these experiments: a tamoxifen-sensitive line, MCF 5-21, and a tamoxifen-resistant line, MCF 5-23. Compared with tamoxifen, ICI 182,780 appeared to be 150 and 1540 times more effective in inhibiting cell growth in the 5-21 and 5-23 sub-lines respectively. ICI 182,780 completely circumvented tamoxifen resistance at a concentration of (5 to 10) x 10(-9) M in this model. Based on IC50 concentrations, the 5-23 line was 22-fold more resistant to tamoxifen than the 5-21 line, but only 2-fold more resistant to ICI 182,780, reducing relative resistance by 10-fold in the resistant line. There were no differences in ER parameters between the 2 lines. ER numbers/cell were: 40500 and 34800 and the KD 0.48 and 0.15 x 10(-9) M in the 5-21 and 5-23 cells respectively. In the 5-23 cells, the concentrations of ICI 182,780 and tamoxifen resulting in a 50% inhibition of 3H-estradiol binding were 2.3 x 10(-8) M and 1 x 10(-6) M, respectively (cf. estradiol 0.89 x 10(-9) M). Thus, one potential mechanism for the increased effectiveness of ICI 182,780 may relate to the increased affinity of this drug for the estrogen receptor as compared with tamoxifen.

Separate mechanisms underlying the multidrug resistant (MDR) phenotype were identified in 2 independent approaches to select tumour cells resistant to low concentrations of doxorubicin (Dox) from the sensitive T cell leukemia cell line CCRF-CEM. The CEM/A7 cell line was selected at an initial concentration of 0.005 microgram/ml of Dox and maintained at 0.07 microgram/ml. In contrast, the CEM/A5 line was selected using an initial concentration of 0.01 microgram/ml and maintained in Dox at a concentration of 0.05 microgram/ml. P-glycoprotein expression was demonstrated in the CEM/A7 line but not the CEM/A5 line. Amplification of the mdrI gene was not observed in the CEM/A7 cell line. Both cell lines showed cross-resistance to a number of structurally unrelated cytotoxic drugs including anthracyclines and etoposide (VP-16), although only the CEM/A7 line was cross resistant to Vinca alkaloids. Immunoblots of total cell lysates of the CEM/A5 line have revealed almost undetectable levels of topoisomerase II alpha and beta in this line. Cytogenetic analyses of both lines revealed numerous karyotypic abnormalities which were present in the parental cell line as well as both resistant cell lines. The CEM/A7 line also demonstrated a duplication of part of the long arm of chromosome 7 which included the region containing the mdrI gene, a finding not seen in the parental or CEM/A5 line. CEM/A5, however, demonstrated an abnormality of chromosome 7, outside the region of the mdrI gene, and it also contained a deletion of the short arm of chromosome 2. Abnormalities in this latter region of genome have been associated with non-P-glycoprotein-mediated MDR.

A study was made of the enzymic degradation of (125)I-labelled parathyroid hormone by rat kidney microsomes. Incubation with microsomes resulted in rapid destruction of the labelled hormone. The microsomal factor was not separable by dialysis, and the reaction was favoured by pH values in the physiological range. Velocity of the reaction varied directly as the substrate concentration, and additional crude parathyroid hormone (trichloroacetic acid-precipitated, 3.68mg./ml.) inhibited destruction of labelled hormone. There was much less inhibition with added trichloroacetic acid-precipitated calcitonin (3.92mg./ml.) and virtually none with added pig insulin (3.80mg./ml.). Gel filtration of control medium on P6 (Bio-Gel) yielded one radioactive peak at the void volume. After incubation with microsomes three further peaks were obtained on gel filtration. Only the void-volume peak contained intact (125)I-labelled parathyroid hormone, indicating that the microsomal enzyme degraded labelled hormone to a number of smaller fragments.

A human lung cancer cell line (BEN cells) with a calcitonin receptor and calcitonin-responsive adenylate cyclase also possesses an insulin receptor. This has been characterized and found to have properties similar to those of other mammalian cell insulin receptors. A receptor number of 58 000 per cell was calculated from curvilinear Scatchard plots, and dissociation of bound labelled insulin by dilution was facilitated by the addition of unlabelled insulin, consistent with negatively co-operative interactions among binding sites. Preincubation of cells with either calcitonin or insulin led to loss of hormone binding in washed cells. In the case of calcitonin this was associated with loss of adenylate cyclase response. For each hormone the state of down-regulation was characterized by a decrease in receptor number, and for calcitonin there was also a low in sensitivity of adenylate cyclase. Down-regulation to calcitonin was more rapid than that to insulin and in each case recovery had occurred by 16 h after removal of the hormone. Induction of down-regulation was specific, in that preincubation with one hormone did not influence the subsequent binding or response of the other. Such data are consistent with independent modulation of peptide receptors in the same cell.