Ok here it comes. Those who are not interested in reading scientific literature and go through details, skip this post, it’s not for you.
First I’ll take a study where good evidence is present that SHBG binds androgens and inactivates them.
https://www.nature.com/articles/srep35539#Fig2
At 24 weeks of age, steroid hormone profiling of serum and urine by LC-MS/MS revealed that male SHBG-Tg mice have increased total serum concentrations of T and DHT (~200-fold)
This is because of HPA stimulation (lack of negative feedback) because the SHBG-Tg mice have SHBG.
As you can see, LH increased because in mice with artificially added SHBG the free T decreases. This decrease gets fully compensated by increasing total T and therefore again free T. So the hypothalamus supports the free hormone hypothesis.
Our findings provide for the first time clear experimental validation of the free hormone hypothesis with respect to sex steroids, i.e. that SHBG-bound T in the circulation is restricted from entering into target tissues and eliciting its physiological functions in vivo . Unexpectedly however, the most striking effect of SHBG is not to decrease free T concentrations, but to increase total androgen and estrogen concentrations. This occurs via at least two mechanisms: hypothalamic-pituitary feedback stimulation (as evidenced by increased LH and concentrations of precursor androgens which themselves do not bind to SHBG) and increased circulating ligand half-life.
Interesting: Blood E2 increased too, normal WT mice don’t have detectable E2 in the blood but rats with SHBG do.
Using a more sensitive, dedicated LC-MS/MS method E2 was undetectable in WT male mice, whereas in SHBG-Tg males (n = 9), values above the limit of quantification (1.3 pg/mL) were observed in four of them. These low to undetectable values could not be attributed to the known fluctuating T concentrations in male mice, as shown by the consistent lack of E2 in WT mice with a s.c. continuous-release T implant. The lack of E2 was also not due to limitations of the LC-MS/MS method, as shown in castrated SHBG-Tg males with s.c. silicone E2 implants, or female mice E2 concentrations measured randomly in the estrous cycle were increased in female SHBG-Tg vs. WT mice
(Also there seems to be a nonsensical statement in the discussion (more on problems here later):
We also found no phenotypic evidence for ligand-independent effects of SHBG, since no differences were seen in gonadectomized or pre-pubertal animals.
Since these animals don’t have SHBG naturally. So taking away something that’s added artificially shouldn’t impact the organism)
My problem with this study in particular is that the mice used don’t have any SHBG normally, so there is no need for mechanisms to use it and so they evolved using “only” free testosterone.
This in my opinion puts a big damper on the findings since the study can’t prove more than a binding of the T in the blood stream.
I think it is more complicated than that though, but I can’t find a critical outlook in this in the study, so I assume they didn’t test for the uptake mechanisms.
The study was published in Nature, one of the biggest and best research publishers. It also was published not long ago (IIRC 2016). I’m wondering why a paper that peer-reviews high level studies routinely didn’t critique that point enough to get the researchers to do one or two more experiments.
An SHBG homologue known as the androgen-binding protein is expressed locally in the testis of mice and rats but hepatic Shbg expression and secretion is lacking postnatally in these rodent models. Therefore a mouse model expressing a human SHBG transgene (SHBG-Tg) has been established previously
So here are some papers trying to go beyond that.
These are from 2005! So the newer paper should have tried to refute or prove the claims.
https://www.sciencedirect.com/science/article/pii/S0092867405008615
In this paper they review a study in which the megalin receptor (a receptor that’s similar in genome and function to the LDL-Receptor, both internalize after binding their substrate, in this case SHBG) is implicated in transporting SHBG and the bound hormone into the cell.
They took mice which have SHBG naturally.
This appears to be true in vivo, at least temporally, as female mice that lack megalin show abnormalities in vaginal opening in response to estrogen, and megalin-deficient male mice exhibit defects in testicular descent in response to androgens.
This could not be compensated by giving more T or E2. So there seems to be evidence for a free T independent pathway.
Here’s the best experiment from the original study:
(RAP blocks the megalin receptor)
To explore the effect of this endocytic pathway for SHBG/ABP on the cellular metabolism of testosterone, we determined uptake of [3H]testosterone in BN16 cells in the presence of unlabeled SHBG. After 5 hr, approximately 11% of the tracer was associated with the cells, a process that was blocked by RAP. Megalin-mediated uptake of testosterone was critically dependent on the amount of SHBG present in the culture medium as shown by varying the molar ratio of carrier and testosterone. At a ratio of carrier to steroid of 5:1 when 95% of the steroid is bound to SHBG, cell association of [3H]testosterone was absolutely dependent on megalin activity as evidenced by RAP inhibitability. At a molar ratio of 1:10 when 98% of [3H]testosterone is free, cell association was independent of megalin and insensitive to RAP. At a molar ratio of 1:1 when 20% of testosterone is complexed, uptake was partially dependent on megalin activity. In the human circulation (adult males), the ratio of SHBG to testosterone is approximately 3:1. SHBG bound testosterone was unable to enter cells when megalin was blocked by RAP and instead accumulated in the medium of the cells.
That quite clearly demonstrates that SHBG bound testosterone
- gets taken up into cells through a specific mechanism
- has defined properties in tissues that miss androgen induced changes when this pathway gets inhibited
But:
Assuming a pivotal role of megalin in sex-steroid signaling, it is intriguing to note that megalin null mice do not share all of the anomalies seen in mice deficient for the AR (e.g., testicular feminization) or estrogen receptors (ER) (e.g., uterine hypoplasia).
So MICE that lack the receptor DONT have all the symptoms of a complete lack of androgens and estrogens ergo there must be other mechanisms for them to enter the cell. These could either be other SHBG receptors or free T diffusing through the plasma membrane of the cell. I think the latter is probable.
To me, all of this in conclusion, looks like both pathways get used by the body to produce different results. The second and third studies I linked look a lot more error prove than the first one, from the get go. Also the argument that if 95+% of T and other hormones is bound to SHBG or ABP and there is efficient uptake needed, that the diffusion might be too slow, is believable. So I’d say that there is not enough evidence to say that only one thing happens but good evidence to suggest that free T as well as SHBG-bound T have their defined roles in the human body. All of the papers here don’t have to contradict each other, they could add to each other.
I’m very open to discussion here. Could be that my analysis is not enough and I’m missing something that points clearly to the one or the other hypothesis. It’s a shame I can’t paint a clearer picture at the moment, maybe I’ll come back to this later.
I don’t know if this is true. I can’t substantiate it and I would not at first sight blame the kinetics of the uptake as that should not be that acute since there’s the mentioned lag time.
This actually is absolutely correct in my opinion.
@dbossa is right if the hypothesis alone is correct. Otherwise it would maybe be an oversimplification or just wrong. But since we don’t know I guess the way to go is the way Mr. Bossa does it, trial and error. The trends physicians see are often there before we have evidence.
So I would not count that out.
Edit: a few months later and I see I forgot one important thing.
Estradiol for example doesn’t only act in the cell but there are membrane receptors (this is very new stuff so for testosterone I don’t know if there are known membrane T receptors) which promote the fast E2 effects. SHBG bound E2 seemingly is functional and acts on these.
https://www.nature.com/articles/s41598-018-36882-3
Another point that supports that the free hormone hypothesis is outdated.
