Isotretinoin and FoxO1: A scientific hypothesis

Abstract

Oral isotretinoin (13-cis retinoic acid) is the most effective drug in the treatment of acne and restores all major pathogenetic factors of acne vulgaris. isotretinoin is regarded as a prodrug which after isomerizisation to all-trans-retinoic acid (ATRA) induces apoptosis in cells cultured from human sebaceous glands, meibomian glands, neuroblastoma cells, hypothalamic cells, hippocampus cells, Dalton's lymphoma ascites cells, B16F-10 melanoma cells, and neuronal crest cells and others. By means of translational research this paper provides substantial indirect evidence for isotretinoin's mode of action by upregulation of forkhead box class O (FoxO) transcription factors. FoxOs play a pivotal role in the regulation of androgen receptor transactivation, insulin/insulin like growth factor-1 (IGF-1)-signaling, peroxisome proliferator-activated receptor-γ (PPArγ)- and liver X receptor-α (LXrα)-mediated lipogenesis, β-catenin signaling, cell proliferation, apoptosis, reactive oxygene homeostasis, innate and acquired immunity, stem cell homeostasis, as well as anti-cancer effects. An accumulating body of evidence suggests that the therapeutic, adverse, teratogenic and chemopreventive effecs of isotretinoin are all mediated by upregulation of FoxO-mediated gene transcription. These FoxO-driven transcriptional changes of the second response of retinoic acid receptor (RAR)-mediated signaling counterbalance gene expression of acne due to increased growth factor signaling with downregulated nuclear FoxO proteins. The proposed isotretinoin→ATRA→RAR→FoxO interaction offers intriguing new insights into the mode of isotretinoin action and explains most therapeutic, adverse and teratogenic effects of isotretinoin in the treatment of acne by a common mode of FoxO-mediated transcriptional regulation.

Keywords: FoxO; acne; apoptosis; isotretinoin; stem cell; transcriptional regulation.

Figure 1

(A) Nuclear exclusion of FoxO proteins into the cytoplasm by growth factor signaling due to Akt kinase-mediated phosphorylation of nuclear FoxO proteins. (B) Isotretinoin-mediated upregulation of FoxO expression as a secondary response of proapoptotic RAR-signaling. FoxO-regulated genes are switched on. IGF-1, insulin-like growth factor-1; FGFs, fibroblast growth factors; PI3K, phosphoinositol-3 kinase; Akt, Akt kinase (protein kinase B); FoxO, forkhead box class O transcription factor; ATRA, all-trans-retinoic acid; CRABP2, cellular retinoic acid binding protein-2; RAR, retinoic acid receptor.

Figure 2

(A) FoxO1-mediated suppression of androgen-receptor (AR) by FoxO1-binding to the AR transcription activation domain (TAD) thereby inhibiting N-/C-terminal interaction of AR resulting in reduced AR transactivation. (B) Direct FoxO1-mediated suppression of PPARγ-regulated target genes. (C) FoxO1-mediated suppression of the PPARγ promoter reducing PPARγ expression. (D) FoxO1-mediated suppression of the SREBP-1c promoter reducing SREBP-1c expression, the key transcription factor of most lipogenic enzymes. DBD, DNA binding domain; DHT, dihydrotestosterone; RXR, retinoid X receptor; PPARγ, peroxisome proliferator-activated receptor-γ.

Figure 3

FoxO-induced G₁/S arrest of the cell cycle. Isotretinoin-mediated upregulation of cell cycle inhibitors p21 and p27 by FoxO binding to their promoters. Growth factor-mediated nuclear export of FoxO proteins with consecutive downregualtion of p21, p27 and p130. ATRA, all-trans-retinoic acid; Akt, Akt kinase; PI3K, phosphoinositol-3 kinase; IGF-1, insulin-like growth factor-1.

Figure 4

Isotretinoin/FoxO1-mediated upregulation of hepatic gene expression at the promoter level: upregulation of phosphoenolpyruvate carboxykinase (PEPCK) results in gluconeogenesis; upregulation of apolipoprotein C-III (Apo CIII) inhibits the activity of lipoprotein lipase (LPL); upregulation of microsomal triglyceride transfer protein (MTP) leads to increased production and secretion of very low density lipoproteins (VLDL). Increased expression of heme oxigenase 1 (Hmox1) results in cytochrome degradation and mitochondrial damage with impaired β-oxidation of fatty acids (FAs) leading to increased formation of hepatic triglycerides (TG).

Figure 5

Isotretinoin-mediated overexpression of FoxO proteins and divergence of β-cateinin signaling from Lef1/Tcf-induced transcription by increased binding of β-catenin to nuclear FoxO proteins. ATRA, all-trans-retinoic acid; CRABP2, cellular retinoic acid binding protein-2; Wnts, Wingless proteins; LRP5/6, low density receptor-related proteins 5/6; Frizzled, Wnt receptor Frizzled; β, β-catenin; Lef1, lymphoid enhancer-binding factor-1; Tcf, T cell factor.

Figure 6

Isotretinoin's effect on muscle homeostasis is mediated by FoxO1-driven upregulation of atrogin-1 (Atg1) and muscle-specific RING finger protein-1 (MuRF1) which both induce autophagy-related protein degradation with muscle loss and release of creatine phosphokinase (CPK).

Figure 7

Isotretinoin's effect on the CNS is mediated by FoxO1 upregulation. in the hypothalamus FoxO1 inhibits neurogenesis associated with the risk of mood changes. FoxO1 suppresses the expression of proopiomelanocortin (POMC) and carboxypeptidase E (Cpe). This results in reduced formation of α-melanocyte stimulating hormone (α-MSH) and general suppression of the hypothalamic-pituitary-axis (HPA) with decreased pituitary hormone secretion.

Figure 8

Isotretinoin-induced and FoxO-mediated apoptosis. FoxO1-induced overexpression of heme oxigenase-1 disrupt the electron transport chain (ETC) with increased release of cytochrome c inducing the intrinsic mitochondrial pathway of apoptosis. Formation of the apoptosome activates caspase 9 which finally activates the excutive caspase 3. The pathway explains isotretinoin's teratogenic effects when neuronal crest and CNS cells are affected during embryonic development.