NAD/NAMPT and mTOR Pathways in Melanoma: Drivers of Drug Resistance and Prospective Therapeutic Targets

Abstract

Malignant melanoma represents the most fatal skin cancer due to its aggressive behavior and high metastatic potential. The introduction of BRAF/MEK inhibitors and immune-checkpoint inhibitors (ICIs) in the clinic has dramatically improved patient survival over the last decade. However, many patients either display primary (i.e., innate) or develop secondary (i.e., acquired) resistance to systemic treatments. Therapeutic resistance relies on the rewiring of multiple processes, including cancer metabolism, epigenetics, gene expression, and interactions with the tumor microenvironment that are only partially understood. Therefore, reliable biomarkers of resistance or response, capable of facilitating the choice of the best treatment option for each patient, are currently missing. Recently, activation of nicotinamide adenine dinucleotide (NAD) metabolism and, in particular, of its rate-limiting enzyme nicotinamide phosphoribosyltransferase (NAMPT) have been identified as key drivers of targeted therapy resistance and melanoma progression. Another major player in this context is the mammalian target of rapamycin (mTOR) pathway, which plays key roles in the regulation of melanoma cell anabolic functions and energy metabolism at the switch between sensitivity and resistance to targeted therapy. In this review, we summarize known resistance mechanisms to ICIs and targeted therapy, focusing on metabolic adaptation as one main mechanism of drug resistance. In particular, we highlight the roles of NAD/NAMPT and mTOR signaling axes in this context and overview data in support of their inhibition as a promising strategy to overcome treatment resistance.

Keywords: NAMPT; cancer therapy; drug resistance; mTOR; melanoma; metabolic reprogramming; signaling.

Figure 1

Metabolic plasticity in melanoma. In normal melanocyte, the energetic metabolism, in the presence of oxygen (O2), starts from the consumption of glucose via glycolysis to the final production of adenosine triphosphate (ATP) mainly via oxidative phosphorylation (OXPHOS). During the development of the tumor, the metabolic pathways drastically change. In melanoma cells, the BRAF-mutated oncogenic pathway, leading to the over-activation of MAPK, sustains mainly aerobic glycolysis/Warburg phenotype with secretion of lactate. Inhibition of MAPK pathway decreases glycolysis, leading to a dependence on mitochondrial metabolism with increased production of reactive oxygen species (ROS). This phenotype switch is a metabolic adaptation mechanism developed by cells to survive under drug’s pressure and is a common feature of melanoma cells resistant to targeted-therapy. Intrinsic (mutations, intracellular pathways) and extrinsic factors (nutrients, O2 levels, acidity, and soluble factors) can contribute to increase the metabolic plasticity of melanoma cells selecting subclones with a different metabolic phenotype. The progressive metabolic reprogramming in melanoma is accompanied by a drastic increase in tumor aggressiveness. +: positive regulation; −: negative regulation.

Figure 2

Summary of the cellular processes regulated by NAD/NAMPT axis and its connections with the BRAF/MEK/ERK signaling cascade. NAMPT is the rate-limiting enzyme in NAD biosynthesis starting from nicotinamide (Nam) that, in turn, is released by NAD-consuming enzymes (PARPs and Sirtuins mainly involved in DNA repair and epigenetics). The BRAF oncogenic signaling and NAMPT/NAD pathway are connected in melanoma: the activation of BRAF-mutated pathway promotes NAMPT transcription and NAD metabolism; on the other hand, NAMPT overexpression support MAPK activation in a positive loop. Increased levels of NAMPT and NAD sustains energetic metabolism and drives drug resistance mechanisms. NAMPT can be also released by melanoma cells, through unknown mechanisms, acting as cytokine binding to TLR4 and/or CCR5 and triggering intracellular signaling. In the microenvironment of melanoma the function of eNAMPT has not been well established, however, in other tumor models it can create immunosuppressive and tumor-promoting conditions modulating immune responses. Note that NAMPT inhibition (using pharmacological agents and/or neutralizing antibody) can play a role in blocking melanoma growth and progression, counteracting BRAF inhibitors resistance and impacting on tumor microenvironment. i: inhibitors; ICI: immune checkpoint inhibitors; NMN: nicotinamide mononucleotide; TLR4: Toll-like receptor 4; CCR5: C–C chemokine receptor type 5; TCL: T cytotoxic lymphocyte; CAFs: cancer-associated fibroblasts; MSCs: mesenchymal stem cell; TAMs: tumor-associated macrophages; MDSCs: myeloid-derived suppressor cells.

Figure 3

Summary of the cellular processes regulated by mTOR pathway and its connections with the BRAF/MEK/ERK signaling cascade. mTOR complexes are activated by various extra- and intracellular stimuli related to the energetic status of the cell and nutrient availability in the extracellular space. mTORC1 functions can be regulated by the BRAF pathway via an ERK-dependent mechanism and mTORC1 itself exerts a negative feedback mechanism on RTKs signaling, inhibiting mTORC2 and BRAF/MEK/ERK pathways. mTOR complexes support therapeutic resistance of melanoma cells by boosting ATP production of the mitochondria, by enhancing protein synthesis. mTORC1 and mTORC2 also promote the expression of Interferon-stimulated genes (ISGs), potentially modulating anti-tumor immune responses. Note that mTORC1 inhibition can play a role in the switch toward therapeutic resistance by removing a brake on autophagy. Additionally, systemic mTOR inhibition can play both immunosuppressive and immunostimulatory roles by impairing CD8+ cytotoxic T cells (TCL) and by favoring the expansion of CD8+ memory T cells (CD8 memory). i: inhibitors; ICI: immune checkpoint inhibitors; AMPK: AMP-activated protein Kinase; TSC: tuberous sclerosis; IFNAR/IFNGR: interferon receptors.