waynakyo
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3.2.2. Progesterone
Progesterone can influence hair follicle growth through central and local action. Central action is referred to as the inhibitory effect on LH secretion, which in turn causes a decrease in ovarian theca cell stimulation (androgen synthesis). At the level of the hair follicle, progesterone decreases the conversion of testosterone to dihydrotestosterone (through the inhibition of 5-alpha reductase activity) [13].
3.3. Prolactin
Prolactin (PRL) is also known as luteotropin and is a polypeptide hormone encoded in the human by the PRL gene on chromosome 6 [14]. It is produced by lactotropic cells in the anterior pituitary gland, and its role is mostly associated with the growth of mammary glands during pregnancy and breast milk production. Recently, many studies concentrated on exploring new functions of prolactin, and now PRL is recognized as playing a role not only in lactation but also in reproduction, angiogenesis, osmoregulation, and hair growth [15–18]. The influence of PRL on hair growth has been intensively investigated in mammals [19–21]; in human scalp skin, PRL and PRL receptors (PRL-R) were identified Int. J. Mol. Sci. 2020, 21, 5342 5 of 13 for the first time in 2006 by Foitzik et al. The luteotropin protein was detected in a thin layer of keratinocytes, while PRL- R was detected in the outer root sheath and in the proximal inner root sheath, as well as in matrix keratinocytes. The immunoreactivity was comparable both in isolated human anagen VI and organ-cultured human hair follicles (HFs) [18]. PRL and PRL receptor immunoreactivity was also demonstrated in non-scalp skin [22], which Slominski et al. failed to prove in previous reports [23]. The very same research study identified the proinflammatory cytokines, IFNγ and TNFα, as regulators of PRL expression in HFs. Interestingly, dopamine, known as an inhibitor of PRL pituitary secretions, has no effect on PRL or PRL-R expression in human HFs [22]. The mechanism whereby prolactin directly regulates hair growth is connected with its inhibitory influence on hair shaft elongation and the premature induction of the catagen phase. Moreover, luteotropin also plays a significant role in the proliferation and apoptosis of keratinocytes in HFs by decreasing the number of Ki-67-positive cells and increasing the quantity of TUNEL + cells [18]. Prolactin is recognized as an androgen metabolism modulator. The luteotropic hormone seems to increase the level of free testosterone and dehydroepiandrosterone sulfate, decreasing at the same time the level of serum testosterone-estradiol-binding globulin [24,25]. PRL also appears to inhibit the activity of 5-alpha reductase [26].