Dietary Vitamin A Dose Dependently Regulates BMP4 and WNT7A in Hair Follicles

Hair cycles through a growing phase (anagen), apoptosis phase (categen), and resting phase (telogen). Telogen is further divided into refractory and competent telogen based on bone morphogenetic protein 4 (BMP4) expression. BMP4 inhibits anagen induction by directly inhibiting wingless‐related MMTV integration site 7A (WNT7A) and other WNTs. Transient activation of WNT signaling in hair follicle stem cells triggers anagen induction; while sustained WNT signaling activation in hair follicle stem cells leads to skin cancer. Previously, we localized a complete set of proteins involved in retinoic acid (RA) synthesis and signaling to the hair follicle, whose expression changed throughout the hair cycle. We also found that dietary vitamin A altered the hair cycle in a dose dependent manner. The purpose of this study was to further define these hair cycle changes. We initially examined localization of RA synthesis proteins retinal dehydrogenase 2 (ALDH1A2), cellular RA binding protein 2 (CRABP2), RA degradation enzyme cytochrome p450 26B1 (CYP26B1), and BMP4 in telogen hair follicles in 70–100 day old female C57BL/6J mice by immunohistochemistry. All proteins localized to BMP4 positive telogen hair follicles. ALDH1A2 was also seen in BMP4 negative telogen hair follicles, but at a lower level. However, CRABP2 and CYP26B1 were barely detectable in BMP4 negative follicles. This suggests that RA synthesis and signaling may be stronger in refractory telogen compared to competent telogen. We next examined the expression of BMP4 and WNT7A by immunohistochemistry in telogen hair follicles from mice fed various levels of vitamin A from our previous studies. In one study female C57BL/6J mice bred an unpurified diet containing 6–28 IU vitamin A/g diet were switched to purified AIN93M diets containing 4, 28, or 56 IU vitamin A/g diet at 12 weeks of age and fed these diets for 16 weeks (High RA bred). In another study, female C57BL/6J mice were bred for two generations on the AIN93 growth diet containing 4 IU vitamin A/g diet then at 6 weeks of age fed AIN93M diets containing 4, 28, or 56 IU vitamin A/g diet for 12 weeks (Adequate RA bred). Breeding mice on the AIN93 diet was previously shown to reduce RA levels in several tissues. In High RA bred mice, consuming excess (56 IU) vitamin A increased BMP4 and reduced WNT7A. However in Adequate RA bred mice the reverse was seen; excess vitamin A (56 IU) reduced BMP4 and increased WNT7A. One side effect of pharmacological doses of oral synthetic retinoids is telogen effluvium – i.e. loss of hair while follicles are trapped in telogen. Our results in High RA bred mice suggest that excess vitamin A arrests hair follicles in refractory telogen, similar to the effects of oral retinoids in humans. However the opposite effects seen in Adequate RA bred mice suggest that physiological levels of dietary vitamin A promote competent telogen, which allows hair follicle stem cells to be activated. Further studies are needed to better define these dose dependent effects. Although, this dose dependent regulation may help explain variable results of retinoids in skin cancer prevention and treatment in mice and humans. This study also highlights the need to breed mice on lower levels of dietary vitamin A to see true physiological effects. Support or Funding Information This work was supported by grants from the National Institutes of Health (AR052009 to HBE, AR052710 to JPS), The Virginia Vivian Endowment Fund (LS), and the T. Kline Hamilton Endowment Fund (LS).