Mechanistic target of rapamycin (mTOR) expression is increased in acne patients' skin

In the last years, a clear link between the endocrine signalling also influenced by Western diet and the pathogenesis of acne has emerged 1, 2. In this context, a crucial role has been attributed to the hyperactivity of the conserved serine/threonine kinase mTORC1 3. mTOR is a nutrient-sensitive regulator of cellular growth, proliferation, lipid synthesis and protein translation, exerting its effects through two distinct signalling complexes: mTORC1 and mTORC2 4. In particular, the rapamycin-sensitive mTORC1 promotes cell growth and proliferation whereas mTORC2, rapamycin-non-sensitive, is involved in the regulation of cell polarity and in the functional phosphorylation of cytoskeleton actin fibres. mTORC1 is able to integrate multiple intra- and extracellular mediators such as growth factors (insulin, IGF-1) and energy-sensing signals (glucose, AMP/ATP-ratio), controlling the adequate availability of amino acids, especially the BCAA leucine, necessary for its own activation. Multiple diseases, either neoplastic, dysmetabolic or inflammatory, show dysregulation of mTOR pathway, which might be also involved in inflammatory skin diseases such as psoriasis, atopic dermatitis and allergic contact dermatitis 5. Enhanced levels of leucine and other amino acids, insulin, IGF-1 and glucose, deriving from meat, milk/dairy intake and high glycaemic load, permit mTORC1 activation through specific mechanisms (s1-s4). Consequently, by the phosphorylation of 4E-BP1, the lipin1 activator of SREBP-1 and S6K1, mTORC1 is able to enhance protein and lipid synthesis, as well as growth and cell proliferation 3. This could be translated into proliferation of acroinfundibular keratinocytes and increased lipid biosynthesis in sebaceous glands, responsible for seborrhoea. In two recent elegant papers, Melnik integrated mTORC1 signalling into the complex scenario of acne pathogenesis, highlighting the antagonistic interaction between the metabolic transcription factor FoxO1 and the nutrient-sensitive kinase mTORC1 (6, 7,s5,s6). Nevertheless, either for the role of FoxO1 or for the modulation of mTOR in the pathophysiology of acne, direct evidences in humans are lacking (8, s7, s8). The aim of this preliminary study was to investigate whether mTOR expression was modified in skin samples from acne patients. A possible relation between mTOR and patients' metabolic profile was also assessed. The study population comprised 10 acne patients (five males and five females) and 10 controls. All subjects underwent anthropometric measurement, fasting serum glucose, insulin, glucose-to-insulin ratio evaluation and HOMA-IR. Skin biopsies were taken from LS and NLS areas; RT-PCR (primers listed in ST-1), IHC and IF were performed to assess mTOR gene and protein expression. IF was also performed to assess S6K1 and its phosphorylated active form P-S6K1. Moreover, blood was collected and TNF-α, IL-6 and IL-8 plasma levels were measured with ELISA, to check the systemic inflammatory profile. Details in SM. Characteristics of the study population are resumed in Table S2 (ST-2). None of the patients was obese or considered insulin resistant, but one of them was classified overweight (BMI: 29.70). Compared to control skin from healthy donors, mTOR gene expression resulted significantly increased either in NLS or LS skin from acne patients, respectively, of 17.96- and 20.77-fold (mean values) (Fig. 1a). No significant difference emerged between NLS and LS mTOR increase. IL-8, not mTOR related (s8), was assessed as positive control and resulted enhanced in acne skin (Fig. 1b). From both techniques, IHC (Fig. 1d–f) and IF (Fig. 1g–i), mTOR gradually increased from HS to LS. The analysis of IHC staining intensity (TMARKER) confirmed this trend, with statistically significant difference between HS and LS (P < 0.05) (data not shown). Gene expression of the upstream regulators of mTOR (TSC1 and TSC2) in LS or NLS from acne patients was analogous to controls (SF-1). Similarly, 4EBP1 and S6K1, the principal effectors modulated by mTOR, did not show any significant differences in gene expression (SF-1). Nonetheless, IF assessment demonstrated that, as expected, P-S6K1 was gradually increased in NLS and LS samples compared to HS (Fig. 2), whereas S6K1 was equally represented in the three samples. P-S6K1 is one of the downstream regulators activated by mTORC1 and controls the activity of SREBP1, which, in turn, is enhanced in acne lesions (s9). P-S6K1 increase in acne skin indirectly proves mTORC1 activation and could explain increased lipogenesis. Unexpectedly, the increment of mTOR was not related to acne severity or to BMI, whereas the only significant correlation was found between mTOR and HOMA-IR (r: 0.7) (Fig. 1c). Although none of the patients could be certainly defined as insulin resistant, as OGTT was not performed (ST-2), it has to be highlighted that subjects with higher HOMA-IR scores also showed higher mTOR expression in the skin (Fig. 1c). This is in line with our recent findings in hidradenitis suppurativa, where mTOR gene expression correlated with disease severity, which, in turn, was linked to higher BMI and HOMA-IR 9. Hence, we hypothesized that in these patients mTOR could represent a marker of future metabolic disease (s10-s12), although plasma levels of TNF-α, IL-6 and IL-8 resulted unmodified (SF-2). Our results provide the first evidence that mTOR gene expression is increased and S6K1 is activated in acne patients, either in involved or non-involved skin, compared to healthy controls, confirming the role of mTORC1 signalling in acne pathogenesis. This is a preliminary study with a small number of patients that needs to be enlarged and further experiments are in programme to adequately investigate the cascade activated by mTOR. Nonetheless, our data highlighted a possible role for mTOR, through mTORC1, in the complex inflammatory scenario of acne and, in absence of defined metabolic disease, such as obesity or ascertained insulin resistance (s10-s12), and it seemed to be related to the glucose–insulin balance (s13). Moreover, the increase in P-S6K1 could represent one of the missing rings in the diet–acne chain. The evaluation of mTOR-mTORC1 in acne patients affected by obesity or by insulin resistance, as well as its modification during therapy, or its relation to FoxO1, represents next investigative steps. Monfrecola, Lembo, Balato and Fabbrocini designed the research study; DeVita and Fabbrocini selected patients and collected samples; Caiazzo and Di Caprio performed IH, IF, RT-PCR and ELISA experiments; Lembo, Monfrecola and Balato analysed data and wrote the manuscript. None. The authors have declared no conflicting interests. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.