Potential role of FoxO1 and mTORC1 in the pathogenesis of Western diet-induced acne

Acne in adolescents of developed countries is an epidemic skin disease and has currently been linked to the Western diet (WD). It is the intention of this viewpoint to discuss the possible impact of WD-mediated nutrient signalling in the pathogenesis of acne. High glycaemic load and dairy protein consumption both increase insulin/insulin-like growth factor-1 (IGF-1) signalling (IIS) that is superimposed on elevated IGF-1 signalling of puberty. The cell's nutritional status is primarily sensed by the forkhead box transcription factor O1 (FoxO1) and the serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1). Increased IIS extrudes FoxO1 into the cytoplasm, whereas nuclear FoxO1 suppresses hepatic IGF-1 synthesis and thus impairs somatic growth. FoxO1 attenuates androgen signalling, interacts with regulatory proteins important for sebaceous lipogenesis, regulates the activity of innate and adaptive immunity, antagonizes oxidative stress and most importantly functions as a rheostat of mTORC1, the master regulator of cell growth, proliferation and metabolic homoeostasis. Thus, FoxO1 links nutrient availability to mTORC1-driven processes: increased protein and lipid synthesis, cell proliferation, cell differentiation including hyperproliferation of acroinfundibular keratinocytes, sebaceous gland hyperplasia, increased sebaceous lipogenesis, insulin resistance and increased body mass index. Enhanced androgen, TNF-α and IGF-1 signalling due to genetic polymorphisms promoting the risk of acne all converge in mTORC1 activation, which is further enhanced by nutrient signalling of WD. Deeper insights into the molecular interplay of FoxO1/mTORC1-mediated nutrient signalling are thus of critical importance to understand the impact of WD on the promotion of epidemic acne and more serious mTORC1-driven diseases of civilization.


Introduction
Acne is a disease of Western civilization with prevalence rates in adolescence of over 85% (1)(2)(3). Western diet (WD), characterized by high glycaemic load and high dairy protein consumption, has been suggested to be a fundamental nutritional factor promoting the acne epidemic (4,5). Proper functioning of the pathways that are involved in sensing of nutrients is central to metabolic homoeostasis (6). Notably, acne is absent in populations consuming less insulinotropic Palaeolithic diets (1,7), which exclude grains, milk and dairy products and exhibit much lower insulin/insulin-like growth factor (IGF-1) signalling (IIS) (4,7).
It is the purpose of this viewpoint to elucidate the molecular pathology of nutrient signalling of WD in the pathogenesis of acne. WD-derived metabolic signals are sensed by the forkhead box class O1 transcription factor (FoxO1) and the nutrient-sensitive kinase mammalian target of rapamycin complex 1 (mTORC1). mTORC1 is regarded as the conductor of the 'cellular signalling symphony' that integrates signals of cellular energy, growth factors and amino acids (8,9). Metabolic regulations mediated by FoxO1 and mTORC1 depend on upstream activation of the IIS cascade, required for adaptive nutrient homoeostasis and endocrine growth regulation (10).

FoxO1: a nutrient-sensing transcription factor
FoxO1 is an important transcription factor that modulates the expression of genes involved in cell cycle control, DNA damage repair, apoptosis, oxidative stress management, cell differentiation, glucose and lipid metabolism, inflammation, and innate and adaptive immune functions (36)(37)(38)(39)(40)(41)(42). FoxO1 is expressed in all mammalian tissues including human SGs (Fig. 2) and plays an important role in the regulation of metabolism (43). Mouse hepatic chromatin exhibited 401 FoxO1-binding locations, regulating metabolic processing of carboxylic acids, fatty acids, steroids and retinoids (44). FoxO1 has been proposed to function as a key regulator in the pathogenesis of acne as FoxO1 senses external nutrient and internal growth factor signals and relays these to FoxO1-dependent gene regulation (45).
Central to the regulation of FoxOs is their shuttling either into the nucleus or into the cytosol. FoxO1 is inhibited by its export into the cytoplasm, which requires specific phosphorylation of FoxO1 in the nucleus by activated Akt kinase (36,37,40). The phosphoinositol-3 kinase (PI3K)/Akt cascade is stimulated by growth factors like insulin and IGF-1 and is negatively regulated by the phosphatase PTEN (46). Thus, increased IIS of WD is superimposed on enhanced IIS of puberty, the two converging in inhibition of FoxO1-dependent gene regulation (4) (Fig. 1).

FoxO1 inhibits hepatic IGF-1 secretion
Nuclear FoxO1 is highly upregulated during fasting. However, in the postprandial state and nutrient overload, enhanced IIS inhibits FoxO1 (43,47,48). It is understandable that FoxO1 closely interacts with regulators of somatic growth, which is suppressed in the absence of nutrients. A major regulatory node of somatic growth and hepatic IGF-1 secretion is the growth hormone receptor (GHR).
Untreated individuals with Laron syndrome, a primary growth hormone (GH) resistance disorder due to a genetic defect of GHR, exhibit diminished congenital IGF-1 serum levels, are of short stature and never develop acne (49,50). IGF-1-deficient serum of Laron individuals increased nuclear FoxO levels and inhibited mTORC1 activity (50). The DKO mouse is an animal model mimicking Laron syndrome with impaired IIS. DKO mice have a double knockout of insulin receptor substrate-1 (IRS)-1 and IRS-2 in the liver, are shorter and exhibit 20% less body mass than control mice (10). Reduced hepatic Akt signalling with increased nuclear FoxO1 levels in DKO mice resulted in decreased expression of GHR, IGF-1 and sterol regulatory element binding protein (SREBP)-1c and increased expression of IGF-binding protein-1 (IGFBP-1) (10).
Isotretinoin's sebum-suppressive effect has recently been associated with upregulated FoxO1 expression (71). Reported reductions in IGF-1 serum levels during isotretinoin treatment (72) are thus well explained by FoxO1-mediated inhibition of hepatic GHR expression resulting in diminished hepatic synthesis of IGF-1.

FoxO1 suppresses androgen signalling
Sebaceous gland growth and acne are androgen dependent (73). The growth of androgen-responsive tissues is coordinated with general somatic growth (74). IGF-1 stimulates gonadal and adrenal androgen synthesis as well as intracutaneous intracrine conversion of testosterone to tenfold more active dihydrotestosterone, the most potent androgen receptor (AR) ligand (11) (Fig. 1). Enhanced hepatic IGF-1 synthesis by WD may thus increase the availability of potent androgens in the skin.
Remarkably, only a few acne patients exhibit hyperandrogenaemia, a fact that points to the predominance of peripheral tissuedependent androgen/AR sensitivity for the manifestation of acne (73). Intriguingly, FoxO1 functions as an AR cosuppressor (75)(76)(77). Nuclear extrusion of FoxO1 by high IIS relieves FoxO1-mediated repression of AR transactivation. Thus, insulinotropic WD may stimulate AR-mediated signalling, which explains enhanced peripheral androgen responsiveness (Fig. 1).

FoxO1 links nutritional status to innate and adaptive immunity
FoxO family members suppress the highly substrate-and energydependent process of T-cell activation (38), whereas FoxO1 deficiency in vivo resulted in spontaneous T-cell activation and effector differentiation (39). Increased CD4+ T-cell infiltration and enhanced IL-1 activity have been detected in acne-prone skin areas prior to comedo formation (88). Thus, FoxO1 links nutrient availability and metabolic conditions to T-cell homoeostasis (89,90) (see Data S1).

FoxOs control antimicrobial peptide synthesis
In Drosophila flies, dFOXO controls the expression of several antimicrobial peptides (AMPs) in the skin (91). AMP induction is lost in dFOXO null mutants but enhanced when dFOXO is overexpressed. In Drosophila, AMP activation can be achieved independently of pathogen-dependent pathways, indicating direct cross-regulation between metabolism and innate immunity (91). As in Drosophila, downregulated FoxO inhibited AMP expression in the polyp Hydra vulgaris (92).
Downregulated FoxO signalling by WD may thus favour an AMP-deficient follicular microenvironment, which may allow overgrowth of P. acnes. WD would not only overstimulate sebum production favouring P. acnes growth but may diminish AMPcontrolled host responses against P. acnes, which may ultimately stimulate inflammatory TLR-mediated innate immune responses against hypercolonized P. acnes (93,94). Upregulated TLR-driven innate immune responses against P. acnes with overexpression of TNF-a may further enhance SG lipogenesis via activated proinflammatory IKKb-TSC1-mTORC1 signalling (see Data S1, Fig. S1) (95). mTORC1: Convergence point of nutrient signalling in acne mTORC1 is an evolutionarily conserved nutrient-sensing kinase that regulates growth and metabolism in all eukaryotic cells (8,9,83). mTORC1 signalling serves as a 'growth checkpoint' surveying the status of the extra-and intracellular milieu of growth factors and nutrients (83,96). mTORC1 signalling stimulates gene transcription, translation, ribosome biogenesis, protein synthesis, cell growth, cell proliferation and lipid synthesis but suppresses autophagy (8,(97)(98)(99)(100)(101). In mammalian cells, two functionally different mTOR complexes exist: mTORC1 and mTORC2. mTORC1 contains the partner protein Raptor that interacts with mTORC1 substrates for their phosphorylation. mTORC2 contains the protein Rictor and activates the kinase Akt. mTORC1 controls the G1/S transition and G2/M progression of the cell cycle (98). The mTORC1 signalling network senses and relays diverse inputs of nutrients, growth factors and cellular energy to a central 'signalling core' that consists of Akt, TSC1/TSC2, Rheb and mTORC1 itself (Fig. 1) (82,102). Liver kinase B1 and the energy sensor AMP-activated protein kinase (AMPK) are critical regulators of mTORC1 (103) (see mTORC1 nutrisome signalling, Data S1).
Western diet overactivates mTORC1 by providing an abundance of dairy-and meat-derived essential amino acids, increased IIS induced by dairy protein consumption and high glycaemic load and suppressed AMPK activity by calorie excess. As protein and lipid biosynthesis, cell growth and proliferation are coordinated by Akt-phosphorylated cytoplasmic FoxO1 dissociates and thereby inhibits the TSC1/TSC2 heterodimer b-Catenin Augmentation of nuclear FoxO1 signalling GSK3 Modulation of GSK3-TSC2-mTORC1 signalling CRM1 Nuclear FoxO1 export Table 1. Important FoxO1-regulated target genes in the pathogenesis of acne Growth hormone receptor (GHR) Suppression of GHR expression with downregulation of hepatic IGF-1 synthesis IGF-binding protein-1 (IGFBP-1) Upregulation of IGFBP-1 expression, reduction in circulating free IGF-1 Eukaryotic initiation factor 4 binding protein-1 (4E-BP-1) Activation of 4E-BP-1 expression inhibiting mRNA translation p21 Activation of p21 expression, cell cycle inhibition, growth inhibition p27 Activation of p27, cell cycle inhibition, growth inhibition Sestrin3 Activation of sestrin3 expression, activation of AMPK-mediated phosphorylation of TSC2 activating the inhibitory function of TSC1/TSC2, thus suppressing mTORC1 mTORC1, it is obvious that mTORC1 plays a key role in acne pathogenesis, characterized by increased proliferation of acroinfundibular keratinocytes, SG hyperplasia and increased SG lipogenesis (5).

Acne and mTORC1-driven insulin resistance
Nutrient signalling of WD results in increased activation of downstream substrates of mTORC1, the S6 kinases S6K1 and S6K2. S6K1-mediated phosphorylation of insulin receptor substrate 1 (IRS-1) downregulates IIS and thus induces insulin resistance. Dietary fatty acids directly activate S6K1 independent of mTORC1 (104). Insulin resistance is considered to be a physiological feature of increased growth during puberty (105). However, pathologically persistent insulin resistance is associated with the metabolic syndrome as well as acne-associated syndromes (106,107). Thus, increased mTORC1/S6K1 signalling explains the reported associations between WD, acne, increased body mass index (BMI) and insulin resistance (32,108,109).

mTORC1 regulates lipid synthesis
Increased SG lipid biosynthesis is responsible for seborrhoea and SG hyperplasia. Importantly, the key transcription factor of lipid biosynthesis SREBP-1 depends on mTORC1 activation (110). mTORC1 phosphorylates lipin-1, which controls the access of SREBP-1 to the promoter region of SREBP-1-dependent lipogenic genes in the nucleus (110).

FoxO1: the rheostat regulating mTORC1
As both mTORC1 and FoxO1 integrate nutrient and growth factor signals, it is conceivable that they interact with each other to coordinate cellular responses to nutrient availability. FoxOs are pivotal inhibitors of mTORC1 and have emerged as important rheostats that modulate the activity of Akt and mTORC1 (111). FoxO1, FoxO3 and FoxO4 induce the expression of sestrin3 that activates AMPK, which inhibits mTORC1 ( Fig. 1) (85). Activated AMPK phosphorylates FoxO3 and facilitates its nuclear localization (112). Furthermore, Akt-phosphorylated cytoplasmic FoxO1 binds to TSC2 and thereby dissociates the TSC1/TSC2 complex, which activates mTORC1 (113). Thus, activated Akt inhibits FoxO1, FoxO3 and FoxO4 through direct phosphorylation and indirectly activates mTORC1, which in turn increases protein and lipid synthesis and induces insulin resistance (113). mTORC1-activated S6K1 via inhibitory IRS-1-phosphorylation elicits a negative feedback loop to inhibit Akt. In contrast, FoxO1 also induces insulin and IGF-1 receptors and IRS-2, a feedback mechanism, which increases insulin sensitivity (111,114). FoxO1 elevates the expression of Rictor, leading to increased mTORC2 activity that consecutively activates Akt.
In summary, FoxO transcription factors, especially FoxO1, inhibit the activity of mTORC1 at multiple levels of cellular regulation (Table 3).

Conclusion
Insulinotropic WD impairs FoxO1-mediated gene regulation in acne (45). FoxO1 controls the somatotropic axis, modifies the magnitude of AR signalling, interacts with important nuclear regulators of SG homoeostasis, metabolism and lipogenesis and most importantly coordinates the activity of mTORC1.
Acne vulgaris with exacerbated pilosebaceous mTORC1 signalling belongs to the family of mTORC1-driven diseases of civilization (5). Dermatologists counselling acne patients, especially the young, should not only focus on the treatment of skin pathology but should advise on means to correct inappropriate systemic mTORC1 signalling that is aggravated by WD. This is essential to prevent more serious mTORC1-driven diseases of civilization like obesity, diabetes and cancer (5,120).
Nutritional therapy of acne should (i) normalize total calorie intake, (ii) lower glycaemic load and (iii) restrict total dairy protein consumption, especially whey protein abuse (5,19,25,(32)(33)(34). The ideal nutritional therapy of acne should favour (i) a Palaeolithic-type diet containing less insulinotropic grains and minimal or no dairy products to avoid increased IIS and androgen precursors present in dairy products, (ii) higher consumption of vegetables, fruits and green tea containing natural plant-derived mTORC1 inhibitors (epigallocatechin gallate, resveratrol and other natural polyphenols) (121) and (iii) increased consumption of fish (lower insulinaemic index than dairy protein; source of antiinflammatory x-3 fatty acids) and adequate intake of vitamin D (see Data S1, Fig. S1).