We have previously reported that insulin receptor expression is increased in human breast cancer specimens (V. Papa et al., J. Clin. Invest., 85: 1503–1510, 1990). In the present study, in order to further understand the role of the insulin receptor in breast cancer, insulin receptor expression and function were characterized in three human breast cancer cell lines, MCF-7, ZR-75-1, and T-47D, and compared to a nonmalignant human breast epithelial cell line, 184B5. Insulin receptor content, measured by radioimmunoassay, was elevated 5- and 3-fold in MCF-7 and ZR-75-1 breast cancer cell lines, respectively, when compared to the nonmalignant cell line 184B5. In contrast, the insulin receptor content of T-47D cells was not increased. The increase in insulin receptor content in MCF-7 and ZR-75-1 cells was not due to amplification of the insulin receptor gene. Also, total insulin receptor mRNA content was not increased in breast cancer cells in respect to nonmalignantly transformed 184B5 breast epithelial cells. However, significant differences in the content of receptor mRNA species were observed.
The insulin receptors in the breast cancer cell lines were functional: (a) In all 4 cell lines, high-affinity insulin-binding sites were detected, and, in concert with the insulin receptor radioimmunoassay data, binding capacity was highest in MCF-7 and then in ZR-75-1 cells. (b) In all cell lines, insulin stimulated insulin receptor tyrosine kinase activity. However, the effect of insulin was greater in breast cancer cell lines than in nonmalignant breast cells. (c) In all cell lines, insulin at concentrations of 1 nm or less stimulated [3H]thymidine incorporation. This effect of insulin was inhibited by 50% in MCF-7 cells and by 60% in 184B5 cells when α-IR3, a monoclonal antibody to the insulin-like growth factor I receptor, was present. In these cells, therefore, insulin was active via both its own receptor and the IGF-I receptor. In contrast, α-IR3 antibody was without effect in T-47D and ZR-75-1 cells, suggesting that in these cell lines insulin acted only via its receptor. In the breast cancer cells, MA-5, an agonist monoclonal antibody to the insulin receptor, stimulated [3H]thymidine incorporation. This present study indicates therefore that in breast cancer cell lines there are functional insulin receptors that regulate breast cancer cell growth.
Supported by the Associazione Italiana per la Ricerca sul Cancro (AIRC), NIH Grant CA24844, The Office of Energy Research, Office of Health and Environmental Research of the United States Department of Energy under Contract DE-AC03-765500098, The John A. Kerner Foundation, The Jay Gershow Cancer Fund, and the Ladies Auxillary of the Veterans of Foreign Wars. G. M. is a recipient of an AIRC fellowship.