CD44 Turns On ROK to Promote Acidification and Invasion

Abstract

Invasion and metastasis--two processes that distinguish cancer from benign tumors and create its chief threat to life--are associated with acidification of the tumor microenvironment. Localized acidification may promote invasion by activating enzymes that degrade the extracellular matrix (ECM), such as hyaluronidase-2 and cathepsin B. In breast cancer, an Na + -H + exchanger isoform (NHE1) that is a substrate of RhoA-activated Rho kinase (ROK) likely plays a role in this extracellular acidification. Bourguignon et al. investigated the involvement of CD44--a transmembrane protein implicated in tumor invasion and metastasis that acts as a receptor for the ECM component hyaluronan (HA)--in regulation of NHE1 and extracellular pH. HA promoted recruitment of CD44, ROK, and NHE1 into lipid rafts isolated from the MDA-MB-231 breast cancer cell line, from which they could be immunoprecipitated in a complex, and stimulated NHE1 phosphorylation. HA stimulated NHE1 activity, as assessed by recovery from intracellular acidification, and acidified the extracellular medium. Pharmacological analysis indicated that HA stimulation of NHE1 and extracellular acidification depended on ROK, as did basal activity of NHE1. Addition of HA decreased the size of endogenous HA polymers in the medium, whereas pharmacological inhibition of NHE1 or ROK or knockdown of hyaluronidase-2 increased HA size. HA enhanced cathepsin B activity, an effect that was mimicked by appropriate changes in extracellular pH and blocked by NHE1 and ROK inhibitors. HA promoted MDA-MB-231 invasion through a Matrigel-coated filter, and this invasion, as well as HA-stimulated extracellular acidification and activation of hyaluronidase-2 and cathepsin-B, was attenuated by CD44 knockdown with small interfering RNA. Thus, HA stimulation of CD44 appears to promote extracellular acidification and tumor invasion through ROK-dependent stimulation of NHE1. L. Y. W. Bourguignon, P. A. Singleton, F. Diedrich, R. Stern, E. Gilad, CD44 interaction with Na + -H + exchanger (NHE1) creates acidic microenvironments leading to hyaluronidase-2 and cathepsin B activation and breast tumor cell invasion. J. Biol. Chem . 279 , 26991-27007 (2004). [Abstract] [Full Text]