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NORE1A Tumor Suppressor Candidate Modulates p21^CIP1 via p53

¹ Institut für Pathologie, Ernst-Moritz-Arndt-Universität, Greifswald, Germany; ² Molecular Targets Group, J.G. Brown Cancer Center, University of Louisville, Louisville, Kentucky; ³ Cell and Cancer Biology Branch, National Cancer Institute, Bethesda, Maryland; and ⁴ Division of Medical Biochemistry, Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Cape Town, South Africa

Requests for reprints: Geoffrey J. Clark, University of Louisville, Room 119C, Baxter II Research Building, 580 S. Preston Street, Louisville, KY 40202. Phone: 502-852-4485; Fax: 502-852-2123; E-mail: gjclar01{at}louisville.edu.

Key Words: Ras • NORE1A • tumor suppressor • p53 • p21^CIP1

NORE1A (RASSF5) is a proapoptotic Ras effector that is frequently inactivated by promoter methylation in human tumors. It is structurally related to the RASSF1A tumor suppressor and is itself implicated as a tumor suppressor. In the presence of activated Ras, NORE1A is a potent inducer of apoptosis. However, when expressed at lower levels in the absence of activated Ras, NORE1A seems to promote cell cycle arrest rather than apoptosis. The mechanisms underlying NORE1A action are poorly understood. We have used microarray analysis of an inducible NORE1A system to screen for physiologic signaling targets of NORE1A action. Using this approach, we have identified several potential signaling pathways modulated by NORE1A. In particular, we identify the cyclin-dependent kinase inhibitor p21^CIP1 as a target for NORE1A activation and show that it is a vital component of NORE1A-mediated growth inhibition. In primary human hepatocellular carcinomas (HCC), loss of NORE1A expression is frequent and correlates tightly with loss of p21^CIP1 expression. NORE1A down-regulation in HCC also correlates with poor prognosis, enhanced proliferation, survival, and angiogenic tumor characteristics. Experimental inactivation of NORE1A results in the loss of p21^CIP1 expression and promotes proliferation. The best characterized activator of p21^CIP1 is the p53 master tumor suppressor. Further experiments showed that NORE1A activates p21^CIP1 via promoting p53 nuclear localization. Thus, we define the molecular basis of NORE1A-mediated growth inhibition and implicate NORE1A as a potential component of the ill-defined connection between Ras and p53. [Cancer Res 2009;69(11):4629–37]