The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress

Abstract

AMP-activated protein kinase (AMPK) is a highly conserved sensor of cellular energy status found in all eukaryotic cells. AMPK is activated by stimuli that increase the cellular AMP͞ATP ratio. Essential to activation of AMPK is its phosphorylation at Thr-172 by an upstream kinase, AMPKK, whose identity in mammalian cells has remained elusive. Here we present biochemical and genetic evidence indicating that the LKB1 serine͞threonine kinase, the gene inactivated in the Peutz-Jeghers familial cancer syndrome, is the dominant regulator of AMPK activation in several mammalian cell types. We show that LKB1 directly phosphorylates Thr-172 of AMPK␣ in vitro and activates its kinase activity. LKB1-deficient murine embryonic fibroblasts show nearly complete loss of Thr-172 phosphorylation and downstream AMPK signaling in response to a variety of stimuli that activate AMPK. Reintroduction of WT, but not kinase-dead, LKB1 into these cells restores AMPK activity. Furthermore, we show that LKB1 plays a biologically significant role in this pathway, because LKB1-deficient cells are hypersensitive to apoptosis induced by energy stress. On the basis of these results, we propose a model to explain the apparent paradox that LKB1 is a tumor suppressor, yet cells lacking LKB1 are resistant to cell transformation by conventional oncogenes and are sensitive to killing in response to agents that elevate AMP. The role of LKB1͞ AMPK in the survival of a subset of genetically defined tumor cells may provide opportunities for cancer therapeutics.

LKB1; linking cell structure and tumor suppression

“…Although LKB1 restoration can block proliferation or metastasis of some LKB1 mutant cancer cell lines, the mechanism may be non-physiological and/or involve targets that are not relevant to the role of LKB1 in the natural history of the tumor. The role of LKB1 in cancers is particularly difficult to conceptualize as LKB1 deficiency renders both primary cells and cancer cell lines sensitive to cell death in response to energy stress (Shaw et al, 2004b;Carretero et al, 2007;Memmott et al, 2008). Therefore it is likely that specific cooperating molecular alterations are required for LKB1-deficient cells to withstand energy stress during tumor progression.…”

“…In response to an increase in the AMP/ATP ratio, AMP binds to the regulatory AMPKg subunit, resulting in a conformation shift that is thought to prevent dephosphorylation of a critical threonine residue in the activation loop of AMPKa (Hardie, 2007). 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).…”

Sugar-free approaches to cancer cell killing

“…Many tumor cells lack LKB1, which is the main kinase involved in activation of AMPK. As cells without LKB1 cannot respond adequately to energetic stress, they are hypersensitive to energydepleting agents (Shaw et al, 2004). Downstream of AMPK, glucose deprivation promotes activation of p53 and p27.…”

AMP-activated protein kinase and human cancer: cancer metabolism revisited

“…During energy depletion, when the AMP/ATP ratio rises, LKB1, the predominant AMPK kinase in mammalian cells, 25 phosphorylates and activates AMPK leading to pleiotropic effects on cellular metabolism. Overall, AMPK activation favors restoration of cellular energy through reduction in macromolecular synthesis, enhancement of glucose transport and fatty acid oxidation, 26,27 and cell cycle arrest in proliferating cells.…”