Impact of gamma-aminobutyric acid receptor-associated protein (GABARAP) on carcinogen-induced tumorigenesis
GABARAP belongs to an evolutionary highly conserved gene family that has a fundamental role in autophagy machinery. There is ample evidence for a crosstalk between autophagy and apoptosis as well as immune response, metabolism and stress response. However, the molecular details for these interactions are not fully characterized. Here we report that the mouse ablation of GABARAP, a member of the Atg8/LC3 family that are central to the autophagosome biogenesis, suppresses the incidence of tumor formation mediated by the carcinogen DMBA and resulted in the enhancement of immune response through increased secretion of IL-1β, IL-6, IL-2 and IFN-γ from stimulated macrophages and lymphocytes. Further, DMBA treatment of GABARAP knockout (KO) mice reduced the cellularity of the spleen and the growth of mammary glands through the induction of apoptosis. Gene expression profiling revealed significantly elevated levels of Xaf1; an apoptotic inducer and tumor suppressor gene, in the mammary glands of KO mice. Furthermore, DMBA treatment triggered the upregulation of pro-apoptotic (Bid, Apaf1, Bax), cell death (Tnfrsf10b, Ripk1) and cell cycle inhibitor (Cdkn1a, Cdkn2c) genes in the mammary glands. GABARAP-deficient fibroblasts treated with DMBA and camptothecin showed impairment in DNA damage repair through accumulation of phosphorylated γH2AX which was accompanied by p62 accumulation and downregulation of Cyclin D1 protein. Reduced tumor growth in KO mice was also evident after B16 melanoma cell inoculation. Finally, mutation analysis of DMBA-induced tumors revealed no H-ras mutation in the tumors of KO mice which may indicate the importance of GABARAP in the glycolysis during Ras-mediated cellular transformation. Together, the data provide strong evidence for the involvement of GABARAP in tumorigenesis in vivo via its implicating in different cellular mechanisms associated with tumor initiation and progression.