E4F1-mediated control of pyruvate dehydrogenase activity is essential for skin homeostasis

Abstract

The multifunctional protein E4 transcription factor 1 (E4F1) is an essential regulator of epidermal stem cell (ESC) maintenance. Here, we found that E4F1 transcriptionally regulates a metabolic program involved in pyruvate metabolism that is required to maintain skin homeostasis. E4F1 deficiency in basal keratinocytes resulted in deregulated expression of dihydrolipoamide acetyltransferase (Dlat), a gene encoding the E2 subunit of the mitochondrial pyruvate dehydrogenase (PDH) complex. Accordingly, E4f1 knock-out (KO) keratinocytes exhibited impaired PDH activity and a redirection of the glycolytic flux toward lactate production. The metabolic reprogramming of E4f1 KO keratinocytes associated with remodeling of their microenvironment and alterations of the basement membrane, led to ESC mislocalization and exhaustion of the ESC pool. ShRNA-mediated depletion of Dlat in primary keratinocytes recapitulated defects observed upon E4f1 inactivation, including increased lactate secretion, enhanced activity of extracellular matrix remodeling enzymes, and impaired clonogenic potential. Altogether, our data reveal a central role for Dlat in the metabolic program regulated by E4F1 in basal keratinocytes and illustrate the importance of PDH activity in skin homeostasis.enewal and wound healing of the epidermis rely on a pool of epidermal stem cells (ESC) located in the basal layer of the interfollicular epithelium (IFE) and in the bulge region of the hair follicle (HF). In the IFE, these long-lived ESC give rise to progenitors with increased proliferative capacities that differentiate into keratinocytes as they migrate upward into suprabasal layers. Numerous studies have addressed the role of several key signaling pathways, such as those implicating bone morphogenetic proteins, TGF-β, Notch, Sonic Hedgehog, or Wnt in skin homeostasis, and how they control ESC maintenance (1-3). The role of these pathways in regulating stemness has been attributed to the regulation of cell proliferation, cell death, cellular senescence, cell adhesion, or differentiation. Although previous data indicate that some of these stem cell regulators also control energy metabolism in the hematopoietic or neuronal lineages (4), very few studies have yet addressed their metabolic functions in keratinocytes. In addition, the potential role of specific metabolic regulators in the control of skin homeostasis remains poorly documented. Nevertheless, previous observations indicate that deregulation of the nutrient-sensing mammalian target of rapamycin pathway in basal keratinocytes occurs as a consequence of prolonged Wnt signaling, leading to the progressive exhaustion of HF bulge stem cells (5). Recent data also indicate that genetic inactivation in mouse epidermis of mitochondrial transcription factor A (Tfam), a gene involved in mitochondrial DNA replication and transcription, impinges on keratinocyte differentiation but does not impair maintenance of basal keratinocytes (6). Although these results suggest that basal keratinocytes display a metabolic st...

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