Oncogenic ras-induced down-regulation of autophagy mediator Beclin-1 is required for malignant transformation of intestinal epithelial cells.
Detachment of non-malignant epithelial cells from the extracellular matrix causes their growth arrest and, ultimately, death. By contrast, cells composing carcinomas, cancers of epithelial origin, can survive and proliferate without being attached to the extracellular matrix. These properties of tumor cells represent hallmarks of malignant transformation and are critical for cancer progression. Previously we identified several mechanisms by which ras, a major oncogene, blocks detachment-induced apoptosis of intestinal epithelial cells, but mechanisms by which Ras promotes proliferation of those cells that remain viable following detachment are unknown. We show here that detachment of non-malignant intestinal epithelial cells promotes formation of autophagosomes, vacuole-like structures that mediate autophagy (a process of cellular self-cannibalization), and that oncogenic ras prevents this autophagosome formation. We also found that ras activates a GTPase RhoA, that RhoA promotes activation of a protease calpain, and that calpain triggers degradation of Beclin-1, a critical mediator of autophagy, in these cells. The reversal of the effect of ras on Beclin-1 (achieved by expression of exogenous Beclin-1) promoted autophagosome formation following cell detachment, significantly reduced the fraction of detached cells in the S phase of the cell cycle and their rate of proliferation without affecting their viability. Furthermore, RNA interference-induced Beclin-1 down-regulation in non-malignant intestinal epithelial cells prevented detachment-dependent reduction of the fraction of these cells in the S phase of the cell cycle. Thus, ras oncogene promotes proliferation of those malignant intestinal epithelial cells that remain viable following detachment via a distinct novel mechanism that involves Ras-induced down-regulation of Beclin-1.
Department of Pediatrics, Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada.
Byong Hoon
BH
Yoo
Xue
X
Wu
Yongling
Y
Li
Mehnaaz
M
Haniff
Takehiko
T
Sasazuki
Senji
S
Shirasawa
Eeva-Liisa
EL
Eskelinen
Kirill V
KV
Rosen
D051017:Apoptosis Regulatory Proteins / Q000096:biosynthesis* / Q000235:genetics; D001343:Autophagy; D000071186:Beclin-1; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism*; D015536:Down-Regulation / Q000235:genetics; D004847:Epithelial Cells / Q000378:metabolism*; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007414:Intestinal Neoplasms / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000235:genetics; D010588:Phagosomes / Q000235:genetics / Q000378:metabolism; D016283:Proto-Oncogene Proteins p21(ras) / Q000235:genetics / Q000378:metabolism*; D016196:S Phase / Q000235:genetics; D014407:Tumor Cells, Cultured; D020742:rhoA GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
D051017:Apoptosis Regulatory Proteins; C491997:BECN1 protein, human; D000071186:Beclin-1; D008565:Membrane Proteins; C066737:RHOA protein, human; D016283:Proto-Oncogene Proteins p21(ras); D020742:rhoA GTP-Binding Protein