Immunocytochemical localization and endogenous synthesis of apolipoprotein E in testicular Leydig cells

Apolipoprotein E (apoE) is synthesized by a wide variety of tissues, including those involved in steroidogenesis; its function in most extrahepatic tissues is unknown. Significant amounts of apoE mRNA have been detected in the testis, but the cellular origin of this material has not yet been determined. The purpose of the present study was to determine whether the steroidogenic cells of the testis synthesize apoE. We localized apoE in the testis of mice by an avidin-biotin peroxidase technique using a cross-reactive anti-rat apoE antibody. ApoE immunoreactivity was strongest in interstitial cells but was also diffusely localized throughout the seminiferous tubules. Acute treatment of mice with hCG diminished apoE immunoreactivity in the testis. A murine Leydig tumor cell line (I-10 cells) also demonstrated apoE immunoreactivity, suggesting that at least one source of interstitial apoE is the Leydig cell. Normal Leydig cells were subsequently isolated from control and hCG-treated mice using Percoll density gradients. Isolated hepatocytes, I-10 cells, and Leydig cells (with or without hCG in vitro) were incubated in the presence of [35S]methionine. A 35S-labeled protein of approximately 33-35 kDa was immunoprecipitated from the cells and media of all three types of preparations using whole antiserum or affinity-purified antibody. Preincubating the antibodies with apoE-containing murine very low-density lipoprotein or purified rat apoE eliminated these bands. Leydig cell apoE synthesis and secretion were decreased by hCG treatment in vivo and/or in vitro. These data suggest that apoE is synthesized by normal and transformed Leydig cells and may play a role in sterol transport in the testis.