Assessment of Mechanisms of Thyroid Hormone Action in Mouse Leydig Cells: Regulation of the Steroidogenic Acute Regulatory Protein, Steroidogenesis, and Luteinizing Hormone Receptor Function**This investigation was supported in part by grants from the Sigrid Jusélius Foundation, Academy of Finland, Foundation for the Finnish Cancer Societies (to I.T.H.), and NIH Grant (HD-1-7481) (to D.M.S.).

Abstract

Recently, we demonstrated that triiodothyronine (T(3)) stimulated steroid hormone biosynthesis and steroidogenic acute regulatory (StAR) protein expression in mLTC-1 mouse Leydig tumor cells through the mediation of steroidogenic factor 1 (SF-1). We now report a dual response mechanism of T(3) on steroidogenesis and StAR expression, and on LH receptor (LHR) expression and binding in mLTC-1 cells. T(3) acutely (8 h), induced a 260% increase in StAR messenger RNA (mRNA) expression over the basal level which was coincident with an increase in progesterone (P) production. In contrast, chronic stimulation with T(3) (beyond 8 h), resulted in an attenuation of StAR expression and P production. This attenuation was most likely caused by a decrease in cholesterol delivery to the inner mitochondrial membrane as demonstrated by incubations with the hydrophilic steroid precursors, 22R hydroxycholesterol and pregnenolone, which restored P synthesis. In similar studies, chronic treatment with T(3) increased the levels of cytochrome P450scc mRNA by 83%, whereas those of cytochrome P450 17alpha-hydroxylase and 3ss-hydroxysteroid dehydrogenase decreased. The diminished response in steroidogenesis following chronic T(3) exposure was not a result of alterations in StAR mRNA stability, but rather was due to inhibition of transcription of the StAR gene. Similar acute stimulatory and chronic inhibitory responses to T(3) were found when LHR mRNA expression and LHR ligand binding were examined. Transfections with an LHR or StAR promoter/luciferase reporter construct demonstrated that a 173-bp fragment of the LHR promoter containing an SF-1 binding motif was involved in T(3) response, as was the SF-1 recognition site at -135 bp in the StAR promoter. Furthermore, the importance of SF-1 in T(3) function was also verified employing mutation in the bases of SF-1 sequences using electrophoretic mobility shift assays. The potential physiological relevance of these findings was demonstrated when similar responses were obtained in mice rendered hypo and hyperthyroid. Collectively, these observations further characterize the thyroid-gonadal connection and provide insights into the mechanisms for a dual regulatory role of thyroid hormone in Leydig cell functions.

Effects of concomitant diabetes mellitus and hyperthyroidism on testicular and epididymal histoarchitecture and steroidogenesis in male animals

“…The numbers of LH receptors and the steroidogenic regulatory proteins and enzymes are directly affected by T3 hormone. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis (Manna et al, 2001;Maran, 2003). T3 has been proven to induce Leydig cell differentiation and to stimulate steroidogenesis in the rat testis (Mendis-Handagama and Siril Ariyaratne, 2005).…”

Regulation of the Steroidogenic Acute Regulatory Protein Expression: Functional and Physiological Consequences

“…Several lines of evidence demonstrate that there is a close correlation between StAR mRNA and StAR protein synthesis via the cAMP mediated pathway and that its expression occurs in a time frame that parallels the acute production of steroids in steroidogenic tissues [14,[83][84][85]. During development, a spatial and temporal relationship between the presence of the StAR protein and the capacity to produce steroid hormones has also been demonstrated.…”

“…Table 1 illustrates a list of agents that play roles in controlling steroidogenesis and StAR expression in different cells. Thus, steroid synthesis and StAR expression are upregulated by LH/human chorionic gonadotropin (hCG) [26,29,76,88,[146][147][148][149][150][151], follicle stimulating hormone (FSH) [68,69,71], cAMP analogs [64,65,68,69,75,83,92,129,152], ACTH [61,[153][154][155], CRH [156,157], thyroid hormone (T3/T4) [63,84,85], prolactin [158], activin [159], GnRH agonist, leuprolide actetale [160], progesterone [145], bone morphogenetic protein (BMP) [159], estradiol-17β [144], GH [161], angiotensin II (Ang II) [86,89,129], insulin like growth factor I & II (IGF-I & II) [68,69,71,147,…”

“…Transcriptional or translational inhibition of the StAR protein results in 90% repression of steroid biosynthesis, whereas approximately 10% of steroid production occurs through StARindependent mechanisms [84,[200][201][202]. However, a number of studies have shown a striking correlation between the synthesis of steroids and the synthesis of the 30-kDa proteins [14,15,21,63].…”

Exploring the Human Chorionic Gonadotropin Induced Steroid Secretion Profile of Mouse Leydig Tumor Cell Line 1 by a 20 Steroid Lc-Ms/Ms Panel

“…We validated the method for its application to the study of the mLTC1 model [31]. mLTC1 cells are permanently expressing the murine Lh receptor, which is structurally similar to the human receptor and capable of binding human LH and hCG [32,33]. Most importantly, human ligands trigger steroidogenic signals mainly activating the synthesis of Δ4 hormones [34], although Δ5 steroid production was described as well [35].…”