Steroid hormone are believed to cross membranes by diffusion. However, Winfried Hanke of the University of Karlruhe (Hanke,W. (1970). Hormone. Fortschr Zool 20. 318–380) had evidence that corticosterone was exported in vesicles. This discussion has relevance since steroid hormones in vesicles would be released when the release is triggered and not continously, while release by diffusion would be continously and regulated by the rate-limiting enzyme in the biosynthetic pathway. And the availability of steroid hormones is one very intruiging question in endocrinology.
An arcticle in Cell of this week demonstrates that ecdysone, that steroid hormone of insects is well released in vesicles. They analyze the machinery of SNARE’s (the proteins who will eventually upon a calcium trigger fuse to the cell membrane release the vesicle content to the outside of the cell) and the loading of ecdysone to the vesicle by the Abet ATP-binding cassette transporter in detail and very convincingly.
One wonders whether other steroids are loaded into vesicles similarily. And old story is to become exciting. Nice paper and very instructive images! Highly recommended!
Endocrinologists are aware of the circadian clock since it determines the release of many hormone likewise cortisol in a daily rhythm. There are other rhythmic hormone releases not dependent on the circadian clock, for example the prolactin release in a circannual fashion, or faster pulses for hormones of the pituitary with one to three hours pulse lengths.
In short, the circadion clock is found in the supraoptic nucleus of the hypothalamus and concists of the RNAs and proteins Per, BMAL, Clock, and cryptochrome(s). These are generated and inactivated in a way that autonomously repeats about every 24 hours. It can also adjust to a light-dark cycle.
What is new in a paper by Liu et al. from the Bradfield labaratory at the Univ. of Wisconsin in PNAS is that steroidogenesis — the synthesis of steroids –is coupled to the clock protein BMAL-1. They show that female mice which fail to express the BMAL protein in steroidogenic cells are not capable to implant an fertilized egg into the uterus and fail to generate progeny. When they transplant one normal uterus into these animals by exchanging one defective with the normal one, these mice will again produce offspring. The defect can, in addition, be rescued by soluble progesterone which shows that progesterone is a determining factor in nidation/implantation.
These experiments are nicely done. The conclusion, however, that the hormone production in the ovar is decisive is too far fetched: They have eliminated the entire steroidogenesis in these mice, therefore the only hormone producing organ of the rescued animals is the transplanted normal ovar. Progestone or other steroid hormones being soluble and acting far away from their place of synthesis could under normal conditions be generated in the adrenal or somewhere else as well. The ovar is by far not the only organ with progesterone synthesis. It will be difficult to answer the question whether the ovar’s progesterone synthesis is required for implantation, since a block in the progesteron synthesis will likely block androgen, estrogen and corticoid synthesis. You would need the 3ß-hydroxysteroid dehydrogenase 1 inactive only in the ovar. And still the animal needs androstendione substitution to allow ongoing testosterone and estrone and thus estradiol synthesis.
Worth to read!