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Properties and Regulation of the Thyrotropin Receptor in the FRTL5 Rat Thyroid Cell Line^*

Charles A. Dana Research Institute and the Haruard-Thorndike Laboratories and the Department of Medicine, Beth Israel Hospital and Harvard Medical School, Boston, Massachusetts 02215

Address requests for reprints to: Dr. Sidney H. Ingbar, Department of Medicine, Beth Israel Hospital, Harvard Medical School, 330 Brookline Avenue, Boston, Massachusetts 02115.

Abstract

Despite extensive use of FRTL5 cells in studies of responses to TSH and anti-TSH receptor antibodies, almost nothing is known of the properties of their TSH receptors, possibly because binding of TSH by these cells is negligible when studied in their usual culture medium. In the present studies, we have demonstrated that specific binding of TSH can readily be demonstrated in confluent monolayers of FRTL5 cells if their culture medium is replaced by Krebs-Ringer bicarbonate (KRB) buffer. In keeping with previous observations concerning the effects of cations on the binding of TSH in other thyroid systems, binding of TSH to FRTL5 was far greater when the medium used was a modified KRB in which an isosmotic substitution of sucrose for NaCl had been made. Kinetic studies of TSH binding in both types of medium suggested the presence of two binding sites, one with a higher affinity and lower maximum binding capacity than the other. The influence of NaCl was to decrease the capacity of both sites, that of the low affinity site to a greater extent than that of the high affinity site, whereas the affinities of the two sites remained unchanged. Correlative studies indicated that physiological responses to TSH were associated mainly with occupancy of the higher affinity sites.

Experiments in which TSH binding was studied in cells grown to confluence in the presence of TSH from which TSH was then withdrawn and in cells maintained in the absence of TSH to which TSH was then added demonstrated the occurrence of upand down-regulation, respectively, of receptor concentrations without a change in their affinities. The reduction in maximum binding capacity induced by TSH was proportionately greater in the case of the high affinity than the low affinity receptor. Down-regulation by TSH was concentration dependent and was demonstrable at a TSH concentration of 10^–11 M, considered to be physiological. Further, maximum down-regulation was induced by 10^–9 M TSH, the approximate concentration at which other responses to TSH in these cells reach their peak. Therefore, down-regulation of TSH receptors can be considered to be one of the physiological responses that TSH elicits. (Endocrinology 118: 1945–1951, 1986)

Footnotes

^* This work was supported in part by Grant AM-18416 from the NIADDK, NIH (Bethesda, MD).

Received September 4, 1985.

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