Binding and degradation of [125I]human growth hormone in rat adipocytes

Iodinated human GH [( 125I]hGH) binds to both specific and nonspecific sites on the surface of adipocytes isolated from the epididymal fat of normal rats. When adipocytes were incubated at 37 C with 1 nM [125I]hGH, specific binding increased for 30-60 min and thereafter remained approximately constant as long as the hormone was present in the medium. When cells that had bound [125I]hGH were removed from the incubation medium and reincubated in hormone-free medium at 37 C, half of the specifically bound 125I was released into the medium about every 30 min, and about half of the nonspecifically bound 125I was released in about 60 min. These rates were seen regardless of whether the time allowed for hormone binding was 15, 30, or 60 min. About 90% of the 125I released was soluble in 5% trichloroacetic acid and was in the form of iodotyrosine. The rate of 125I release from specific binding sites decreased by a factor of 4 when the temperature was lowered from 37 to 17 C. Replacement of some of the sodium chloride in the buffer with 25 mM ammonium chloride had little or no effect on the amount on 125I that bound to cells when [125I]hGH was present in the medium, but virtually completely blocked the release of 125I from cells transferred to hormone-free medium. Ammonium chloride also significantly reduced both the release of 125I from nonspecific binding sites and the amount of 125I recovered in trichloroacetic acid-soluble form. Cloroquine, leupeptin, or colchicine nearly doubled the specific binding of [125I]hGH after 180 min and markedly slowed the release of 125I when cells were transferred to hormone-free medium. All of these agents also significantly reduced the rate of release of 125I from nonspecific binding sites. Incubation of adipose tissue from hypophysectomized rats with ammonium chloride, leupeptin, or colchicine failed to alter the ability of GH to increase glucose oxidation, induce refractoriness, or promote lipolysis in the presence of theophylline. We conclude that GH binds virtually irreversibly to both specific and nonspecific sites on the adipocyte surface and is then internalized and degraded in the lysosomones. These events appear to be independent of the cellular processes that lead to expression to GH responses.