Receptors for D-Trp6-luteinizing hormone-releasing hormone, somatostatin, and insulin-like growth factor I in MXT mouse mammary carcinoma.

AbstractMembrane receptors for d-Trp6-luteinizing hormone-releasing hormone (d-Trp6-LH-RH), somatostatin-14 (SS-14), and insulin-like growth factor I (IGF-I) were estimated in MXT mammary cancers of mice using sensitive multipoint micromethods. The receptors were characterized in untreated animals and following in vivo treatment with microcapsules of the agonist d-Trp6-LH-RH and the somatostatin analog RC-160, which strongly inhibited tumor growth. In the control group, d-Trp6-LH-RH was bound to the single class of saturable, specific, noncooperative receptor sites (Kd, = 29.3 ± 8.48 × 10-9M; Bmax = 4.55 ± 0.31 pmol/mg membrane protein). Treatment with d-Trp6-LH-RH alone or in combination with RC-160 produced down-regulation of membrane receptors for d-Trp6-LH-RH on MXT mammary tumor cells. RC-160 alone and ovariectomy were without effect on d-Trp6-LH-RH receptors. On the membrane surface of MXT mammary cells, we found one class of high affinity, specific, saturable binding sites for SS-14 (Kd = 4.4 ± 1.9...