Transglutaminase 2 contributes to a TP53-induced autophagy program to prevent oncogenic transformation.

Genetic alterations which impair the function of the TP53 signaling pathway in TP53 wild-type human tumors remain elusive. To identify new components of this pathway, we performed a screen for genes whose loss-of-function debilitated TP53 signaling and enabled oncogenic transformation of human mammary epithelial cells. We identified transglutaminase 2 (TGM2) as a putative tumor suppressor in the TP53 pathway. TGM2 suppressed colony formation in soft agar and tumor formation in a xenograft mouse model. The depletion of growth supplements induced both TGM2 expression and autophagy in a TP53-dependent manner, and TGM2 promoted autophagic flux by enhancing autophagic protein degradation and autolysosome clearance. Reduced expression of both CDKN1A, which regulates the cell cycle downstream of TP53, and TGM2 synergized to promote oncogenic transformation. Our findings suggest that TGM2-mediated autophagy and CDKN1A-mediated cell cycle arrest are two important barriers in the TP53 pathway that prevent oncogenic transformation.

FHL2 regulates cell cycle-dependent and doxorubicin-induced p21Cip1/Waf1 expression in breast cancer cells.

The transcriptional cofactor FHL2 interacts with a broad variety of transcription factors and its expression is often deregulated in various types of cancer. Here we analyzed for the first time the molecular function of FHL2 in breast cancer. FHL2 is overexpressed in almost all human mammary carcinoma samples tested but not in normal breast tissues and only low levels of FHL2 expression were present in four premalignant ductal carcinoma in situ (DCIS). Cell cycle analysis revealed an upregulation of endogenous FHL2 towards G2/M in MDA-MB 231 cells and an accelerated G2/M transition when FHL2 expression was suppressed in these cells. In search for G2/M specific target genes regulated by FHL2, we found that expression of the cell cycle inhibitor p21Cip1/Waf1 (hereafter p21) is dependent on FHL2 in MDA-MB 231 breast cancer cells. Downregulation of FHL2 by shRNA abrogated the cell cycle dependent upregulation of p21 as well as the induction of p21 in response to treatment with the DNA damaging agent doxorubicin. FHL2-dependent p21 expression occurs in a p53-independent manner and p21 expression can be downregulated by specific inhibition of mitogen-activated protein kinases (MAPKs), implicating an involvement of MAPK signaling in this regulation. Analysis of FHL2 contribution to the MAPK signaling identified FHL2 as an important downstream effector of MAPKs in breast cancer cells, capable of transactivating endogenous AP1 target genes as well as AP1 dependent reporter genes. Finally, downregulation of FHL2 reduces the ability of MDA-MB 231 cells to form colonies in soft agar, while FHL2 overexpression enhances colony formation of breast cancer cells. Thus, our findings indicate that overexpression of the transcriptional cofactor FHL2 contributes to breast cancer development by mediating transcriptional activation of MAPK target genes known to be involved in cancer progression, such as p21.

The biological relevance of FHL2 in tumour cells and its role as a putative cancer target.

The LIM-only protein FHL2 (four-and-a-half LIM-domain protein 2) belongs to the FHL protein family of transcriptional cofactors present in various cell lines. FHL2 interacts with a variety of transcription factors known to be involved in tumour development. Furthermore, FHL2 expression is often deregulated in cancer including overexpression and down-regulation in various types of tumours. The function of FHL2 in cancer is particularly intriguing, since it may act as an oncoprotein or as a tumour suppressor in a tissue-dependent fashion. This dual nature of FHL2 is also reflected by the finding that it can function as repressor or activator of transcriptional activity depending on the cell-type. The ability of FHL2 to exert functional diversity lies within its structural composition as a LIM-only protein. LIM-domains are enzymatically inactive protein-to-protein interaction domains, which determine the function of LIM-only proteins as adaptor molecules or scaffolding proteins. By selectively using different LIM-domains for protein-to-protein interactions, FHL2 is capable to interact with a broad spectrum of functionally unrelated proteins, thereby triggering different signalling pathways. In this review, the current knowledge of FHL2 expression in different cancers was summarized and the interaction of FHL2 with transcription factors and other proteins involved in cancer development was examined. Since transcription factors control all fundamental developmental and homeostatic processes, transcriptional cofactors like FHL2 are likely to contribute to human carcinogenesis and are of clinical importance in various forms of cancer.