Paired-like homeoprotein ESXR1 acts as a sequence-specific transcriptional repressor of the human K-ras gene.

Gain-of-function mutation of the K-ras gene is one of the most common genetic changes in human tumors. In tumors carrying K-ras mutation, the presence of oncogenic K-Ras is necessary for maintenance of the transformed phenotype. ESXR1 is a human paired-like homeodomain-containing protein expressed primarily in the testis. In cells, the 65-kDa full-length ESXR1 protein is proteolytically processed into an N-terminal 45-kDa fragment containing the homeodomain, which localizes exclusively within the nucleus, and a C-terminal 20-kDa fragment consisting of a proline-rich repeat region, which is located in the cytoplasm. In this work, we demonstrated that the N-terminal ESXR1 fragment specifically recognizes the TAATNNNATTA P3 consensus sequence for the paired-like homeodomain and functions as a sequence-specific transcriptional repressor. We also showed that the N-terminal ESXR1 fragment binds to the TAATGTTATTA sequence present within the first intron of the human K-ras gene and inhibits its expression at both mRNA and protein levels. Ectopic expression of the N-terminal ESXR1 fragment in human carcinoma cells that carry mutated K-ras reduces the level of K-Ras and thereby inhibits the tumor cell proliferation. Identification of ESXR1 as a transcriptional repressor of K-ras has an important implication for the development of cancer therapy that inhibits oncogenic K-Ras expression.