The lethal giant larvaetumour suppressor mutation requires dMyc oncoprotein to promote clonal malignancy

Abstract

BackgroundNeoplastic overgrowth depends on the cooperation of several mutations ultimately leading to major rearrangements in cellular behaviour. Precancerous cells are often removed by cell death from normal tissues in the early steps of the tumourigenic process, but the molecules responsible for such a fundamental safeguard process remain in part elusive. With the aim to investigate the molecular crosstalk occurring between precancerous and normal cells in vivo, we took advantage of the clonal analysis methods that are available in Drosophila for studying the phenotypes due to lethal giant larvae (lgl) neoplastic mutation induced in different backgrounds and tissues.ResultsWe observed that lgl mutant cells growing in wild-type imaginal wing discs show poor viability and are eliminated by Jun N-terminal Kinase (JNK)-dependent cell death. Furthermore, they express very low levels of dMyc oncoprotein compared with those found in the surrounding normal tissue. Evidence that this is a cause of lgl mutant cells elimination was obtained by increasing dMyc levels in lgl mutant clones: their overgrowth potential was indeed re-established, with mutant cells overwhelming the neighbouring tissue and forming tumourous masses displaying several cancer hallmarks. Moreover, when lgl mutant clones were induced in backgrounds of slow-dividing cells, they upregulated dMyc, lost apical-basal cell polarity and were able to overgrow. Those phenotypes were abolished by reducing dMyc levels in the mutant clones, thereby confirming its key role in lgl-induced tumourigenesis. Furthermore, we show that the eiger-dependent Intrinsic Tumour Suppressor pathway plays only a minor role in eliminating lgl mutant cells in the wing pouch; lgl-/- clonal death in this region is instead driven mainly by dMyc-induced Cell Competition.ConclusionsOur results provide the first evidence that dMyc oncoprotein is required in lgl tumour suppressor mutant tissue to promote invasive overgrowth in larval and adult epithelial tissues. Moreover, we show that dMyc abundance inside versus outside the mutant clones plays a key role in driving neoplastic overgrowth.

Tumor suppression by cell competition through regulation of the Hippo pathway

“…Similar effects are observed for lgl − and dlg − clones, although they may not be eliminated very efficiently (11,14,15). Thus, the presence of wild-type cells prevents scrib − , lgl − , and dlg − cells from manifesting their tumorigenic potential (2)(3)(4)(5)(6)(7)(11)(12)(13)(14)(15). Several groups have shown that the JNK stress-response pathway is activated in scrib − clones, leading to engulfment and death or extrusion of mutant cells from the epithelium (2-4, 6, 11, 16).…”

“…When scrib − cells are produced in patches (clones) of mutant cells that are surrounded by normal cells, they do not hyperproliferate, remain small, and eventually are eliminated (2)(3)(4)(5)(6)(7)(11)(12)(13). Similar effects are observed for lgl − and dlg − clones, although they may not be eliminated very efficiently (11,14,15). Thus, the presence of wild-type cells prevents scrib − , lgl − , and dlg − cells from manifesting their tumorigenic potential (2)(3)(4)(5)(6)(7)(11)(12)(13)(14)(15).…”

“…S6). Notably, the hinge region has been proposed to be a less competitive environment than the wing pouch, and the posterior region of eye discs may similarly be a less competitive environment, since cells in that region start to differentiate earlier than those located more anteriorly (15,26,27). Therefore, some scrib − clones may face less cell competition in these regions, allowing them to elevate ex- lacZ levels.…”

“…However, upon quantification we found that the majority of scrib − clones have normal or reduced levels of ex-lacZ expression, and only a small percentage of scrib − clones have elevated levels of ex-lacZ expression. Clones with elevated ex-lacZ expression were observed mainly in the hinge region of wing discs, which may provide an environment of reduced cell competition (15,26,27). Thus, outcompeted scrib − clones do not have elevated levels of Yki activity.…”

“…The modulation of Yki activity in scrib − cells thus is a critical effect of the JNK-dependent cell-competition process that removes such tumorigenic cells from imaginal discs. Finally we show that the Myc and Ras oncogenes, which can rescue scrib − clones from elimination (2,4,15), do so by conferring competitive fitness to scrib − cells and thereby prevent the down-regulation of Yki activity in scrib − cells. Our results thus further characterize the effects of cell-competition pathways in removing tumorigenic scrib − cells from imaginal discs.…”