## 5 Discussion
### What Should Be the Next Research Topics to Advance Treatments for Human Hair Graying? PKA and MAPKs play important roles in directly or indirectly regulating MITF expression and activity [167; 168]. PKA activates CREB, which increases MITF expression in melanocytes stimulated by \(\alpha\)-MSH [169]. Of the MAPKs, p38 MAPK directly phosphorylates MITF at Ser307 to increase its transcriptional activity [170] or activates PKA or other protein kinases, such as MAP kinase-activated protein kinase 2 (MAPKKPZ), to contribute to increased MITF expression through CREB [43; 171]. ERK phosphorylates MITF at Ser73, enhancing both the transcriptional activity and proteasomal degradation of MITF, up- or downregulated MMITF activity and expression depending on the cellular context [167]. JNK phosphorylates CREB-regulated transcription coactivator 3 (CRTC3), thereby inhibiting nuclear translocation and transcriptional activity, and suppressing MITF expression [172]. Thus, p38 MAPK has a consistent and greater influence than JNK and ERK in regulating MITF expression and activity. The PM extract and TSG increased p38 MAPK activation and MITF expression, and this effect was neutralized by a selective inhibitor of p38 MAPK (SB203580). Additionally, placental sphingolipid and recombinant human IL-33 increased MITF expression through a similar mechanism [32; 155]. These observations highlight the importance of p38 MAPK in regulating MITF expression and activity. Therefore, future studies should focus on the selective targeting of p38 MAPK as a strategy for treating hair graving due to melanocyte dysfunction. NRF2 is a transcription factor regulating the gene expression of Phase II metabolism/ antioxidant enzymes by binding the antioxidant-responsive elements (AREs) in the promoter of target genes, enhancing the cellular antioxidant defense capacity [173]. In basal state cells, cytosolic NRF2 is captured by dimeric KEAP1 or other proteins, which promote ubiquitylation and proteasomal degradation of NRF2. Under stimulated conditions, NRF2 escapes from KEAP1 or proteasomal degradation and translocates to the nucleus where it induces the transcription of target genes [174]. Topical administration of a fraction of Fuzhuan brick tea extract or an apocarotenoid compound bixin to mice increased the expression of NRF2 and its target genes in the skin and attenuated hair graving induced by hydroquinone treatment or PUVA phototherapy, respectively [156; 158]. Therefore, an effective NRF2 activator could help attenuate hair graving due to oxidative stress. Consequently, future studies need to systemically examine the effects on hair graving of many known and unknown NRF2 modulators [174]. The rapid growth or turnover of hair might cause hair graving due to accelerated differentiation and depletion of MSCs [47; 52; 53]. Supporting this notion, less pigmented hair grew faster than pigmented black hair; in other words, hair that grew faster was less pigmented [51]. Although the cause of accelerated differentiation and depletion of MSCs during rapid hair growth is unclear, it might be due to excessive activation of the \(\beta\)-catenin-dependent WNT signaling pathway [60]. If so, the \(\beta\)-catenin-dependent WNT signaling pathway cannot be an ideal therapeutic target for alleviating hair graying, although it could be a promising therapeutic target for promoting hair growth [175; 176]. Resiniferatoxin injection and repeated hair plucking commonly induce hair graying, probably by stimulating the release of norepinephrine, a stress hormone [104; 153]. Interestingly, norepinephrine promoted hair follicle growth in an organotypic skin culture model [177], despite accelerating the differentiation and depletion of MSCs [106]. Therefore, proper regulation of the norepinephrine-\(\beta\)2 AR-PKA signaling pathway is important for maintaining a balance between hair growth and pigmentation. Understanding how to control the secretion and action of norepinephrine to treat hair graying and hair loss will be an intriguing topic of future research.