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Background: NORE1A and RASSF1A are growth and tumour suppressors inactivated in a variety of cancers. Methylation of NORE1A and RASSF1A promoters is the predominant mechanism for downregulation of these proteins; however, other mechanisms are likely to exist.

Methodology/Principal Findings: Here we describe a proteolysis of NORE1A and RASSF1A by calpains as alternative mechanism of their downregulation. Extracts of H358 cell line, a human bronchoalveolar carcinoma, and H460, a large cell carcinoma, were capable of proteolysis of NORE1A protein in the calpain-dependent manner. Likewise, RASSF1A tumor suppressor was proteolyzed by the H358 cell extract. Addition of calpain inhibitor to H358 and H460 cells growing in tissue culture resulted in re-expression of endogenous NORE1A. A survey of 10 human lung tumours revealed that three of them contain an activity capable of inducing NORE1A degradation.

Conclusions/Significance: Thus, degradation by calpains is a novel mechanism for downregulation of NORE1A and RASSF1A proteins and might be the mechanism allowing cancer cells to escape growth suppression.