The atypical RhoU/Wrch1 Rho GTPase controls cell proliferation and apoptosis in the gut epithelium.

BACKGROUND INFORMATION: The mammalian gut epithelium displays among the highest rates of self-renewal, with a turnover time of less than five days. Renewal involves concerted proliferation at the bottom of the crypt, migration and differentiation along the crypt-villus axis and anoïkis/shedding in the luminal epithelium. Renewal is controlled by interplay between signaling pathways, among which canonical and non-canonical Wnt signals play prominent roles. 92% of colon tumors show increased canonical Wnt signaling resulting from mutations, established as major driver steps towards carcinogenesis.

RESULTS: Here we examined the physiological role of RhoU/Wrch1 in gut homeostasis. RhoU is an atypical Rho GTPase related to Cdc42/Rac1 and identified as a transcriptional target of non-canonical Wnt signaling. We found that RHOU expression is reduced in human colorectal tumor samples. We show that RhoU is mainly expressed in the differentiated compartment of the gut epithelium. Rhou specific invalidation in the mouse gut elicits cell hyperplasia and is associated in the colon with a highly disorganized luminal epithelium. Hyperplasia affects all cell types in the small intestine and colon and has a higher impact on goblet cells. Hyperplasia is associated with a reduction of apoptosis and an increased proliferation. RhoU knockdown in human DLD-1 colon cancer cells also elicits a higher growth index and reduces cell apoptosis. Last, loss of RhoU function in the mouse gut epithelium or in DLD-1 cells increases RhoA activity and the level of phosphorylated Myosin Light Chain-2, which may functionally link RhoU activity to apoptosis.

CONCLUSION: RhoU is mostly expressed in the differentiated compartment of the gut. It plays a role in homeostasis as its specific invalidation elicits hyperplasia of all cell types. This mainly results from a reduction of apoptosis, through actomyosin-dependent mechanisms.

SIGNIFICANCE: RhoU negatively controls cell growth in the intestinal epithelium. Since its expression is sensitive to non-canonical Wnt signals and is reduced in colorectal tumors, down-regulating RhoU may thus have an instrumental role in tumor progression. This article is protected by copyright. All rights reserved.