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

Abstract

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 sequencespecific transcriptional repressor. We also showed that the N-terminal ESXR1 fragment binds to the TAATGTTAT TA 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.

Rapid evolution of primate ESX1 , an X-linked placenta- and testis-expressed homeobox gene

“…Thus, it has been proposed that the C-terminal fragment of ESX1 plays a role in spermatogenesis, functioning as a checkpoint in male germ cell division (32). In contrast, the N-terminal fragment, including the homeodomain, functions as a transcriptional repressor in nucleus (32,33). Our observations of conserved sequences in the N-terminal fragment but rapid sequence changes in the C-terminal fragment are explainable by the distinct functions of the two regions.…”

Clinical Relevance of KRAS in Human Cancers

“…ESXR1 is a human protein with an N-terminal homeodomain in the nucleus and a C-terminal proline-rich repeat region I in the cytoplasm. The N-terminal fragment of ESXR1 binds to the TAATGTTATTA consensus sequence in exon-1 of the KRAS gene, thus inhibiting its mRNA expression [57]. miRNAs contain a 21-22 nucleotide long noncoding sequence that is able to regulate gene expression [58].…”

ESX1 gene as a potential candidate responsible for male infertility in nonobstructive azoospermia

“…The human ESX1 gene is present on the X chromosome at position Xq22.2 and has 4 exons. It is a homeotic gene that is only expressed in the male gonad of adult males 13 , 14 . This gene, which encodes a dual-function 65 kDa protein, contains a homeodomain in the N-terminal fragment and a proline-rich region in the C-terminal fragment 15 .…”