Inactivation of the Dlc1 Gene Cooperates with Downregulation of p15INK4b and p16Ink4a, Leading to Neoplastic Transformation and Poor Prognosis in Human Cancer

Abstract

The tumor suppressor gene deleted in liver cancer-1 (DLC1), which encodes a protein with strong RhoGAP (GTPase activating protein) activity and weak Cdc42GAP activity, is inactivated in various human malignancies. Following Dlc1 inactivation, mouse embryo fibroblasts (MEF) with a conditional Dlc1 knockout allele reproducibly underwent neoplastic transformation. In addition to inactivation of Dlc1 and increased activity of Rho and Cdc42, transformation depended on the subsequent decreased expression of the Cdk4/6 inhibitors p15Ink4b and p16Ink4a together with increased expression and activation of Cdk4/6. The level of expression of these cell-cycle regulatory genes was relevant to human tumors with low DLC1 expression. Analysis of publicly available annotated datasets of lung and colon cancer with gene expression microarray profiles indicated that, in pairwise comparisons, low DLC1 expression occurred frequently together (P < 0.01) with downregulation of p15Ink4b or p16Ink4a or upregulation of CDK4 or CDK6. In addition, an unfavorable prognosis (P < 0.05) was associated with low DLC1 and low p15Ink4b in lung cancer and colon cancer, low DLC1 and low p16Ink4a in lung cancer, low DLC1 and high CDK4 in lung cancer, and low DLC1 and high CDK6 in colon cancer. Thus, several genes and biochemical activities collaborate with the inactivation of DLC1 to give rise to cell transformation in MEFs, and the identified genes are relevant to human tumors with low DLC1 expression. Cancer Res; 72(22); 5900–11. ©2012 AACR.

Rho regulation: DLC proteins in space and time

“…Consistent with GAP domain conservation, all DLC proteins show GAP activity towards RhoA and, at least in vitro, weak activity was also reported for Cdc42, but not for Rac [10,[23][24][25]89]. For DLC1 in vitro substrate specificity towards the highly related RhoA homologs RhoB and RhoC has also been shown [23] and recently in vivo GAP activity towards Cdc42 was demonstrated for DLC1 and DLC2 [64,84]. Active Rho is known to induce the assembly of stress fibers and focal adhesions, whereas Rac and Cdc42 promote the formation of specialized membrane protrusions called lamellipodia and filopodia, respectively [67].…”

DLC1 is the principal biologically-relevant down-regulated DLC family member in several cancers

“…DLC1 was the first family member identified, and a considerable amount of clinical and experimental evidence has established it as a bona fide tumor suppressor gene. Overexpression of DLC1 inhibits several biological parameters of neoplastic growth [13], and inactivation of endogenous DLC1 can, in conjunction with other genetic and/or epigenetic changes, lead to cell transformation and tumor formation [14, 15]. DLC2 and DLC3 have been studied less extensively, but they also appear to be tumor suppressors that are down-regulated in malignancies [9, 16].…”

Deleted in Liver Cancer-1 (DLC1): An Emerging Metastasis Suppressor Gene

“…In addition to inactivation of DLC1 and concomitant increased activity of Rho and Cdc42, neoplastic transformation depended on the activation of Cdk4/6 resulting from decreased expression of the Cdk4/6 inhibitors p15 Ink4b and p16 Ink4a . Several of the genes in these pathways are known to be altered in human cancers [24]. Experiments with human and mouse hepatoma cells investigating upstream factors have provided conclusive evidence that protein kinase A (PKA) can regulate the Rho-GAP activity and tumor suppressive capacity of DLC1 [10].…”