mTOR signaling in growth control and disease

Abstract

The mechanistic target of rapamycin (mTOR) signaling pathway senses and integrates a variety of environmental cues to regulate organismal growth and homeostasis. The pathway regulates many major cellular processes and is implicated in an increasing number of pathological conditions, including cancer, obesity, type 2 diabetes, and neurodegeneration. Here, we review recent advances in our understanding of the mTOR pathway and its role in health, disease, and aging. We further discuss pharmacological approaches to treat human pathologies linked to mTOR deregulation.

Figure 1. Overview of mTORC1 and mTORC2

The mTOR kinase nucleates two distinct protein complexes termed mTORC1 and mTORC2. mTORC1 responds to amino acids, stress, oxygen, energy, and growth factors and is acutely sensitive to rapamycin. It promotes cell growth by inducing and inhibiting anabolic and catabolic processes, respectively, and also drives cell cycle progression. mTORC2 responds to growth factors and regulates cell survival and metabolism, as well as the cytoskeleton. mTORC2 is insensitive to acute rapamycin treatment but chronic exposure to the drug can disrupt its structure. The middle panel describes the known functions of the protein components that make up the mTOR complexes and the bottom panel schematically depicts their interaction sites. Refer to the text and abbreviation list for details about the complete name of proteins.

Figure 2. The mTOR signaling pathway

(A) The key signaling nodes that regulate mTORC1 and mTORC2. (B) The key outputs of the mTORC1 and mTORC2 pathways. See text for details. Refer to the text and abbreviation list for details about the complete name of proteins.

Figure 3. Connections of mTOR to cancer

(A) mTOR signaling promotes tumorigenesis. Oncogenes (red) or tumor suppressors (green) implicated in the control of mTOR signaling are indicated. Asterisk (*) denotes proteins currently targeted for cancer therapy. (B) mTORC1 controls many negative feedback loops that regulate RTK-PI3K signaling. (C) The inhibition of mTORC1 by rapalogs reduces the intensity of the negative feedback loops on RTK signaling, which promotes PI3K activation and cell survival. Because the rapalogs only partially inhibit 4E-BP1 phosphorylation, their impact on eiF4E-mediated protein translation is limited. (D) By completely blocking mTORC1, mTOR kinase inhibitors strongly inhibit the 4E-BP1/eIF4E axis and protein synthesis. Additionally, mTOR kinase inhibitors can affect cell survival and proliferation by blocking mTORC2-mediated Akt phosphorylation. The elevation in RTK-PI3K-PDK1 activity in response to mTOR kinase inhibitors can potentially reactivate Akt phosphorylation on Thr308, which may be sufficient to drive cell survival. (E) Dual PI3K/mTOR inhibitors block all known outputs of the PI3K, mTORC1, and mTORC2 pathways. Refer to the text and abbreviation list for details about the complete name of proteins.

Figure 4. mTOR signaling and metabolism

The roles of mTOR signaling in the regulation of tissue metabolism in the normal (left side) or obese/nutrient overload state (right side). (A) In the hypothalamus, mTORC1 activation reduces the expression of orexigenic peptides (NPY, AgRP) through an unclear mechanism that involves S6K1. High fat diets reduce the ability of leptin and insulin to promote mTORC1 activity and reduce food intake. (B) In adipose tissue, mTORC1 activation promotes adipogenesis by activating PPAR-γ. mTORC2-Akt activation reduces lipolysis and promote glucose uptake. High circulating nutrients and cytokines promote mTORC1 activity in obesity, which inhibits insulin signaling and cause insuline resistance (IR) through various mechanisms. (C) In muscles, mTORC1 play crucial role in regulating protein synthesis, mitochondrial biogenesis, and oxidative metabolism. Muscle contractions increase mTORC1 activity. mTORC2-Akt activation induces glucose uptake and blocks protein catabolism. Similar to adipose tissue, the elevation of mTORC1 activity by obesity/nutrient overload blocks insulin signaling. The reduction in mTORC2-Akt action promotes protein catabolism and reduces glucose uptake, contributing to muscle mass loss and systemic IR. (D) In the liver, mTORC1 activation reduces ketone body production by inhibiting PPAR-α activity. mTORC1 also promotes hepatic lipogenesis by activating SREBP1. Alternatively, mTORC2-Akt blocks FoxO1 activity and the activation of gluconeogenesis. Liver mTORC1 activity is elevated in obesity/overfeeding, which promotes hepatic IR, unrestrained gluconeogenesis, and hyperglycemia. (E) In the pancreas, mTORC1 regulates β-cell mass by promoting β-cell growth and proliferation. mTORC1 is also important for insulin production/secretion. The mTORC2-Akt axis positively affects β-cell mass by promoting proliferation and survival. Obesity/nutrient overload drives mTORC1 activity in β-cells. Sustained activation of mTORC1 ultimately cause β-cell apoptosis by inhibiting Akt signaling. The loss of β-cells favors progression towards diabetes.

Figure 5. Rapamycin and the treatment of metabolic diseases

(A) Overview of the impact of rapamycin on organ and systemic metabolism. Rapamycin induces a diabetes-like syndrome by impairing the function of the muscles, liver, adipose tissue and pancreatic β-cells. The downregulated processes are in red and those upregulated in green. (B) Illustration of the hypothesized relation between mTORC1 activation and insulin sensitivity/metabolic profile in vivo. The relation between mTORC1 activity and insulin sensitivity/metabolic profile probably follows a U-shaped curve, where too little or too much mTORC1 activity has a negative impact on systemic metabolism.

Figure 6. mTORC1 and aging

The activation of mTORC1 by growth factors and nutrients inhibits autophagy and promotes protein synthesis. Over time, this may promote cellular stress (protein aggregation, organelle dysfunction, oxidative stress), which might lead to damage accumulation, a reduction in cell function and thus promote the development of aging-related diseases. Also, mTORC1 activation induces stem cell exhaustion, which reduces tissue repair and promotes tissue dysfunction. Dietary restriction and rapamycin may delay aging and increase longevity by regulating these processes downstream of mTORC1.