Regulation of the TSC pathway by LKB1: evidence of a molecular link between tuberous sclerosis complex and Peutz-Jeghers syndrome

Abstract

Tuberous sclerosis complex (TSC) and Peutz-Jeghers syndrome (PJS) are dominantly inherited benign tumor syndromes that share striking histopathological similarities. Here we show that LKB1, the gene mutated in PJS, acts as a tumor suppressor by activating TSC2, the gene mutated in TSC. Like TSC2, LKB1 inhibits the phosphorylation of the key translational regulators S6K and 4EBP1. Furthermore, we show that LKB1 activates TSC2 through the AMP-dependent protein kinase (AMPK), indicating that LKB1 plays a role in cell growth regulation in response to cellular energy levels. Our results suggest that PJS and other benign tumor syndromes could be caused by dysregulation of the TSC2/mTOR pathway.Supplemental material is available at http://www.genesdev.org.Received February 29, 2004; revised version accepted April 27, 2004. Tuberous sclerosis complex (TSC) is an autosomal dominant syndrome characterized by the development of benign tumors termed hamartomas in a wide range of tissues. The majority of TSC cases are caused by mutation in either the tsc1 or tsc2 tumor suppressor genes (Young and Povey 1998). The proteins TSC1 and TSC2 form a functional complex and inhibit cell growth by negatively regulating the mammalian target of rapamycin (mTOR; Kwiatkowski 2003). mTOR controls cell growth by phosphorylating key substrates such as p70 ribosomal S6 kinase 1 (S6K) and eukaryote initiation factor 4E binding protein 1 (4EBP1; Jacinto and Hall 2003). Peutz-Jeghers syndrome (PJS), another dominantly inherited genetic disorder, is characterized by the formation of gastrointestinal hamartomas that happen to be histologically similar to those observed in TSC patients (Devroede et al. 1988). PJS is associated with mutations in the lkb1 tumor suppressor gene, which codes for a serine/threonine kinase. Although extensive work has been performed on the molecular pathogenesis of TSC, the molecular mechanism of LKB1 as a tumor suppressor has remained elusive.Recent studies from several laboratories have demonstrated that LKB1 phosphorylates and activates AMPK, representing the first convincing physiological target of LKB1 (Hawley et al. 2003;Hong et al. 2003;Woods et al. 2003, Shaw et al. 2004. AMPK is a multimeric protein, and its kinase activity is enhanced by both phosphorylation and high intracellular AMP levels. The amount of AMP in the cell is inversely proportional to the amount of ATP, and high levels of AMP are present under low energy conditions. Under such conditions, AMPK is activated and phosphorylates numerous substrates to suppress anabolism and enhance catabolism, thereby regulating cellular energy homeostasis (Hardie and Hawley 2001). LKB1 potentiates the effect of AMP on AMPK by phosphorylating AMPK on Thr 172, a residue found in the AMPK activation loop.Previous studies in our laboratory have demonstrated that activated AMPK phosphorylates and activates the TSC2 tumor suppressor protein (Inoki et al. 2003). This AMPK-dependent regulation of TSC2 is especially important for cellular energy response because cells e...

LKB1; linking cell structure and tumor suppression

“…In human umbilical vein endothelial cells, the presence of LKB1 downregulates AKT (S473) phosphorylation under oxidative stress, consistent with an LKB1-PTEN-AKT pathway (Song et al, 2007). However, in other cell types, levels of downstream AKT signaling do not appear to be regulated by LKB1 (Corradetti et al, 2004;Shaw et al, 2004a). Wnt/b-catenin signaling is also modulated by Lkb1 (Ossipova et al, 2003;Spicer et al, 2003;Lin-Marq et al, 2005).…”

“…LKB1 is the major upstream kinase that phosphorylates and activates AMPK (Shaw et al, 2004b); activated AMPK phosphorylates a number of proteins resulting in a decrease in ATP-consuming processes and an increase in ATP production through inhibition of protein synthesis, fatty acid and glucose metabolism and enhancement of glucose transport (Hardie, 2007). AMPK turns off mTOR (mammalian target of rapamycin) by signaling to the tuberous sclerosis (TSC2/TSC1) tumor suppressor complex, as well by directly phosphorylating the mTOR-binding partner, raptor (regulatory associated protein of mTOR) (Corradetti et al, 2004;Shaw et al, 2004a;Gwinn et al, 2008). TSC2/TSC1, in response, inactivates the small GTPase Rheb (Ras homology enriched in brain), which positively signals to mTOR.…”

JNK pathway mediates apoptotic cell death induced by tumor suppressor LKB1 in Drosophila

“…Recent reports on the TSC tumor suppressor suggested that LKB1 suppresses TOR activity via the TSC complex, ultimately resulting in inhibition of cell growth and proliferation. 17,18 Likewise, we observed that TSC overexpression dramatically reduces cell size in Drosophila clonal overexpression analyses (Figure 2). However, unexpectedly, LKB1 overexpression did not induce any cell size reduction (Figure 2), in stark contrast to the case of TSC.…”

“…39 Recent mammalian studies suggested that LKB1 negatively regulates TOR signaling via the AMPK-TSC pathway. 17,18 Since reduction in cell size is a typical outcome of TSC overexpression, we attempted to test whether the LKB1 overexpression phenotype results from cell size reduction. 39,40 As expected, the overexpression clones of TSC (Figure 2i) displayed a dramatic reduction in wing marginal bristle size.…”

“…[14][15][16] It has been recently proposed that LKB1 also regulates cellular growth by controlling another tumor suppressor tuberous sclerosis complex (TSC) via the AMP kinase (AMPK)-dependent pathway. 17,18 In addition, studies showing the absence of apoptosis in PJS polyps revealed that LKB1 is an inducer of apoptosis in vivo. 19 Another tumor suppressor p53, the first identified in vitro substrate of LKB1, was proposed as a mediator of this apoptosis.…”