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. 2004 Jul 1;18(13):1533–1538. doi: 10.1101/gad.1199104

Figure 5.

Figure 5.

Cells null for TSC2 or LKB1 display similar phenotypes for certain biological responses. (A) TSC2 and LKB1 play essential roles in protecting cells from glucose depletion-induced cell death. LEF cells (TSC2-/-) stably expressing empty vector or TSC2, and MEF cells (LKB1-/-) stably expressing empty vector or LKB1 were cultured in 25 mM glucose (glucose+) or glucose-free medium (glucose-). Photos were taken after 72 h in culture for LEF cells and 48 h for MEF cells. (B) Rapamycin inhibits glucose depletion-induced caspase-3 activation in LKB-/- MEF cells. MEF (LKB-/-) cells stably expressing vector or LKB1 were cultured for 30 h in media containing various concentrations of glucose (1–25 mM) in the presence or absence of rapamycin (20 nM). Immunoblots of caspase-3, cleaved caspase-3, and actin are shown. (C) Rapamycin inhibits VEGF secretion in LKB1-/- MEF cells. Equal numbers of LKB-/- MEF cells stably expressing empty vector or LKB1 were cultured for 36 h with or without rapamycin. The concentration of secreted VEGF in the medium was determined by ELISA. Data are expressed as a mean, and one standard deviation is indicated by the error bars (n = 3). The asterisk indicates that the reduction in VEGF secretion induced by rapamycin in LKB1-null cells is significant, as well as the increased expression of VEGF in the LKB1-null cells relative to the reexpressed cells (p < 0.0001). (D) Rapamycin inhibits VEGF expression in LKB1-/- MEF cells. Equal numbers of MEFs stably expressing empty vector or LKB1 were cultured for 36 h with or without rapamycin. VEGF expression in cell lysates was monitored by VEGF immunoblot, and protein level was monitored by β-tubulin immunoblot. (E) A proposed model for a role of LKB1 in the TSC pathway. Loss of LKB1 is a hallmark of Peutz-Jeghers syndrome. Less LKB1 would cause a decrease in the activation status of AMPK on T172 and of TSC2 on S1227 and S1335, relieving TSC2's inhibitory effect on Rheb. Higher Rheb activity would subsequently drive mTOR activation. Two key targets of mTOR—S6K and 4EBP1—are shown. We propose a model in which dysregulated increases in the activity S6K, 4EBP1, and other mTOR targets could lead to hamartoma formation in PJS in an analogous way to that of TSC. In a similar way, hamartoma-causing mutations in PTEN affect the activation status of AKT, a molecule that also acts on TSC2. Thus, the TSC2/mTOR pathway could represent a common pathway for hamartoma formation. Pointed arrowheads indicate activation, and flat arrowheads indicate inhibition.