Mammalian target of rapamycin signaling and autophagy: roles in lymphangioleiomyomatosis therapy

Abstract

The pace of progress in lymphangioleiomyomatosis (LAM) is remarkable. In the year 2000, TSC2 gene mutations were found in LAM cells; in 2001 the tuberous sclerosis complex (TSC) genes were discovered to regulate cell size in Drosophila via the kinase TOR (target of rapamycin); and in 2008 the results were published of a clinical trial of rapamycin, a specific inhibitor of TOR, in patients with TSC and LAM with renal angiomyolipomas. This interval of just 8 years between a genetic discovery for which the relevant signaling pathway was as yet unknown, to the initiation, completion, and publication of a clinical trial, is an almost unparalleled accomplishment in modern biomedical research. This robust foundation of basic, translational, and clinical research in TOR, TSC, and LAM is now poised to optimize and validate effective therapeutic strategies for LAM. An immediate challenge is to deduce the mechanisms underlying the partial response of renal angiomyolipomas to rapamycin, and thereby guide the design of combinatorial approaches. TOR complex 1 (TORC1), which is known to be active in LAM cells, is a key inhibitor of autophagy. One hypothesis, which will be explored here, is that low levels of autophagy in TSC2-null LAM cells limits their survival under conditions of bioenergetic stress. A corollary of this hypothesis is that rapamycin, by inducing autophagy, promotes the survival of LAM cells, while simultaneously arresting their growth. If this hypothesis proves to be correct, then combining TORC1 inhibition with autophagy inhibition may represent an effective clinical strategy for LAM.

Figure 1.

Simplified model of the tuberous sclerosis complex signaling pathway. Tuberin and hamartin physically interact and function as a heterodimeric complex to inhibit the mammalian target of rapamycin (mTOR) complex 1 (TORC1), which includes mTOR, Raptor, mLST8, PRAS40, and DEPTOR. TORC1 integrates mitogenic signals and nutrient availability with protein synthesis via substrates including p70 S6 kinase (S6K). Tuberin inhibits TORC1 via the Ras homologue Rheb, which is a key target of tuberin's highly conserved guanosine triphosphatase activating protein (GAP) domain. Tuberin stimulates the conversion of Rheb–guanosine triphosphate (active) to Rheb–guanosine diphosphate (inactive), thereby inhibiting TORC1.

Figure 2.

Potential mechanisms underlying the partial regression of angiomyolipomas with rapamycin treatment. There are two possible outcomes of mammalian target of rapamycin complex 1 (TORC1) inhibition that could result in the approximately 50% decrease in angiomyolipoma size that has been observed in clinical studies: cell size could be decreased (upper panel) or cell number could be decreased (lower panel). If a decrease in cell size is accompanied by sustained TORC1 inhibition, then a further decrease in tumor size and/or elimination of angiomyolipoma cells could be achieved, hypothetically, through inhibition of autophagy or inhibition of tuberous sclerosis complex-2–dependent, rapamycin-independent pathways. If a decrease in cell number is accompanied by evidence of TORC1 activation, then a further decrease in tumor size and/or elimination of angiomyolipoma cells could be achieved by targeting TORC1 via other agents. In human lymphangioleiomyomatosis, these two scenarios may coexist, based on regional differences in bioenergetic stress and hypoxia.

Figure 3.

Effects of dysregulation of autophagy in tumorigenesis and in lymphangioleiomyomatosis (LAM). LAM cells are predicted to have low levels of autophagy, because of mammalian target of rapamycin complex 1 activation. This may have a dual effect in tuberous sclerosis complex and LAM, by enhancing tumorigenesis via increased production of reactive oxygen species, and inhibiting tumorigenesis via decreased survival under conditions of bioenergetic stress, including hypoxia.

Figure 4.

Potential therapeutic strategy combining mammalian target of rapamycin complex 1 (TORC1) inhibition and autophagy inhibition in lymphangioleiomyomatosis (LAM). We hypothesize that rapamycin blocks the further growth of LAM cells by inhibiting protein translation, but simultaneously promotes the survival of LAM cells by inducing autophagy. If this hypothesis is correct, then combined inhibition of TORC1 and an autophagy inhibitor such as chloroquine could have synergistic efficacy. Other therapeutic combinations also need to be considered, as outlined in Figure 2.