Epidermal Growth Factor Receptors in Human Breast Carcinoma Cells: A Potential Selective Target for Transforming Growth Factor α-Pseudomonas Exotoxin 40 Fusion Protein

Abstract

Epidermal growth factor (EGF) receptors are expressed in high levels by some poor prognosis breast tumors. We have examined the cytotoxic effect of the tumor growth factor α (TGFα)-ΔCys-Pseudomonas exotoxin (PE40) recombinant fusion protein on normal and tumorigenic human breast epithelial cells in vitro and in vivo. The MDA-468, MDA-231, BT-20, and MCF-7_ADR estrogen receptor-negative, EGF receptor-rich breast cancer lines were exquisitely sensitive in vitro to TGFα-ΔCys-PE40 with a 50% inhibitory concentration of ≤0.02 nm. The estrogen receptor-positive, low EGF receptor MCF-7, ZR75-1, and T47D cells were less sensitive to the fusion toxin with a 50% inhibitory concentration of >0.2 nm. The nontumorigenic cell lines 184, 184A1, and 184B5 were relatively resistant to TGFα-ΔCys-PE40 despite exhibiting high levels of EGF receptors. Continuous i.p. administration of TGFα-ΔCys-PE40 via an osmotic minipump at a dose of 0.4 µg/g/day over 7 days inhibited MDA-468, MA-231, and BT-20 but not MCF-7 tumor growth in female athymic mice. Host tissue toxicity was not observed with this dose of TGFα-ΔCys-PE40. Mixed MDA-468/MCF-7 tumors were established in nude mice after coinoculation of both cell types in estrogen-supplemented animals. EGF receptor immunohistochemistry and immunoblot procedures indicated that TGFα-PE40 eliminated the MDA-468 cells while sparing the adjacent MCF-7 cells. By immunoblot, EGF receptors were consistently more abundant in tumor tissue than in adjacent nontumor tissue from the same mastectomy specimen (n = 7). These data support the notion that EGF receptors can be selectively targeted in human breast cancer cells for the delivery of antitumor agents. Further clinical studies with TGFα-ΔCys-PE40 and other chimeric toxins using the same cellular target will address this possibility.