DOI: 10.1111/exd.12504
Viewpoint
www.wileyonlinelibrary.com/journal/EXD
Parathyroid hormone-related peptide and the hair cycle – is it the agonists or the antagonists that cause hair growth? Robert C. Gensure Pediatric Endocrinology, Children’s Hospital at Montefiore and Albert Einstein College of Medicine, Bronx, NY, USA Correspondence: Robert C. Gensure, MD/PhD, Pediatric Endocrinology, Children’s Hospital at Montefiore and Albert Einstein College of Medicine, 3415 Bainbridge Ave, Bronx, NY 10467, USA, Tel.: 718-920-4664, Fax: 718-405-5609, e-mail: [email protected] Abstract: While the effects of PTHrP have been studied for almost 20 years, most of these studies have focused on effects on the termination of the anagen phase, giving an incomplete picture of the overall effect of PTHrP on the hair cycle. PTHrP was determined in several experimental models to promote transition of hair follicles from anagen to catagen phase, which by itself would suggest that PTHrP blockade might prolong the anagen phase and promote hair growth. However, clinical trials with topically applied PTHrP antagonists have been disappointing, leading to a reconsideration of this model. Additional studies performed in mouse models where hair follicles are damaged (alopecia areata, chemotherapy-induced alopecia) suggest that
PTHrP has effects early in the hair cycle as well, promoting hair follicles’ entry into anagen phase and initiates the hair cycle. While the mechanism of this has yet to be elucidated, it may involve activation of the Wnt pathway. Thus, the overall effect of PTHrP is to stimulate and accelerate the hair cycle, and in the more clinically relevant models of hair loss where hair follicles have been damaged or become quiescent, it is the agonists, not the antagonists, which would be expected to promote hair growth.
The effects of parathyroid hormone-related peptide (PTHrP) agonists and antagonists on hair growth have been studied for over almost 20 years (1,2). Two reports appearing in 1994 used different models to examine effects of PTHrP agonists on hair. In the first, a transgenic model was employed, linking PTHrP to a K14 promotor for selective expression in the skin (2). The result was disappearance of the hair follicles from the ventral skin and delay in follicular development on the dorsal skin. In the second study, effects of PTHrP agonists and antagonists on hair growth were studied (1). In vitro studies showed hPTH(1-34) inhibited keratinocyte proliferation, and this effect was blocked by the antagonist bPTH(7-34). When administered daily to SKH-1 ‘hairless’ mice, PTHrP antagonism with bPTH(7-34) increased hair number and hair length after 7 days, while the agonist hPTH(134) had no effect on hair number or hair length. Taken together, these two studies indicate a role for PTHrP agonists to block formation of hair follicles, while PTHrP antagonists promote follicular activation, and, given the effects on hair length, may prolong the growth phase (Fig. 1a). A follow-up study examined effects of PTH(7-34) on the hair cycle in C57BL6 mice, showing evidence that the antagonist PTH(7-34) promoted entry into anagen phase from telogen phase and prevented progression from anagen to catagen phase (3). Based on these observations, an experiment was performed to test the effects of PTHrP agonists and antagonists in a mouse model of chemotherapy alopecia (4). Hair follicles were synchronised by waxing, and cyclophosphamide was administered 9 days later when hair follicles are in the anagen VI phase and are maximally sensitive. Two antagonists, PTH(7-34) and PTHrP(7-34), and one agonist, PTH(1-34), were evaluated. Results with two antagonists were not consistent at all time points, but generally showed increases in anagen VI hair follicles at 2 weeks, and
showed increases in visible hair at 2 weeks and 30 days compared with chemotherapy controls. Effects of the agonist PTH(1-34) appear to be biphasic – there was an initial increase in hair loss on day 14 in the PTH(1-34) group compared with the chemotherapy control group, and histological examination at this time point shows fewer anagen VI and more catagen hair follicles. However, by day 30, the PTH(1-34)-treated group showed increased regrowth of hair compared with chemotherapy controls, attributed to a more rapid hair cycle induced by the accelerated transition to catagen phase. The overall conclusion was that a paired treatment with PTH antagonists before chemotherapy to slow the hair cycle and prevent hair loss, followed by treatment with a PTH agonist after chemotherapy to accelerate the hair cycle and speed regrowth, would be optimal. A later study examined the spatial and temporal expression of PTHrP and PTH/PTHrP receptor expression after depilation (5). PTHrP was expressed throughout the early anagen phase throughout the follicle, but expression was increased in the proximal outer root sheath and isthmus during late anagen. Interestingly, PTH/ PTHrP receptor expression could only be observed during early anagen phase. Mating the K14-PTHrP mouse with the FGF-5 knockout mouse, which has extraordinarily long hair, resulted in a mouse with a relatively normal appearing coat, indicating that PTHrP could ‘rescue’ the long coat phenotype. This was confirmed in another mouse model of delayed catagen transition, the TSP1-KO mouse, where PTHrP overexpression again rescued the long hair phenotype (6). These findings again indicate that PTHrP induces anagen to catagen transition of hair follicles. As PTHrP promotes anagen to catagen transition, blocking PTHrP effect should prolong the anagen phase and promote hair growth (Fig. 1a). A topical formulation of a PTHrP antagonist was shown to increase hair growth in the SKH-1 ‘hairless’ mouse
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
Key words: alopecia – anagen – hair follicle – parathyroid hormone – parathyroid hormone-related peptide
Accepted for publication 13 July 2014
865
Gensure
(a)
Catagen
Telogen PTHrP
+
–
PTHrP
Anagen Embroynic epidermis
(b)
PTHrP + Nipple epithelium lineage (ventral surface only, no hair follicles)
Hair follicle lineage (Cycling hair follicles)
Telogen PTHrP
Catagen +
+
– Anagen –
Hairless mouse (impaired transition)
–
PTHrP
FGF-5 KO Mouse (impaired transition)
Chemotherapy, alopecia areata (damaged anagen follicles) Figure 1. Models of PTHrP effects on the hair cycle. Shown in panel (a) is the original model of PTHrP effects on the hair cycle, which suggested that PTHrP promotes transition of hair follicles from anagen to catagen stage, and can prevent formation of anagen phase hair follicles. PTHrP antagonists have the opposite effects (not shown). Shown in panel (b) is a revised model of PTHrP effects on the hair cycle, taking into account more recent data supporting a role for PTHrP to also promote transition of hair follicles from telogen to anagen phase. PTHrP antagonists have the opposite effects (not shown). Also shown in panel (b) are the defects in the various model systems in which PTHrP agonists and antagonists have been tested. In the SKH-1 ‘hairless’ mouse, defects in the gene hairless impair telogen to anagen transition. In the FGF-5 KO mouse, there is impaired transition from anagen to catagen phase. With chemotherapy and alopecia areata, there is direct damage to anagen hair follicles.
(7). Another topical formulation of the antagonist PTH(7-34) was tested in chemotherapy-induced alopecia in humans by IGI Pharmaceuticals. However, these trials were discontinued in phase 2, according to the company’s annual reports, because of lack of observed efficacy. The final results were never published. The failure of PTHrP antagonists in clinical trials may have been a result of the treatment regimen. Periodic application of an antagonist can be problematic; as the antagonist wears off, escape and even rebound effects can occur. We attempted to address this problem by linking both PTHrP agonists (PTH-CBD) and antagonists (PTH(7-33)-CBD) to a collagen binding domain, prolonging retention in skin (8). We tested these compounds using the experimental protocol of Peters et al. (4), expecting to see improved hair growth-promoting effects with our PTHrP antagonists. However, the actual results showed quite the opposite; PTHrP agonists linked to a collagen binding domain were effective at speeding regrowth of hair, while PTHrP antagonists appeared to have little effect (9). Histological examination showed that animals treated with the PTHrP agonist increased the number of anagen VI hair follicles and led to resolution of the dystrophic changes from the chemotherapeutics, while the PTHrP antagonist had little, and perhaps even deleterious, effects. We think the explanation for this surprising result can be found by re-examining some of the earlier studies. The disappearance of hair follicles on the ventral surface first noticed in
866
the K14 PTHrP mouse has since, by the same group of investigators, been attributed to development of ectopic nipple skin in this region rather than by direct inhibition of folliculogenesis (10). Nipple skin is normally devoid of hair follicles. In the study by Cho et al., both PTHrP and PTH/PTHrP receptor were expressed during the early anagen phase, with receptor expression at higher levels than that seen in the late anagen phase (5), which suggests a role for PTHrP for during the initiation of the anagen phase as well. The actual role of PTHrP would likely depend on which mediators are activated. In bone, PTHrP induces bone morphogenic proteins (11), which when produced by adipocytes in skin (12) function to prevent the transition of hair follicles from telogen to anagen phase (13). On the other hand, in skin, PTHrP overexpression has been shown to cause upregulation of beta-catenin and LEF-1 within the ectopic primary mammary mesenchyme (10), two signallers responsible for the transition of hair follicles into the anagen phase (14). Thus, in addition to promoting transitions from anagen to catagen phase, PTHrP might also be predicted to promote entry into anagen phase, thus having an overall effect of accelerating the hair cycle (Fig. 1b). Interestingly, such a dual regulatory role is similar to the well-described effects of thyroid hormone on the hair cycle (15), where thyroid hormone administration speeds the hair cycle and speeds regrowth after depilation, while hypothyroidism slows the hair cycle and ultimately results in thinning of the hair. Likewise, lepitin has been recently shown to stimulate transition of hair follicles to the anagen phase, through a different mechanism dependent on STAT3, and promotes more rapid regrowth of hair after shaving (16). If PTHrP does indeed function by inducing both entry and termination of the anagen phase of the hair cycle, the ultimate effects of PTHrP stimulation or blockade on hair growth would be dependent on the model under study. The ‘hairless’ mouse has a defect which results in upregulation of the Wnt inhibitor WISE, thus blocking hair follicles from entering the anagen phase (17). Thus, potentially adverse effects of PTHrP antagonism on slowing this transition in the hair cycle would be masked, leaving only the beneficial effects of prolonging the anagen phase by inhibiting anagen to catagen transitions. One might predict that in this model, PTHrP agonists would have a greater beneficial effect, if they can overcome the blockade imposed by WISE. This becomes critical when one considers applications to androgenic alopecia (18), where hair loss is thought to be mediated by inhibition of Wnt signalling, including upregulation of the Wnt inhibitor dickkopf 1 (19). Indeed, a mouse model of Marie Unna hereditary hypotrichosis (‘hairpoor’) was recently shown to overexpress dickkopf 1 (20). A transgenic mouse overexpressing the androgen receptor using the keratin 5 promotor mimics the phenotype of androgenic alopecia (21) and may provide an interesting test system. In models of chemotherapy-induced alopecia, the hair follicle dystrophy induced by cyclophosphamide damages anagen phase follicles and slows the hair cycle, which may again mask potentially deleterious effects of PTHrP antagonism on transitions to the anagen phase, leaving only the beneficial effects of delayed transition to the catagen phase. Over time, however, inhibiting the replacement of damaged anagen follicles would diminish these beneficial effects, consistent with the findings that after 30 days,
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
PTHrP and the hair cycle
Table 1. Effects of parathyroid hormone-related peptide (PTHrP) agonists and antagonists in several models of defective hair cycling Model
Hair cycle defect
Treatment effects
SKH-1 ‘hairless’ mouse
Impaired transition from telogen to anagen phase Impaired transition from anagen to catagen phase Pharmacological damage to anagen hair follicles
PTHrP antagonists stimulate hair growth by inhibiting transition of hair follicles from anagen to catagen phase PTHrP agonists shorten the coat by overcoming impaired anagen to catagen transition
FGF-5 KO mouse Cyclophosphamide administration (chemotherapy) C3H/HeJ engrafted mouse (alopecia areata) Waxing
Autoimmune damage to anagen hair follicles Physical damage to hair follicles, physical removal of hair shaft
PTHrP agonists promote hair growth by stimulating replacement of damaged anagen hair follicles PTHrP agonists promote hair growth by stimulating replacement of damaged anagen hair follicles PTHrP agonists promote more rapid regrowth of hair by stimulating transition of hair follicles to anagen phase. PTHrP antagonists cause limited initial hair growth by prolonging anagen of remaining hair follicles, but later prevent normal regrowth of hair by preventing hair follicles from re-entering the anagen phase
treatment with PTHrP agonists had more beneficial effects on hair growth than did PTHrP antagonists (4,9). To date, there are no reports on effects of PTHrP agonists and antagonists on radiationinduced hair loss, but as this is another model of hair loss resulting from damage to anagen phase follicles (22), it would be an interesting model for future investigation. We have tested the effects of PTHrP agonists and antagonists in another mouse model with damage to anagen phase hair follicles, the engrafted C3H/HeJ mouse model for alopecia areata. In this model, hair loss occurs due to autoimmune destruction of anagen hair follicles. Evaluation at the injection site showed beneficial effects of PTHrP agonists, but not of PTHrP antagonists, on hair growth (23). Importantly, as PTHrP agonists act by modulating the hair cycle, they were shown to be effective even when given after the alopecia had begun to develop. This is in contrast to most immune modulators, which are more effective if given before the hair loss develops (24). If PTHrP is important for telogen–anagen transition of hair follicles, PTHrP antagonism might be predicted to slow regrowth of hair after depilation by waxing, where the regrowth is highly dependent on generating a large number of anagen hair follicles. Indeed, while PTHrP agonists linked to a collagen binding domain accelerated hair regrowth after depilation, the effects of PTHrP antagonists can be seen in two phases. There is an initial eruption of a few hair shafts which precedes that seen in normal control mice. However, the long-term effect was to prevent the normal regrowth of hair after waxing (25). These data thus provide evidence that PTHrP blockade can inhibit entry into the anagen phase. The
References
1 Holick M F, Ray S, Chen T C et al. Proc Natl Acad Sci U S A 1994: 91: 8014–8016. 2 Wysolmerski J J, Broadus A E, Zhou J et al. Proc Natl Acad Sci U S A 1994: 91: 1133–1137. 3 Schilli M B, Ray S, Paus R et al. J Invest Dermatol 1997: 108: 928–932. 4 Peters E M, Foitzik K, Paus R et al. J Invest Dermatol 2001: 117: 173–178. 5 Cho Y M, Woodard G L, Dunbar M et al. J Invest Dermatol 2003: 120: 715–727. 6 Diamond A G, Gonterman R M, Anderson A L et al. J Invest Dermatol 2006: 126: 2127–2134. 7 Safer J D, Ray S, Holick M F. Endocrinology 2007: 148: 1167–1170. 8 Ponnapakkam T, Katikaneni R, Miller E et al. Calcif Tissue Int 2011: 88: 511–520. 9 Katikaneni R, Ponnapakkam T, Suda H et al. Int J Cancer 2012: 131: E813–E821.
effects of PTHrP agonists and antagonists in the above model systems are summarised in Table 1. Overall, we propose a hypothesis of PTHrP effects on the hair cycle which are quite similar to those seen with thyroid hormone. PTHrP causes initiation of the anagen phase, likely by upregulating beta-catenin and LEF-1. PTHrP also accelerates the transition of the hair follicle from the anagen to the catagen phase, the overall effect being to accelerate the hair cycle (Fig. 1b). In models where anagen to catagen transition is impaired, PTHrP agonists will reduce hair growth by re-establishing the normal termination of the hair cycle (6). In models where hair follicles have been damaged, PTHrP agonists compounds can accelerate regrowth and repair of damaged hair follicles by accelerating the hair cycle. While PTHrP antagonism can produce a paradoxical short-term increase in hair growth, caused by prolonging the anagen phase of existing follicles, the long-term effect is to inhibit the hair cycle and reduce hair growth.
Acknowledgements Robert Gensure wrote the report. I would like to acknowledge Tulasi Ponnapakkam, Ph.D., and Ranjitha Katikaneni, MBBS, for providing much of the experimental data which form the bases for the hypotheses presented in this viewpoint, as well as for their insightful discussions in helping to formulate these ideas.
Conflict of interests One of the compounds mentioned in this report, PTH-CBD, is patented, licensed to BiologicsMD, and currently under development as a therapy for fracture repair and for osteoporosis. Robert Gensure is the Chief Medical Officer of BiologicsMD.
10 Foley J, Dann P, Hong J et al. Development 2001: 128: 513–525. 11 Yu B, Zhao X, Yang C et al. J Bone Miner Res 2001: 27: 2001–2014. 12 Schmidt B, Horsley V. Exp Dermatol 2012: 21: 827–830. 13 Zhang J, He X C, Tong W G et al. Stem Cells 2006: 24: 2826–2839. 14 Huelsken J, Vogel R, Erdmann B et al. Cell 2001: 105: 533–545. 15 Safer J D. Dermatoendocrinol 2011: 3: 211– 215. 16 Sumikawa Y, Inui S, Nakajima T et al. Exp Dermatol 2014: 23: 27–32. 17 Beaudoin G M III, Sisk J M, Coulombe P A et al. Proc Natl Acad Sci U S A 2005: 102: 14653– 14658.
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
18 Inui S, Itami S. Exp Dermatol 2013: 22: 168– 171. 19 Kwack M H, Sung Y K, Chung E J et al. J Invest Dermatol 2008: 128: 262–269. 20 Kim B K, Lee H Y, Kim I et al. J Dermatol Sci 2014: 74: 81–87. 21 Crabtree J S, Kilbourne E J, Peano B J et al. Endocrinology 2010: 151: 2373–2380. 22 Wu X J, Jing J, Zhu J W et al. Exp Dermatol 2012: 21: 881–883. 23 Katikaneni R, Gulati R, Suh D et al. J Investig Dermatol Symp Proc 2013: 16: 25. 24 Kinori M, Bertolini M, Funk W et al. Exp Dermatol 2012: 21: 223–226. 25 Katikaneni R, Ponnapakkam T, Matsushita O et al. Anticancer Drugs 2014: 25: 30–38.
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www.wileyonlinelibrary.com/journal/EXD
Parathyroid hormone-related peptide and the hair cycle – is it the agonists or the antagonists that cause hair growth? Robert C. Gensure Pediatric Endocrinology, Children’s Hospital at Montefiore and Albert Einstein College of Medicine, Bronx, NY, USA Correspondence: Robert C. Gensure, MD/PhD, Pediatric Endocrinology, Children’s Hospital at Montefiore and Albert Einstein College of Medicine, 3415 Bainbridge Ave, Bronx, NY 10467, USA, Tel.: 718-920-4664, Fax: 718-405-5609, e-mail: [email protected] Abstract: While the effects of PTHrP have been studied for almost 20 years, most of these studies have focused on effects on the termination of the anagen phase, giving an incomplete picture of the overall effect of PTHrP on the hair cycle. PTHrP was determined in several experimental models to promote transition of hair follicles from anagen to catagen phase, which by itself would suggest that PTHrP blockade might prolong the anagen phase and promote hair growth. However, clinical trials with topically applied PTHrP antagonists have been disappointing, leading to a reconsideration of this model. Additional studies performed in mouse models where hair follicles are damaged (alopecia areata, chemotherapy-induced alopecia) suggest that
PTHrP has effects early in the hair cycle as well, promoting hair follicles’ entry into anagen phase and initiates the hair cycle. While the mechanism of this has yet to be elucidated, it may involve activation of the Wnt pathway. Thus, the overall effect of PTHrP is to stimulate and accelerate the hair cycle, and in the more clinically relevant models of hair loss where hair follicles have been damaged or become quiescent, it is the agonists, not the antagonists, which would be expected to promote hair growth.
The effects of parathyroid hormone-related peptide (PTHrP) agonists and antagonists on hair growth have been studied for over almost 20 years (1,2). Two reports appearing in 1994 used different models to examine effects of PTHrP agonists on hair. In the first, a transgenic model was employed, linking PTHrP to a K14 promotor for selective expression in the skin (2). The result was disappearance of the hair follicles from the ventral skin and delay in follicular development on the dorsal skin. In the second study, effects of PTHrP agonists and antagonists on hair growth were studied (1). In vitro studies showed hPTH(1-34) inhibited keratinocyte proliferation, and this effect was blocked by the antagonist bPTH(7-34). When administered daily to SKH-1 ‘hairless’ mice, PTHrP antagonism with bPTH(7-34) increased hair number and hair length after 7 days, while the agonist hPTH(134) had no effect on hair number or hair length. Taken together, these two studies indicate a role for PTHrP agonists to block formation of hair follicles, while PTHrP antagonists promote follicular activation, and, given the effects on hair length, may prolong the growth phase (Fig. 1a). A follow-up study examined effects of PTH(7-34) on the hair cycle in C57BL6 mice, showing evidence that the antagonist PTH(7-34) promoted entry into anagen phase from telogen phase and prevented progression from anagen to catagen phase (3). Based on these observations, an experiment was performed to test the effects of PTHrP agonists and antagonists in a mouse model of chemotherapy alopecia (4). Hair follicles were synchronised by waxing, and cyclophosphamide was administered 9 days later when hair follicles are in the anagen VI phase and are maximally sensitive. Two antagonists, PTH(7-34) and PTHrP(7-34), and one agonist, PTH(1-34), were evaluated. Results with two antagonists were not consistent at all time points, but generally showed increases in anagen VI hair follicles at 2 weeks, and
showed increases in visible hair at 2 weeks and 30 days compared with chemotherapy controls. Effects of the agonist PTH(1-34) appear to be biphasic – there was an initial increase in hair loss on day 14 in the PTH(1-34) group compared with the chemotherapy control group, and histological examination at this time point shows fewer anagen VI and more catagen hair follicles. However, by day 30, the PTH(1-34)-treated group showed increased regrowth of hair compared with chemotherapy controls, attributed to a more rapid hair cycle induced by the accelerated transition to catagen phase. The overall conclusion was that a paired treatment with PTH antagonists before chemotherapy to slow the hair cycle and prevent hair loss, followed by treatment with a PTH agonist after chemotherapy to accelerate the hair cycle and speed regrowth, would be optimal. A later study examined the spatial and temporal expression of PTHrP and PTH/PTHrP receptor expression after depilation (5). PTHrP was expressed throughout the early anagen phase throughout the follicle, but expression was increased in the proximal outer root sheath and isthmus during late anagen. Interestingly, PTH/ PTHrP receptor expression could only be observed during early anagen phase. Mating the K14-PTHrP mouse with the FGF-5 knockout mouse, which has extraordinarily long hair, resulted in a mouse with a relatively normal appearing coat, indicating that PTHrP could ‘rescue’ the long coat phenotype. This was confirmed in another mouse model of delayed catagen transition, the TSP1-KO mouse, where PTHrP overexpression again rescued the long hair phenotype (6). These findings again indicate that PTHrP induces anagen to catagen transition of hair follicles. As PTHrP promotes anagen to catagen transition, blocking PTHrP effect should prolong the anagen phase and promote hair growth (Fig. 1a). A topical formulation of a PTHrP antagonist was shown to increase hair growth in the SKH-1 ‘hairless’ mouse
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
Key words: alopecia – anagen – hair follicle – parathyroid hormone – parathyroid hormone-related peptide
Accepted for publication 13 July 2014
865
Gensure
(a)
Catagen
Telogen PTHrP
+
–
PTHrP
Anagen Embroynic epidermis
(b)
PTHrP + Nipple epithelium lineage (ventral surface only, no hair follicles)
Hair follicle lineage (Cycling hair follicles)
Telogen PTHrP
Catagen +
+
– Anagen –
Hairless mouse (impaired transition)
–
PTHrP
FGF-5 KO Mouse (impaired transition)
Chemotherapy, alopecia areata (damaged anagen follicles) Figure 1. Models of PTHrP effects on the hair cycle. Shown in panel (a) is the original model of PTHrP effects on the hair cycle, which suggested that PTHrP promotes transition of hair follicles from anagen to catagen stage, and can prevent formation of anagen phase hair follicles. PTHrP antagonists have the opposite effects (not shown). Shown in panel (b) is a revised model of PTHrP effects on the hair cycle, taking into account more recent data supporting a role for PTHrP to also promote transition of hair follicles from telogen to anagen phase. PTHrP antagonists have the opposite effects (not shown). Also shown in panel (b) are the defects in the various model systems in which PTHrP agonists and antagonists have been tested. In the SKH-1 ‘hairless’ mouse, defects in the gene hairless impair telogen to anagen transition. In the FGF-5 KO mouse, there is impaired transition from anagen to catagen phase. With chemotherapy and alopecia areata, there is direct damage to anagen hair follicles.
(7). Another topical formulation of the antagonist PTH(7-34) was tested in chemotherapy-induced alopecia in humans by IGI Pharmaceuticals. However, these trials were discontinued in phase 2, according to the company’s annual reports, because of lack of observed efficacy. The final results were never published. The failure of PTHrP antagonists in clinical trials may have been a result of the treatment regimen. Periodic application of an antagonist can be problematic; as the antagonist wears off, escape and even rebound effects can occur. We attempted to address this problem by linking both PTHrP agonists (PTH-CBD) and antagonists (PTH(7-33)-CBD) to a collagen binding domain, prolonging retention in skin (8). We tested these compounds using the experimental protocol of Peters et al. (4), expecting to see improved hair growth-promoting effects with our PTHrP antagonists. However, the actual results showed quite the opposite; PTHrP agonists linked to a collagen binding domain were effective at speeding regrowth of hair, while PTHrP antagonists appeared to have little effect (9). Histological examination showed that animals treated with the PTHrP agonist increased the number of anagen VI hair follicles and led to resolution of the dystrophic changes from the chemotherapeutics, while the PTHrP antagonist had little, and perhaps even deleterious, effects. We think the explanation for this surprising result can be found by re-examining some of the earlier studies. The disappearance of hair follicles on the ventral surface first noticed in
866
the K14 PTHrP mouse has since, by the same group of investigators, been attributed to development of ectopic nipple skin in this region rather than by direct inhibition of folliculogenesis (10). Nipple skin is normally devoid of hair follicles. In the study by Cho et al., both PTHrP and PTH/PTHrP receptor were expressed during the early anagen phase, with receptor expression at higher levels than that seen in the late anagen phase (5), which suggests a role for PTHrP for during the initiation of the anagen phase as well. The actual role of PTHrP would likely depend on which mediators are activated. In bone, PTHrP induces bone morphogenic proteins (11), which when produced by adipocytes in skin (12) function to prevent the transition of hair follicles from telogen to anagen phase (13). On the other hand, in skin, PTHrP overexpression has been shown to cause upregulation of beta-catenin and LEF-1 within the ectopic primary mammary mesenchyme (10), two signallers responsible for the transition of hair follicles into the anagen phase (14). Thus, in addition to promoting transitions from anagen to catagen phase, PTHrP might also be predicted to promote entry into anagen phase, thus having an overall effect of accelerating the hair cycle (Fig. 1b). Interestingly, such a dual regulatory role is similar to the well-described effects of thyroid hormone on the hair cycle (15), where thyroid hormone administration speeds the hair cycle and speeds regrowth after depilation, while hypothyroidism slows the hair cycle and ultimately results in thinning of the hair. Likewise, lepitin has been recently shown to stimulate transition of hair follicles to the anagen phase, through a different mechanism dependent on STAT3, and promotes more rapid regrowth of hair after shaving (16). If PTHrP does indeed function by inducing both entry and termination of the anagen phase of the hair cycle, the ultimate effects of PTHrP stimulation or blockade on hair growth would be dependent on the model under study. The ‘hairless’ mouse has a defect which results in upregulation of the Wnt inhibitor WISE, thus blocking hair follicles from entering the anagen phase (17). Thus, potentially adverse effects of PTHrP antagonism on slowing this transition in the hair cycle would be masked, leaving only the beneficial effects of prolonging the anagen phase by inhibiting anagen to catagen transitions. One might predict that in this model, PTHrP agonists would have a greater beneficial effect, if they can overcome the blockade imposed by WISE. This becomes critical when one considers applications to androgenic alopecia (18), where hair loss is thought to be mediated by inhibition of Wnt signalling, including upregulation of the Wnt inhibitor dickkopf 1 (19). Indeed, a mouse model of Marie Unna hereditary hypotrichosis (‘hairpoor’) was recently shown to overexpress dickkopf 1 (20). A transgenic mouse overexpressing the androgen receptor using the keratin 5 promotor mimics the phenotype of androgenic alopecia (21) and may provide an interesting test system. In models of chemotherapy-induced alopecia, the hair follicle dystrophy induced by cyclophosphamide damages anagen phase follicles and slows the hair cycle, which may again mask potentially deleterious effects of PTHrP antagonism on transitions to the anagen phase, leaving only the beneficial effects of delayed transition to the catagen phase. Over time, however, inhibiting the replacement of damaged anagen follicles would diminish these beneficial effects, consistent with the findings that after 30 days,
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
PTHrP and the hair cycle
Table 1. Effects of parathyroid hormone-related peptide (PTHrP) agonists and antagonists in several models of defective hair cycling Model
Hair cycle defect
Treatment effects
SKH-1 ‘hairless’ mouse
Impaired transition from telogen to anagen phase Impaired transition from anagen to catagen phase Pharmacological damage to anagen hair follicles
PTHrP antagonists stimulate hair growth by inhibiting transition of hair follicles from anagen to catagen phase PTHrP agonists shorten the coat by overcoming impaired anagen to catagen transition
FGF-5 KO mouse Cyclophosphamide administration (chemotherapy) C3H/HeJ engrafted mouse (alopecia areata) Waxing
Autoimmune damage to anagen hair follicles Physical damage to hair follicles, physical removal of hair shaft
PTHrP agonists promote hair growth by stimulating replacement of damaged anagen hair follicles PTHrP agonists promote hair growth by stimulating replacement of damaged anagen hair follicles PTHrP agonists promote more rapid regrowth of hair by stimulating transition of hair follicles to anagen phase. PTHrP antagonists cause limited initial hair growth by prolonging anagen of remaining hair follicles, but later prevent normal regrowth of hair by preventing hair follicles from re-entering the anagen phase
treatment with PTHrP agonists had more beneficial effects on hair growth than did PTHrP antagonists (4,9). To date, there are no reports on effects of PTHrP agonists and antagonists on radiationinduced hair loss, but as this is another model of hair loss resulting from damage to anagen phase follicles (22), it would be an interesting model for future investigation. We have tested the effects of PTHrP agonists and antagonists in another mouse model with damage to anagen phase hair follicles, the engrafted C3H/HeJ mouse model for alopecia areata. In this model, hair loss occurs due to autoimmune destruction of anagen hair follicles. Evaluation at the injection site showed beneficial effects of PTHrP agonists, but not of PTHrP antagonists, on hair growth (23). Importantly, as PTHrP agonists act by modulating the hair cycle, they were shown to be effective even when given after the alopecia had begun to develop. This is in contrast to most immune modulators, which are more effective if given before the hair loss develops (24). If PTHrP is important for telogen–anagen transition of hair follicles, PTHrP antagonism might be predicted to slow regrowth of hair after depilation by waxing, where the regrowth is highly dependent on generating a large number of anagen hair follicles. Indeed, while PTHrP agonists linked to a collagen binding domain accelerated hair regrowth after depilation, the effects of PTHrP antagonists can be seen in two phases. There is an initial eruption of a few hair shafts which precedes that seen in normal control mice. However, the long-term effect was to prevent the normal regrowth of hair after waxing (25). These data thus provide evidence that PTHrP blockade can inhibit entry into the anagen phase. The
References
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effects of PTHrP agonists and antagonists in the above model systems are summarised in Table 1. Overall, we propose a hypothesis of PTHrP effects on the hair cycle which are quite similar to those seen with thyroid hormone. PTHrP causes initiation of the anagen phase, likely by upregulating beta-catenin and LEF-1. PTHrP also accelerates the transition of the hair follicle from the anagen to the catagen phase, the overall effect being to accelerate the hair cycle (Fig. 1b). In models where anagen to catagen transition is impaired, PTHrP agonists will reduce hair growth by re-establishing the normal termination of the hair cycle (6). In models where hair follicles have been damaged, PTHrP agonists compounds can accelerate regrowth and repair of damaged hair follicles by accelerating the hair cycle. While PTHrP antagonism can produce a paradoxical short-term increase in hair growth, caused by prolonging the anagen phase of existing follicles, the long-term effect is to inhibit the hair cycle and reduce hair growth.
Acknowledgements Robert Gensure wrote the report. I would like to acknowledge Tulasi Ponnapakkam, Ph.D., and Ranjitha Katikaneni, MBBS, for providing much of the experimental data which form the bases for the hypotheses presented in this viewpoint, as well as for their insightful discussions in helping to formulate these ideas.
Conflict of interests One of the compounds mentioned in this report, PTH-CBD, is patented, licensed to BiologicsMD, and currently under development as a therapy for fracture repair and for osteoporosis. Robert Gensure is the Chief Medical Officer of BiologicsMD.
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ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 865–867
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