Metabolic studies on isolated hair follicles: hair follicles engage in aerobic glycolysis and do not demonstrate the glucose fatty acid cycle

Intermediate Hair Follicles from Patients with Female Pattern Hair Loss Are Associated with Nutrient Insufficiency and a Quiescent Metabolic Phenotype

Female pattern hair loss (FPHL) is a non-scarring alopecia resulting from the progressive conversion of the terminal (t) scalp hair follicles (HFs) into intermediate/miniaturized (i/m) HFs. Although data supporting nutrient deficiency in FPHL HFs are lacking, therapeutic strategies are often associated with nutritional supplementation. Here, we show by metabolic analysis that selected nutrients important for hair growth such as essential amino acids and vitamins are indeed decreased in affected iHFs compared to tHFs in FPHL scalp skin, confirming nutrient insufficiency. iHFs also displayed a more quiescent metabolic phenotype, as indicated by altered metabolite abundance in freshly collected HFs and release/consumption during organ culture of products/substrates of TCA cycle, aerobic glycolysis, and glutaminolysis. Yet, as assessed by exogenous nutrient supplementation ex vivo, nutrient uptake mechanisms are not impaired in affected FPHL iHFs. Moreover, blood vessel density is not diminished in iHFs versus tHFs, despite differences in tHFs from different FPHL scalp locations or versus healthy scalp or changes in the expression of angiogenesis-associated growth factors. Thus, our data reveal that affected iHFs in FPHL display a relative nutrient insufficiency and dormant metabolism, but are still capable of absorbing nutrients, supporting the potential of nutritional supplementation as an adjunct therapy for FPHL.

Human hair follicles operate an internal Cori cycle and modulate their growth via glycogen phosphorylase

Hair follicles (HFs) are unique, multi-compartment, mini-organs that cycle through phases of active hair growth and pigmentation (anagen), apoptosis-driven regression (catagen) and relative quiescence (telogen). Anagen HFs have high demands for energy and biosynthesis precursors mainly fulfilled by aerobic glycolysis. Histochemistry reports the outer root sheath (ORS) contains high levels of glycogen. To investigate a functional role for glycogen in the HF we quantified glycogen by Periodic-Acid Schiff (PAS) histomorphometry and colorimetric quantitative assay showing ORS of anagen VI HFs contained high levels of glycogen that decreased in catagen. qPCR and immunofluorescence microscopy showed the ORS expressed all enzymes for glycogen synthesis and metabolism. Using human ORS keratinocytes (ORS-KC) and ex vivo human HF organ culture we showed active glycogen metabolism by nutrient starvation and use of a specific glycogen phosphorylase (PYGL) inhibitor. Glycogen in ORS-KC was significantly increased by incubation with lactate demonstrating a functional Cori cycle. Inhibition of PYGL significantly stimulated the ex vivo growth of HFs and delayed onset of catagen. This study defines translationally relevant and therapeutically targetable new features of HF metabolism showing that human scalp HFs operate an internal Cori cycle, synthesize glycogen in the presence of lactate and modulate their growth via PYGL activity.

Inhibition of pyruvate oxidation as a versatile stimulator of the hair cycle in models of alopecia

Hair follicle stem cells (HFSCs) are known to be responsible for the initiation of a new hair cycle, but typically remain quiescent for very long periods. In alopecia, or hair loss disorders, follicles can be refractory to activation for years or even permanently. Alopecia can be triggered by autoimmunity, age, chemotherapeutic treatment, stress, disrupted circadian rhythm or other environmental insults. We previously showed that hair follicle stem cells and the hair cycle can be manipulated by regulation of pyruvate entry into mitochondria for subsequent oxidation to fuel the TCA cycle in normal adult mice with typical hair cycling. Here, we present new data from our efforts to develop murine models of alopecia based on environmental triggers that have been shown to do the same in human skin. We found that inhibition of pyruvate transport into mitochondria can accelerate the hair cycle even during refractory hair cycling due to age, repeated chemotherapeutic treatment and stress. Hair cycle acceleration in these alopecia models led to the formation of histologically normal hair follicles within 30-40 days of treatment without any overt signs of toxicity or deleterious effects. Therefore, we propose inhibition of pyruvate entry into mitochondria as a versatile treatment strategy for alopecia in humans.

Alopecia in Harlequin mutant mice is associated with reduced AIF protein levels and expression of retroviral elements

We investigated the contribution of apoptosis-inducing factor (AIF), a key regulator of mitochondrial biogenesis, in supporting hair growth. We report that pelage abnormalities developed during hair follicle (HF) morphogenesis in Harlequin (Hq) mutant mice. Fragility of the hair cortex was associated with decreased expression of genes encoding structural hair proteins, though key transcriptional regulators of HF development were expressed at normal levels. Notably, Aifm1 (R200 del) knockin males and Aifm1(R200 del)/Hq females showed minor hair defects, despite substantially reduced AIF levels. Furthermore, we cloned the integrated ecotropic provirus of the Aifm1Hq allele. We found that its overexpression in wild-type keratinocyte cell lines led to down-regulation of HF-specific Krt84 and Krtap3-3 genes without altering Aifm1 or epidermal Krt5 expression. Together, our findings imply that pelage paucity in Hq mutant mice is mechanistically linked to severe AIF deficiency and is associated with the expression of retroviral elements that might potentially influence the transcriptional regulation of structural hair proteins.

Computational derivation of a molecular framework for hair follicle biology from disease genes

Knowledge about genetic drivers of disease increases the efficiency of interpreting patient DNA sequence and helps to identify and prioritize biological points of intervention. Discoveries of genes with single mutations exerting substantial phenotypic impact reliably provide new biological insight, although such approaches tend to generate knowledge that is disjointed from the complexity of biological systems governed by elaborate networks. Here we sought to facilitate diagnostic sequencing for hair disorders and assess the underlying biology by compiling an archive of 684 genes discovered in studies of monogenic disorders and identifying molecular annotations enriched by them. To demonstrate utility for this dataset, we performed two data driven analyses. First, we extracted and analyzed data implicating enriched signaling pathways and identified previously unrecognized contributions from Hippo signaling. Second, we performed hierarchical clustering on the entire dataset to investigate the underlying causal structure of hair disorders. We identified 35 gene clusters representing genetically derived biological modules that provide a foundation for the development of a new disease taxonomy grounded in biology, rather than clinical presentations alone. This Resource will be useful for diagnostic sequencing in patients with diseases affecting the hair follicle, improved characterization of hair follicle biology, and methods development in precision medicine.

The nutritional biochemistry of wool and hair follicles

The rôle of various classes of nutrients (energy substrates, vitamins, minerals, amino acids) in the production of wool and hair from follicles, is considered for a variety of animal species. The wool and hair follicle have evolved a number of interesting features of carbohydrate metabolism including glutaminolysis, aerobic glycolysis, significant activity of the pentose phosphate pathway, and storage and mobilisation of glycogen. Presumably the necessity to continue to produce fibre despite fluctuations in the supply of oxygen and nutrients has resulted in some of these unique features, while others reflect the high level of DNA and protein synthesis occurring in the follicle. While it is considered that energy does not normally limit fibre growth, the relative contributions of aerobic and anerobic metabolism will greatly influence the amount of ATP available for follicle activity, such that energy availability may at times alter fibre growth. Alopecia and deficient fibre growth are consistent outcomes of deficiencies of biotin, riboflavin, pyridoxine, folate and pantothenic acid, but the precise rôles of these vitamins in follicle function await elucidation. Folate, in particular appears to play an important rôle in wool production, presumably reflecting its involvement in methionine metabolism. Cholecalciferol (vitamin D) significantly alters fibre growth in cultured follicles; vitamin D receptors are located in the outer root sheath, bulb, and dermal papilla of the follicle; and alopecia occurs in humans with defects in the vitamin D receptor. Retinol (vitamin A), too, appears to influence follicle function by altering keratinocyte proliferation and differentiation, with direct effects on the expression of keratin genes. The receptors for the retinoids are present in the keratogenous zone, the outer root sheath, the bulb, and the sebaceous glands. Vitamin A may also act indirectly on follicle function by influencing the activity of the insulin-like and epidermal growth factors and by altering vitamin D activity. At present there is little evidence implicating alpha-tocopherol (vitamin E) or phytylmenaquinone (vitamin K) in follicular events. Of the minerals, only copper and zinc have been shown to have direct effects on follicle function, independent of effects on food intake. Copper has direct effects on the activity of an unidentified enzyme on oxidation of thiol groups to form disulphide linkages. Wool produced by copper-deficient sheep lacks crimp, is weak and lustrous. Copper is also necessary for the activity of tyrosinase and the tyrosinase-related proteins involved in melanin synthesis. Zinc, like copper, is required for the normal keratinization of fibres but again, the precise rôle has yet to be elucidated. While the importance of amino acid supply for wool growth has long been established, there are still some unaswered questions such as; what are the effects of amino acids on fibre growth in animals other than sheep; what are the characteristics of the amino acid transport genes and proteins operating in the wool and hair follicle; and what are the specific rôles for amino acids in follicle function.

Adipocytes in skin health and disease

Adipocytes are intimately associated with the dermal compartment of the skin, existing in a specialized dermal depot and displaying dynamic changes in size during tissue homeostasis. However, the roles of adipocytes in cutaneous biology and disease are not well understood. Traditionally, adipocytes within tissues were thought to act as reservoirs of energy, as thermal, or as structural support. In this review, we discuss recent studies revealing the cellular basis of the dynamic development and regenerative capacity of dermal adipocytes associated with the hair cycle and following injury. We discuss and speculate on potential roles of dermal adipocytes in cutaneous biology with an emphasis on communication during hair follicle growth and wound healing. Finally, we explore how alterations in the dermal adipose tissue may support clinical manifestations of cutaneous diseases such as lipodystrophy, obesity, and alopecia.

Human hair follicles are an extrarenal source and a nonhematopoietic target of erythropoietin

Erythropoietin primarily serves as an essential growth factor for erythrocyte precursor cells. However, there is increasing evidence that erythropoietin (EPO)/EPO receptor (EPO-R) signaling operates as a potential tissue-protective system outside the bone marrow. Arguing that growing hair follicles (HF) are among the most rapidly proliferating tissues, we have here explored whether human HFs are sources of EPO and targets of EPO-R-mediated signaling. Human scalp skin and microdissected HFs were assessed for EPO and EPO-R expression, and the effects of EPO on organ-cultured HFs were assessed in the presence/absence of a classical apoptosis-inducing chemotherapeutic agent. Here, we show that human scalp HFs express EPO on the mRNA and protein level in situ, up-regulate EPO transcription under hypoxic conditions, and express transcripts for EPO-R and the EPO-stimulatory transcriptional cofactor hypoxia-inducible factor-1alpha. Although EPO does not significantly alter human hair growth in vitro, it significantly down-regulates chemotherapy-induced intrafollicular apoptosis and changes the gene expression program of the HFs. The current study points to intriguing targets of EPO beyond the erythropoietic system: human HFs are an extrarenal site of EPO production and an extrahematopoietic site of EPO-R expression. They may recruit EPO/EPO-R signaling e.g., for modulating HF apoptosis under conditions of hypoxia and chemotherapy-induced stress.

Biochemistry of human skin--our brain on the outside

The skin, our body's largest organ, is located at the interface between the external and internal environments, and so is strategically placed to provide not only a barrier against a range of noxious stressors (UV radiation, mechanical, chemical and biological insults) but also to act as the periphery's 'sensing' system. Recent developments suggest that this organ is much more critical to maintaining body homeostasis than previously thought. This tutorial review introduces the reader to some of the biochemistry that underpins the skin's enormous multi-functionality.

Human sebaceous glands engage in aerobic glycolysis and glutaminolysis

BACKGROUND

The skin and its appendages support aerobic glycolytic and glutaminolytic metabolism. Their major fuels are glucose and glutamine, which are, however, largely catabolized anaerobically.

OBJECTIVES

For the human sebaceous gland it has been reported that glucose, lactate and acetate provide good lipogenic substrates but that glutamine does not. Therefore, we have investigated the intermediary metabolism in vitro of freshly isolated human sebaceous glands to determine if their metabolism of glutamine is anomalous relative to the rest of the skin.

METHODS

Glycolytic rate, glucose and glutamine oxidation, and glucose metabolism by the pentose phosphate pathway were determined in freshly isolated human chest sebaceous glands. Further, sebaceous intermediary metabolites were analysed using spectrophotometry and high-performance liquid chromatography. Moreover, glands were maintained in vitro as whole organs to investigate the effects of precursors and inhibitors of polyamine synthesis on rates and patterns of lipogenesis and DNA synthesis.

RESULTS

We confirm that the human sebaceous gland is a glycolytic and glutaminolytic tissue. Glucose is mainly converted to lactate, with only 6% of glucose being oxidized to CO(2). Glutamine is largely converted to glutamate, alanine, serine, glycine, aspartate, threonine, lactate and ammonia, with only 12% being oxidized. We have also shown that exogenous glutamine is required for cellular proliferation and lipogenesis by human sebaceous glands. However, in its absence spermidine could fully restore rates of DNA synthesis and lipogenesis.

CONCLUSIONS

Although glutamine is a poor substrate for sebaceous lipogenesis, this cannot be attributed to its lack of catabolism. We have shown that glutamine is an essential fuel, but that it can be replaced by exogenous spermidine. Therefore, we suggest that in sebocytes both glutamine and spermidine may act as essential purine and pyrimidine precursors.

Lectin histochemistry of glycoconjugates in the feline hair follicle and hair

The distribution of glycoconjugate in the feline hair follicle and hair was studied by light and electron microscopic histochemical methods. The hair apparatus was found to contain considerable amounts of complex carbohydrates with different saccharide residues (alpha-D-mannose, beta-D-glucose, alpha-L-fucose, beta-N-acetyl-D-glucosamine). Variations of those were detected in the plasma membrane of the hair follicle cells during the course of their differentiation and keratinization, namely, alph-D-glucose, alpha-L-fucose and beta-N-acetyl-D-glucosamine in the suprabulbar and bulbar regions. The reaction level of sialic acid residues in the plasma membrane decreased in some cell layers during the course of differentiation. The results obtained from the present study indicated that interaction between saccharide residues of neutral carbohydrates and sialyl groups during the anagen phase might contribute to cell keratinization in hair follicles and hairs. It is discussed whether the existence of glycogen in outer root sheath cells might enable these cells to provide other hair apparatus cells with energy when necessary. Moreover, it became obvious from variations in sialyl residue distribution that cell differentiation processes terminate first of all in Huxley's and Henle's layers within the suprabulbar region of the hair follicle, as followed by the hair cortex.

Human hair growth in vitro: a model for the study of hair follicle biology

The factors that regulate hair follicle growth are still poorly understood. In vitro models may be useful in elucidating some aspects of hair follicle biology. We have developed an in vitro human hair growth model that enables us to maintain isolated human hair follicles for up to 10 days, during which time they continue to grow at an in vivo rate producing a keratinised hair fibre. We have shown that epidermal growth factor (EGF) in our system mimics the in vivo depilatory action of EGF in sheep, and suggest that this occurs as a result of EGF stimulating outer root sheath (ORS) cell proliferation which results in the disruption of normal mechanisms of cell-cell interaction in the hair follicle. We identify transforming growth factor-beta (TGF-beta) as a possible negative regulator of hair follicle growth and show that physiological levels of insulin-like growth factor-I (IGF-I) can support the same rates of hair follicle growth as supraphysiological levels of insulin. Furthermore, in the absence of insulin hair follicles show premature entry into a catagen-like state. This is prevented by physiological levels of IGF-I. Finally we demonstrate that the hair follicle is an aerobic glycolytic, glutaminolytic tissue and discuss the possible implications of this metabolism.

Growth of wool follicles in culture

A procedure for the culture of isolated wool follicles from Merino sheep is described. Follicles were microdissected from midside skin samples of 2-yr-old wethers and transferred, individually, to 24-well tissue culture plates. When maintained in supplemented Williams' E medium containing 5 to 10% fetal bovine serum (FBS), insulin, hydrocortisone, and a trace element mixture, fibre growth rates of 40 to 80 microns/day were observed. Follicles maintained their morphologic integrity for up to 7 days, incorporated [methyl-3H]thymidine into DNA and [35S]methionine into intermediate-filament keratins of the growing fiber. Insulin and hydrocortisone stimulated fiber growth at concentrations of 10 micrograms/ml and 50 ng/ml, respectively, but higher doses were inhibitory. The growth of fibers in response to hydrocortisone and the changes in follicle morphology was similar to those induced in skin after systemic administration of cortisol in vivo. A positive interaction between hydrocortisone and trace elements for follicle survival and hydrocortisone, insulin, and FBS for fiber growth was also found. The successful culture of Merino sheep follicles provides a model with which to study the direct influence of endocrine, nutritional and local factors on wool keratin synthesis independently of systemic shifts in the animals' metabolism.

Enhanced in vitro hair growth at the air-liquid interface: minoxidil preserves the root sheath in cultured whisker follicles

Inasmuch as hair follicles are difficult to maintain in culture, the study of hair biology using cultured hair follicles has met with only limited success. In our attempts to solve the problem of follicle degeneration, we cultured follicles at the air-surface interface on a modified collagen matrix (Gelfoam). In follicles cultured at the air-surface or submerged, we examined follicular morphology, hair shaft growth, sulfotransferase levels, cysteine incorporation, an expression of a tissue inhibitor of metalloproteinase (TIMP), and ultra-high sulfur keratin (UHSK). Follicles cultured at the air-liquid interface produced a 2.7-fold increase in hair growth and maintained an anagen-like morphology. Substrates such as nylon mesh seeded with fibroblasts, Full Thickness Skin, or 5-microns polycarbonate filter also supported hair growth, whereas Gelfilm, GF-A glass filter, filter paper, or 1-micron polycarbonate filter did not. The UHSK expression was significantly higher in the air-liquid interface cultures compared to the submerged culture. Several potassium channel openers, including minoxidil, a minoxidil analog, and the pinacidil analog (P-1075), all stimulated significant cysteine incorporation in follicles. Minoxidil and its analog specifically preserved the follicular root sheath, in contrast to P-1075 which did not, indicating a difference in the two drug types. The preservation of the root sheath was measured by increased TIMP expression and sulfotransferase activity and indicates that the root sheath is a target tissue for minoxidil. Our results show that follicles cultured at the air-liquid interface maintain a better morphology and produced greater hair growth than follicles cultured on tissue culture plastic.

Metabolism of freshly isolated human hair follicles capable of hair elongation: a glutaminolytic, aerobic glycolytic tissue

The metabolism of the human hair follicle was investigated in vitro under conditions that maintained glycogen and adenosine triphosphate (ATP) content and the growth rate of the follicle at values observed in vivo. We have shown that only 10% of the total glucose utilized was oxidized to CO2 and 40% of this was oxidized via the pentose phosphate shunt. Although fatty acids and ketone bodies were oxidized by the hair follicle, they are poor energetic substitutes for glucose. Nor will fatty acids or ketone bodies sustain hair growth in vitro. Glutamine, however, was shown, both biochemically and by comparing growth rates, to be an important fuel with 23% of uptake being oxidized, generating a possible 2.16 +/- 0.32 nmoles ATP/follicle/h (mean +/- SEM) (glucose metabolism generates 4.54 +/- 0.61 nmoles ATP/follicle/h). Sixty-four percent of the glutamine taken up was calculated to be metabolized to lactate, showing that the hair follicle engages in both glycolysis and glutaminolysis. The glucose-fatty acid cycle appears to be unimportant in the hair follicle but our data indicates that a glucose-glutamine cycle does operate.