Endocrinology, Vol 130, 3246-3256, Copyright © 1992 by Endocrine Society

ARTICLES

Characterization of multidrug-resistant pituitary tumor cells

EJ Nelson and PM Hinkle
Department of Pharmacology, University of Rochester School of Medicine and Dentistry, New York 14642.

These studies were designed to investigate the role of P-glycoprotein in an endocrine cell line. Drug-resistant pituitary cells were obtained by growing GH4C1 cells in the presence of increasing concentrations of colchicine. Cells resistant to colchicine at 0.4 micrograms/ml, termed GH4C1/RC.4, exhibited the multidrug-resistance phenotype, as the LD50 values for colchicine, puromycin, actinomycin D, and doxorubicin were between 8 and 30 times greater than the corresponding values for the parental GH4C1 cells. Verapamil at 10 microM increased the sensitivity of GH4C1/RC.4 cells to colchicine, puromycin, and actinomycin D, almost completely reversing the drug resistance. Flow cytometry and fluorescence microscopy were used to demonstrate that GH4C1/RC.4 cells retained less rhodamine 123 than GH4C1 cells, and that the rate of efflux of rhodamine 123 was much faster for GH4C1/RC.4 cells. Immunocytochemical staining with a monoclonal antibody, C219, to the 170-kilodalton P-glycoprotein showed directly that GH4C1/RC.4 cells overexpress P-glycoprotein. We used drug-resistant pituitary cells to assess the possible role of P-glycoprotein in uptake and efflux of several hormones. At equilibrium, GH4C1 and GH4C1/RC.4 cells bound similar amounts of [125I]L-triiodothyronine and [125I]L-thyroxine, and verapamil did not alter either equilibrium binding or thyroid hormone efflux kinetics. Multidrug-resistant GH4C1/RC.4 cells retained less [3H]hydrocortisone than parental GH4C1 cells at equilibrium, and verapamil increased the equilibrium concentration of [3H]hydrocortisone 3.6-fold. The effect of verapamil was due to its ability to reverse multidrug resistance, since two other chemosensitizers, quinidine and vinblastine, increased [3H]hydrocortisone retention as effectively as verapamil but another calcium channel blocker, nifedipine, had no effect. The drug-resistant GH4C1/RC.4 line synthesized more GH (290%) and much less PRL (5%) than the parent. Hydrocortisone stimulated GH synthesis and inhibited PRL similarly in GH4C1 and GH4C1/RC.4 cells. The results show that the GH4C1/RC.4 line is multidrug-resistant and overexpresses the 170-kilodalton P-glycoprotein and suggest that the P- glycoprotein pump contributes to hydrocortisone kinetics.

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