Forskolin up-regulates aromatase (CYP19) activity and gene transcripts in the human adrenocortical carcinoma cell line H295R.
Watanabe M, Nakajin S.
Department of Biochemistry, Hoshi University School of Pharmacy and Pharmaceutical Sciences, 2-4-41 Ebara, Shinagawa, Tokyo 142-8501, Japan.
[email protected]
A number of conditions related to sex-reversal in boys and men and precocious puberty in girls are caused by estrogen-secreting adrenal tumors. In these tumors, cytochrome P450 aromatase (aromatase) that is encoded in the CYP19 gene is expressed at high levels. To investigate the molecular mechanism of aromatase expression in these adrenal tumors, we characterized the activity, gene transcript and genomic promoter region of aromatase in the human adrenocortical carcinoma cell line H295R. Aromatase activity and the transcript of the CYP19 gene were highly up-regulated by forskolin, but not by dexamethasone. The results from exon I-specific reverse transcriptase (RT)-PCR and the transfection of reporter constructs suggested that promoter I.3 and promoter II were activated in H295R. Deletion and mutation analysis suggested that cAMP response element-like sequence (CLS) and steroidogenic factor-1 (SF-1) motif, were critical for the activation of promoter II. The results of this work should provide the basis for the molecular analysis of aromatase expression in adrenocortical cells.
PMID: 14709151 [PubMed - indexed for MEDLINE
That testosterone exerts negative feedback inhibition on StAR was detected coincidentally by researchers investigating the action of so-called Müllerian inhibiting substance (MIS) on StAR mRNA expression (3). MIS is produced in the developing testes of an embryo once it commits to male development. MIS is responsible for the regression and ultimate disappearance of incipient female reproductive tract organs in the developing male fetus. MIS continues to be secreted in small quantities in adult males.
The authors of (3), Houk et.al., noted that MIS augmented cAMP induced expression of StAR mRNA (messenger RNA). Previous studies had show that MIS was capable of suppressing testicular steroidogenesis, and the authors speculated that perhaps this MIS induced testosterone suppression might indirectly be responsible for the upregulation of StAR by MIS. So if lowering levels of testosterone might increase StAR, the converse might be true as well; namely that increasing testosterone would suppress StAR. This would imply negative feedback inhibition of StAR by testosterone. As it turns out, this is exactly what the researchers found. In other words, MIS indirectly induces StAR mRNA expression by reducing androgen levels, which normally exert a feedback inhibition on StAR expression.
To test their hypothesis the researchers first stimulated StAR in culture by adding cAMP to Leydig cells. As expected, StAR activity increased. Then the authors added increasing concentrations of DHT to the cells, and the amount of StAR was reduced in a dose dependent manner.
Progesterone and synthetic progestins are known to suppress testosterone production (4). The authors incubated the cAMP stimulated Leydig cells with progesterone and indeed noted a decline in StAR levels. This may explain one aspect of progesterone’s suppression of the HPTA. Similar results were obtained with estradiol.
At the beginning of the paper we discussed the classical picture of how androgens and estrogens act to suppress the HPTA by acting on the hypothalamus and pituitary to suppress gonadotropin secretion. To quote Houk et.al.,
"The in vitro and in vivo demonstration that testosterone directly inhibits StAR expression in Leydig cells suggests an autocrine mechanism through which steroid hormones can regulate their own production at the crucial, rate-limiting step of cholesterol transfer to the inner mitochondrial membrane. This feedback may provide a local mechanism for modulating Leydig cell steroidogenesis in addition to the well-known feedback inhibition of sex steroids on the hypothalamic-pituitary axis to suppress gonadotropin secretion.
I found a piece of the lessor of the two articles I read on forskolin below. I couldn't find the whole article nor the second one. The other article I read was more certain about slowing down the feed-back loop
The authors did not address the question of whether increasing cAMP levels could override the negative feedback effect of testosterone on StAR. This leaves open the intriguing possibility that forskolin, a popular supplement known to elevate cAMP, might increase testosterone production by taking the brakes off of testosterone’s inhibition of StAR. Indirect research does exist however that lends credence to this idea. Luo et.al (5) noted that testosterone production decreased as a function of age and declining levels of StAR protein in Norway rats. When Leydig cells from the aged rats were cultured with dbcAMP (Dibutyryl cyclic AMP), an analog of naturally occurring cAMP, testosterone production was restored to youthful levels.