Cannabadiol, CBD, is a low affinity ligand of the cannabinoid 1 receptor, CB1. It may also act and a mild antagonist. CBD is also marketed as a remedy for hair loss or a hair growth enhancer.
Is CB1 even expressed in human hair follicles?
Hair follicle growth and loss occurs in three phases. Telek and coworkers (2007) used antibodies to measure CB1 expression in the phases of the hair cycle. Anagen phase is the growth phase in which the hair hair shaft is attached to the dermal papilla and blood supply. The hair shaft becomes separated from the dermal papilla and blood supply in catagen phase. In telogen phase the hair shaft is dead. The follicle that produced it is in a resting phase.
Cannabinoids: Growth and death in the hair follicle
Figures 1a and 3a from Telek (2007). Amandamide (AEA) and 2-Arachidonoylglycerol (2-AG) are endogenous cannabinoids. (−)-trans-Δ⁹-tetrahydrocannabinol (THC) is a plant cannabinoid.
Telek and coworkers (2007) also used basic histology to assess the percent of cells in catagen and anagen phases. The hair bulb is more narrow and depigmented in catagen (Mescalchin 2005).
Proliferation was measured by the presence of the Ki-67 protein
- In order to translate messenger RNA into proteins, the cell needs ribosomes.
- Ribosomes are made up of proteins and ribosomal RNA.
- Ribosomal RNA is encoded by chromosomal DNA.
- Ki-67 is a protein involved in transcribing DNA into rRNA.
- Sections of hair follicles were incubated with a primary antibody against Ki-67 produced in a mouse.
- Red emitting fluorescent probe tagged secondary antibodies against mouse antibodies that was made in a rabbit.
Apoptosis is a form of programmed cell death that is a normal part of the catagen phase. DNA fragmentation is one aspect of this programed cell death.
- A enzyme called terminal deoxynucleotidyl transferase is used to incorporate the nucleotide dUTP in the nicked end.
- This dUTP is conjugated with a unique small molecule.
- Sections were incubated with a primary antibody raised in the rabbit against the unique small molecule.
- A secondary antibody raised in the goat against all rabbit antibodies was conjugated with a green emitting fluorescent probe.
The blue dots are nuclei stained with the blue flourophore DAPI. *P < 0.05 when compared to control. DP, dermal papilla, MK, matrix keratinocytes.
Cross talk between CB1 and TRPV1
Background. G-protein coupled receptors (GPCR), of which CB1 is one, bind heterotrimeric G proteins. This link explores the different flavors of the Gα subunits as well as the βγ subunit. In simplistic terms, when an agonist binds to the GPCR, GDP is replaced by GTP. The α and βγ subunits and do their separate things.
Information was not found on the varieties of Gα subunits in the hair follicles and layers therein. Some possibilities are presented based on cross talk between GPCR and the TRPV1 cation ion channel. Acidic pH, heat, anandamide, and capsaicin gate the flow of sodium and calcium through the TRPV1 channel.
Fig 3
The first question asked by figure 3a is, “Can anandamide and capsaicin synergize to inhibit elongation?” The answer is yes. (* significantly different at p<0.05).
The second question asked by Fig 3a is, “In the presence of anandamide and the absence of capsaicin, can inhibiting either CB1 or TRPV1 stimulate elongation?” Inhibiting CB1 allows for elongation in the presence of anandamide. Inhibiting TRPV1 makes no difference.
The third question figure 3a asks is , “In the presence of capsaicin and the absence of amandamide, can inhibiting either CB1 or TRPV1 stimulate elongation?” Inhibiting TRPV1 allows for elongation in the presence of capsaicin. Inhibiting CB1 makes no difference.
Figure 3b puts everything together and asks if the combination of anandamide and capsaicin can decrease the number of cells actively dividing and increase the number of cells undergoing apoptosis.
A capsaicin caveat
Sanz-Salvador and coworkers (2012) have demonstrated that over stimulation of TRPV1 with capsaicin targets these channels for lysosomal degradation. The following are short snap shots of a publication that is available free.
The experiments were performed in triplicate or quadruplicate in human embryonic kidney HEK293 cells expressing TRPV1. Note the relative effectiveness of the capsaicin analog resiniferatoxin (RTX). Iodoresiniferatoxin (I-TRX) used in the Telek (2007) is an antagonist. RTX is a potent agonist.
Chloroquine eners the lysosomes and increases their pH making them inactive. Leupeptin is an inhibitor of the proteases (Pac men) in the lysosomes.
This is a rather generic cartoon combined with some data from Sanz-Salvador (2012). The caveat to the previous Telek (2007) report is that the 10 μM capsaicin that they used could have been acting as a long term down regulator.
CBD cream for hair restoration
Talek (2007) did a nice job of demonstrating only CB1 in human hair follicles. Cannabidiol (CBD) is marketed in creams for hair restoration, mechanism unknown. Thomas and coworkers (2007) conducted an in depth study on CBD interaction with CB1 and CB2. They were prompted by earlier studies showing that CBD produces antagonism at concentrations below those which it binds to CB1 and CB2 receptors. This observation was suggestive of competing with agonists WIN55212 and CP55940 for non-CB1 pharmacological targets on nerve terminals. Thomas and coworkers used mouse brain membrane vesicles from wild type mice and CB1 knock out mice. Thomas and coworkers performed direct competition assays as wells as assays of receptor activation using [35S] GTPγS, a radioactive analog of GTP which cannot be hydrolyzed to GDP.
This is Fig 3 of Thomas (2007) with some background information. GTγS binding was compared to unstimulated brain membranes. The agonists CP55940 and Win55940 increase GTγS binding above basal levels. Note that CBD and rimonabant decrease basal activity at lower concentrations than does O-2654.
An inverse agonist is an agent that binds to the same receptor as an agonist but induces a pharmacological response opposite to that agonist.
A neutral antagonist has no activity in the absence of an agonist or inverse agonist but can block the activity of either.
A negative allosteric modulator reduces the affect of the primary ligand.
The slight 10 μM CBD decrease in basal activity is seen in brain vesicles is with and without the presence of CB1. The same can be said for 10 μM rimonabant, a CB1 inverse agonist. In CB1 CHO cell membrane vesicles, CBD slightly stimulated basal activity at sub micromolar concentrations. This study quickly turned into being all about CB2 related mechanisms.
A negative allosteric modulator, NAM
LaPrairie and coworkers (2015) closely examined CBD’s ability to modulate CB1. HEK239A cells were used for expressing wild type and mutant forms of CB1. STHdhQ7/Q7 cells were derived from conditionally immortalized striatal progenitor cells of embryonic day 14 of C57BL/6J mice.
Phospholipase C (PLC) is an inactive enzyme until it is phosphorylated on Ser537. Like anandamide, 2-AG is an endocannabinioid agonist of CB1. LaPrairie also tested the plant cannabinoid THC and neutral antagonist O-2050. Two of many concentration response curves are shown. ERK is also activated by phosphorylation. Antibodies against these two enzymes and specific phosphorylated activation sites were used to measure activation.
Only the results of activated (phosphorylated) PLCβ3 and ERK1/2 are presented for brevity. This publication is available free online. Because the discussion is on hair growth, only inhibition of 2-AG is presented. ANOVA * p<0.01 compared to DMSO. Non overlapping confidence intervals, † p<0.01 compared to DMSO.
The Hill coefficient n is a measure of cooperativity: n<1 implies that the second molecule decreases the affinity of the first, n>1 implies that the second molecule increases the affinity of the first.
LaPrairie and coworkers, in their extensive study, tested the hypothesis that Cys98 and Cys107 were necessary for NAM activity. Mutation of these residues to Serine resulted in no change in NAM activity whereas mutation of these residues to alanine resulted in a 50% decrease in NAM activity.
Concluding Comments
Telek and coworkers (2007) demonstrated that the endocannabinoid anandamide and the exocannabinoid THC slow hair growth and promote entrance into the catagen phase. Capsaicin and anandamide act synergistically to promote this process. Mercati and coworkers (2012) found CB2 in canine hair follicles in addition to CB1. Perhaps he presence of CB1 and CB2 needs to be examined in the blood vessels that nourish the growing hair shaft.
A caveat to capsaicin (Sanz-Salvador, 2012) is that too much capsaicin stimulation of the TRPV1 cation channel results in its uptake. How CB1 stimulation or inhibition in the hair follicle affects TRPV1 uptake (± capsaicin) is an unresolved matter.
In an early study Thomas and coworkers (2007) In the absence of agonists, 10 μM CBD acts as an inverse agonist of CB1 in CB1-CHO cell membranes, decreasing agonist free activity to below baseline. They quickly moved onto more tangible interactions (in their system) of CB2 and CBD.
LaPrairie and coworkers (2015) presented convincing arguments that CBD is a negative allosteric modulator of CB1. They also demonstrated a probable site of action to the N-terminus of the orthosteric agonist binding site.
References
All of the papers cited in this blog are available free on line. Educational material has been added to some of the figures to make them more understandable to a general audience.
Laprairie RB, Bagher AM, Kelly ME, Denovan-Wright EM.(2015) Cannabidiol is a negative allosteric modulator of the cannabinoid CB1 receptor. Br J Pharmacol.172(20):4790-805. PMID
Mercati F, Dall’Aglio C, Pascucci L, Boiti C, Ceccarelli P. (2012) Identification of cannabinoid type 1 receptor in dog hair follicles. Acta Histochem. 114(1):68-71. PMID
Sanz-Salvador L, Andrés-Borderia A, Ferrer-Montiel A, Planells-Cases R.(2012)Agonist- and Ca2+-dependent desensitization of TRPV1 channel targets the receptor to lysosomes for degradation. J Biol Chem.287(23):19462-71. PMID
Thomas A, Baillie GL, Phillips AM, Razdan RK, Ross RA, Pertwee RG. (2007) Cannabidiol displays unexpectedly high potency as an antagonist of CB1 and CB2 receptor agonists in vitro. Br J Pharmacol. 150(5):613-23.
Telek A, Bíró T, Bodó E, Tóth BI, Borbíró I, Kunos G, Paus R. (2007)Inhibition of human hair follicle growth by endo- and exocannabinoids. FASEB J. 21(13):3534-41. PMID
