Rho GTPase Transcriptome Analysis Reveals Oncogenic Roles for Rho GTPase-Activating Proteins in Basal-like Breast Cancers

Abstract

The basal-like breast cancer (BLBC) subtype accounts for a disproportionately high percentage of overall breast cancer mortality. The current therapeutic options for BLBC need improvement; hence, elucidating signaling pathways that drive BLBC growth may identify novel targets for the development of effective therapies. Rho GTPases have previously been implicated in promoting tumor cell proliferation and metastasis. These proteins are inactivated by Rho-selective GTPase-activating proteins (RhoGAPs), which have generally been presumed to act as tumor suppressors. Surprisingly, RNA-Seq analysis of the Rho GTPase signaling transcriptome revealed high expression of several RhoGAP genes in BLBC tumors, raising the possibility that these genes may be oncogenic. To evaluate this, we examined the roles of two of these RhoGAPs, ArhGAP11A (also known as MP-GAP) and RacGAP1 (also known as MgcRacGAP), in promoting BLBC. Both proteins were highly expressed in human BLBC cell lines, and knockdown of either gene resulted in significant defects in the proliferation of these cells. Knockdown of ArhGAP11A caused CDKN1B/p27-mediated arrest in the G1 phase of the cell cycle, whereas depletion of RacGAP1 inhibited growth through the combined effects of cytokinesis failure, CDKN1A/p21-mediated RB1 inhibition, and the onset of senescence. Random migration was suppressed or enhanced by the knockdown of ArhGAP11A or RacGAP1, respectively. Cell spreading and levels of GTP-bound RhoA were increased upon depletion of either GAP. We have established that, via the suppression of RhoA, ArhGAP11A and RacGAP1 are both critical drivers of BLBC growth, and propose that RhoGAPs can act as oncogenes in cancer.

miR‐4324‐RACGAP1‐STAT3‐ESR1 feedback loop inhibits proliferation and metastasis of bladder cancer

“…As a GTPase activating protein, RACGAP1 was originally identified in testis and male germ cells . RACGAP1 has been reported to be an essential regulator in several cancers, including uterine carcinosarcoma, hepatocellular carcinoma, and breast cancer, and it is linked to aggressive clinical behavior. However, the involvement of RACGAP1 in BCa has not yet been investigated.…”

Filling GAPs in our knowledge: ARHGAP11A and RACGAP1 act as oncogenes in basal-like breast cancers

“…In our recent study, 12 we aimed to identify potential oncogenic or tumor suppressor roles for the 20 RHO GTPases, 79 Dbl or DOCK family RHOGEFs, 64 RHO-GAPs, and 3 RHOGDIs in human breast cancer. Breast tumors can be categorized into one of 5 major subtypes (luminal A, luminal B, HER2-enriched, basal-like, and normal breast-like) depending on their gene expression and, of these, basal-like tumors are particularly aggressive and have a poor prognosis.…”

“…We were therefore greatly surprised when our RNA-Seq data analysis revealed a number of RHOGAP genes that were highly expressed in basal-like tumors relative to normallike or luminal A tumors, which have a better prognosis. 12 This unexpected result prompted us to hypothesize that, in BLBC, certain RHOGAPs can act as oncogenes.…”

“…Strikingly, shRNA-mediated knockdown of ARHGAP11A or RACGAP1 from BLBC cell lines abolished the ability of these cells to proliferate under anchorage-dependent conditions, consistent with an oncogenic role for these GAPs in BLBC. 12 We next set about identifying the mechanism(s) through which ARHGAP11A and RACGAP1 supported BLBC proliferation.…”

“…Instead, ARHGAP11A-knockdown caused these cells to arrest in the G 1 phase of the cell cycle, as determined using flow cytometry analysis of propidium iodide-stained cells. 12 By examining the expression of proteins involved in G 1 to S phase progression, we established that the G 1 arrest of ARHGAP11Adeficient cells was caused by an induction of the cyclindependent kinase (CDK) inhibitor CDKN1B/p27 and the hypophosphorylation of the cell cycle regulator RB1. In RACGAP1-depleted cells, the CDK inhibitor CDKN1A/p21 was upregulated and there was an associated increase in senescence in these cells.…”