Construction of a Leydig cell line synthesizing testosterone under gonadotropin stimulation: a complex endocrine function immortalized by cell hybridization

Leydig cells: formation, function, and regulation

Herein we summarize important discoveries made over many years about Leydig cell function and regulation. Fetal Leydig cells produce the high levels of androgen (testosterone or androstenedione, depending upon the species) required for differentiation of male genitalia and brain masculinization. Androgen production declines with loss of these cells, reaching a nadir at postpartum. Testosterone then gradually increases to high levels with adult Leydig cell development from stem cells. In the adult, luteinizing hormone (LH) binding to Leydig cell LH receptors stimulates cAMP production, increasing the rate of cholesterol translocation into the mitochondria. Cholesterol is metabolized to pregnenolone by the CYP11A1 enzyme at the inner mitochondrial membrane, and pregnenolone to testosterone by mitochondria and smooth endoplasmic reticulum enzymes. Cholesterol translocation to the inner mitochondrial membrane is mediated by a protein complex formed at mitochondrial contact sites that consists of the cholesterol binding translocator protein, voltage dependent anion channel, and other mitochondrial and cytosolic proteins. Steroidogenic acute regulatory protein acts at this complex to enhance cholesterol movement across the membranes and thus increase testosterone formation. The 14-3-3γ and ε adaptor proteins serve as negative regulators of steroidogenesis, controlling the maximal amount of steroid formed. Decline in testosterone production occurs in many aging and young men, resulting in metabolic and quality-of-life changes. Testosterone replacement therapy is widely used to elevate serum testosterone levels in hypogonadal men. With knowledge gained of the mechanisms involved in testosterone formation, it is also conceivable to use pharmacological means to increase serum testosterone by Leydig cell stimulation.

Testicular cell lines

The range of in vivo or in vitro immortalized cell lines currently available provides a variety of model systems for studies of normal and pathological cell functions. The cell lines have been derived from spontaneous or experimentally induced tumors, or through in vitro immortalization. The transgenic (TG) techniques provide a powerful approach, allowing the production of in vivo animal models for a variety of diseases, including malignant tumors, through tissue-specific expression of oncogenes or other tumor-promoting genes. The TG techniques also enable the production of cell lines with specific characteristics, through insertion of desired genes into specific cell types, which can then be immortalized upon cell culture. The use of temperature-sensitive immortalizing genes offers an additional advantage of controlling gene expression, including the proliferation and differentiation of the cells to be immortalized. As regards the male reproductive system, a number of cell lines of testicular somatic cells are currently available. This review covers mainly the immortalized cell lines of testicular Leydig and Sertoli cells, with special reference to murine cell lines for the study of testicular endocrine function and tumorigenesis. These cell lines also provide useful tools to investigate the molecular basis of hormone actions and testicular cell interactions.

Cloning of thyrotropin-releasing hormone precursor and receptor in rat thymus, adrenal gland, and testis

TRH is a hypophysiotropic peptide that acts mainly via the hypothalamic-pituitary-thyroid axis, but TRH immunoreactivity is also detected in several peripheral tissues. PCR with two pairs of primers enabling amplification of three fragments of TRH complementary DNA (cDNA) was used to demonstrate local production of TRH. Products of the expected size were detected in the testis, adrenal gland, lymphoid organs, thymus, and spleen. The amplified cDNA fragments were cloned and sequenced to show that the TRH gene is expressed in the thymus, spleen, and adrenal gland. Competitive RT-PCR showed that the TRH messenger RNA content of the testis was about one third that of the hypothalamus, whereas the adrenal gland contained 2% and the thymus 6%. HPLC analysis of thymus and spleen extracts showed small amounts of TRH, with a particular processing pattern of pro-TRH in lymphoid organs. The expression of the TRH receptor gene in peripheral organs was investigated to determine whether TRH had an autocrine or a paracrine action. cDNA fragments that encompassed the coding region of the receptor were identified in the testis, adrenal gland and thymus. No signal was detected in the spleen. These findings indicate that TRH may have a biological activity in extrapituitary organs and may act locally in the testis, adrenal gland, and thymus.

Retinoic acids up-regulate steroidogenic acute regulatory protein gene

The steroidogenic acute regulatory (StAR) protein plays essential roles in the delivery of cytosolic cholesterol into the mitochondrial inner membrane, which is an acute regulated and rate-limiting step for the steroid hormone synthesis. Since retinoic acids (RAs) are known to induce the synthesis of steroid hormones in mouse Leydig cells in vitro, mouse Leydig tumour cells, K28, were used to determine the effect of RAs on the level of StAR mRNA by Northern blot analysis. The level of StAR mRNA reached the maximum in a 4-8 h treatment with all-trans-RA (atRA) or 9-cis-RA (9cRA), and the effects were dose-dependent. The effect of 9cRA on the levels of StAR mRNA was blocked by actinomycin D, which indicates that 9cRA might exert a direct effect on the transcription of the gene. Promoter/reporter constructs containing a 5'-flanking region of the mouse or rat StAR gene were prepared, and luciferase activity was assayed following transient transfection into K28 or adrenal tumour cells, Y1. The result revealed that the luciferase activity was increased by 4-5-fold in response to the treatment of 9cRA, which indicated that 9cRA participates transcriptional activation of the StAR protein gene.