Ki-1/57 and CGI-55 ectopic expression impact cellular pathways involved in proliferation and stress response regulation

Abstract

Ki-1/57 (HABP4) and CGI-55 (SERBP1) are regulatory proteins and paralogs with 40.7% amino acid sequence identity and 67.4% similarity. Functionally, they have been implicated in the regulation of gene expression on both the transcriptional and mRNA metabolism levels. A link with tumorigenesis is suggested, since both paralogs show altered expression levels in tumor cells and the Ki-1/57 gene is found in a region of chromosome 9q that represents a haplotype for familiar colon cancer. However, the target genes regulated by Ki-1/57 and CGI-55 are unknown. Here, we analyzed the alterations of the global transcriptome profile after Ki-1/57 or CGI-55 overexpression in HEK293T cells by DNA microchip technology. We were able to identify 363 or 190 down-regulated and 50 or 27 up-regulated genes for Ki-1/57 and CGI-55, respectively, of which 20 were shared between both proteins. Expression levels of selected genes were confirmed by qRT-PCR both after protein overexpression and siRNA knockdown. The majority of the genes with altered expression were associated to proliferation, apoptosis and cell cycle control processes, prompting us to further explore these contexts experimentally. We observed that overexpression of Ki-1/57 or CGI-55 results in reduced cell proliferation, mainly due to a G1 phase arrest, whereas siRNA knockdown of CGI-55 caused an increase in proliferation. In the case of Ki-1/57 overexpression, we found protection from apoptosis after treatment with the ER-stress inducer thapsigargin. Together, our data give important new insights that may help to explain these proteins putative involvement in tumorigenic events.

Intracellular hyaluronic acid-binding protein 4 (HABP4): a candidate tumor suppressor in colorectal cancer

“…In accordance with these findings, the tumor cells from the knockout mice had a higher expression of the proliferation markers PCNA, Cyclin D1 and CDK4. In a microarray study with an over-expression of HABP4 in comparison to control cells with very low expression level [11], we observed a general tendency of a down-regulation of the expression of target genes in about 90% of the identified genes. Among the down regulated genes are several important regulators of cellular proliferation, including: MAP2K5, MAPK12, PLAU, PDXK, NRG1, TXLNA (alpha-taxilin).…”

“…Finally, we need to consider the fact that the HABP4 protein forms a family of proteins that consists in vertebrates of two members [11], with the gene CGI-55 or SERBP1 or PAI-RNABP1, being the other member. SERBP1 shares most of the characteristics of HABP4 at least partially [10].…”

“…Previous data have shown that HABP4 overexpression in HEK293 cells decreases the expression of genes related to the proliferative activity [11]. To investigate in vivo the involvement of HABP4 in cell proliferation activity, the incorporation of bromodeoxyuridine (BrdU) by the colon epithelium cells of the wild-type and Habp4 -/-mice was evaluated.…”

The RNA-binding protein SERBP1 interacts selectively with the signaling protein RACK1

“…Also like HABP4, SERBP1 is a substrate for PRMT1-mediated arginine methylation, which may regulate the nuclear-cytoplasmic distribution of the protein (79;80). SERBP1 and HABP4 target similar mRNA populations in cells, clearly suggesting some overlap in their physiological functions (81). SERBP1 also interacts with progesterone receptor membrane component 1 (PGRMC1), linking it to progesterone-signaling pathways (82).…”

SERBP1 affects homologous recombination-mediated DNA repair by regulation of CtIP translation during S phase

“…Although there is little evidence for a functional relationship, amino acid sequence homology suggests that SERBP1 is a member of the HABP4 family which has the hyaluronan binding domain and an RNA binding motif in common. SERBP1 does not contain RNA recognition motif (RRM) or K homology (KH) domain, but has an arginine-glycine (RG)-rich and arginine-glycine-glycine (RGG) box for target mRNA binding ( 24 ). SERBP1 interacting proteins were identified using the yeast two-hybrid system.…”