DLC-1:a Rho GTPase-activating protein and tumour suppressor

Abstract

The deleted in liver cancer 1 (DLC-1) gene encodes a GTPase activating protein that acts as a negative regulator of the Rho family of small GTPases. Rho proteins transduce signals that influence cell morphology and physiology, and their aberrant up-regulation is a key factor in the neoplastic process, including metastasis. Since its discovery, compelling evidence has accumulated that demonstrates a role for DLC-1 as a bona fide tumour suppressor gene in different types of human cancer. Loss of DLC-1 expression mediated by genetic and epigenetic mechanisms has been associated with the development of many human cancers, and restoration of DLC-1 expression inhibited the growth of tumour cells in vivo and in vitro. Two closely related genes, DLC-2 and DLC-3, may also be tumour suppressors. This review presents the current status of progress in understanding the biological functions of DLC-1 and its relatives and their roles in neoplasia.

1

Tumour suppressor activity of DLC1. (A) The human DLC-1 gene is located in band 8p22, a chromosomal region of DNA copy-number losses in a number of cancers, as demonstrated by conventional (left) and array-based (right) comparative genom-ic hybridization. Re-expression of the DLC-1 cDNA in human tumour cells that lack expression of the endogenous gene results in suppression of colony formation (B), suppression of cell invasion (C), reduction of actin stress fibres (D), diminution of vin-culin-containing focal adhesions (E), reduction of tumour size in nude mice (F), induction of apoptosis (as demonstrated by cell accumulation of sub-G1 phase and chromatin fragmentation) (G), and suppression of formation of lung metastases in nude mice (H). In each pair of photographs, DLC-1-expressing cells are below (C, D, E, H) or to the right (B, F) of cells transfected with control vectors. Images in H are reproduced with permission from Cancer Research.

2

Comparison of the mammalian and Drosophila DLC family proteins. Schematic representation of the structure of human DLC-1, DLC-2α and DLC-3α, the rat DLC-1 orthologue p122RhoGAP and Drosophila RhoGAP88 C (cv-c). The SAM, serine-rich (SR), RhoGAP and START domains are indicated. The amino acid length of the polypeptide is given at the right. Beneath each domain is the percent identity to the corresponding domain of human DLC-1, and in parenthesis is the percent identity of the full-length polypeptide.

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Origin of human DLC-1 and DLC-2 transcriptional variants. (A). Diagram of the genomic region at the 5′ end of DLC1, with exons represented by boxes. The five exons comprising the novel 5′sequence of the KIAA1723 transcript are labelled A–E, and the alternative first exon of the AK025544 transcript is denoted as 1*. The three transcripts share exons 2–14, and the distances between the putative transcription start sites (marked with arrows) and exon 2 are indicated. (B). Diagram of the 5’end of the DLC-2 gene, showing the first exons of the DLC-2α (1α), DLC-2β (1β) and DLC-2γ (1γ) isoforms. Exons 2–14 are common to all three transcripts, and the distances between exon 2 and the putative transcription start sites are indicated. The genomic DNA distances in A and B were obtained from the human genome sequence compilation (NCBI Build 36) and are not drawn to scale. The structures of the human DLC-3 gene and its transcripts were described in Ref 22.

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Structural and functional features of the serine-rich domain of DLC-1. (A). Graphical representation of the predicted secondary structure of the human DLC-1 protein, performed using the FoldIndex program (http://bioportal.weizmann.ac.il/fldbin/findex). The serine-rich region between the SAM and RhoGAP domains is predicted to have a largely unfolded conformation. (B). Features of the serine-rich region. The amino acid sequence of the region between the SAM and RhoGAP domains (residues 79–638) of human DLC-1 is shown, and residues that were found to be phosphorylated in the mouse [51] and rat [49,50] DLC-1 proteins are indicated in yellow. Phosphorylation of Ser549 was presumed, since the equivalent serine in a highly conserved region of mouse DLC-2 was phosphorylated [51]. The tensin-binding site (SIYDNV) and the LD motif (LDDILYHV) are shown in blue.

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Co-localization of DLC-1 and tensin. Confocal photomicrographs showing endogenous tensin (red, A) and transfected GFP-DLC1 fusion protein (green, B) in human fibroblasts. The merged image (C) shows co-localization of tensin and DLC-1 in focal adhesions, indicated in yellow. The bar represents 10 μm. (Images courtesy of Dr. Guorong Li, Laboratory of Cellular Oncology, National Cancer Institute).