Functional Interactions of the KLF6 transcription factor, a potencial tumor supresor, with the c-Jun and E1A oncoproteins.

BOCCO JOSE LUIS

Hotel Amancay – Bariloche – Province of Río Negro – Argentina

Congreso; Combined meetings: Gene Expression and RNA Processing – Cell Biology, Signaling and Alternative Splicing; 2007

International Centre for Genetic Engineering and Biotechnology (ICGEB) - Facultad de Ciencias Exactas y

KLF6 is a member of an evolutionary conserved family of gene regulators known as Krüppel-like transcription factors (KLFs) characterized by a common Zn-finger DNA-binding domain although each KLF regulates a differential and specific panel of target genes. It has been postulated that KLF6 is a potential tumor suppressor whose expression is responsive to external cell stimulation mediated by growth factors, tumor promoters and DNA-damage agents.  Biochemical evidence of our laboratory demonstrated that this factor interacts with the c-Jun proto-oncoprotein and induces c-Jun degradation by the proteasome-mediated pathway leading to inhibition of cell proliferation. Interestingly, KLF6 expression is also targeted during microbial infections. Thus, endogenous KLF6 protein is substantially induced in lung-derived cells after 6 h of Adenovirus infection but then decreased dramatically until barely detected levels following 24 h of infection.  The KLF6 decline occurs simultaneously with increased expression of the Adenovirus E1A oncoprotein indicating an active virus replication.  Reduced KLF6 protein level is also accompanied by enhanced c-Jun phosphorylation and by a decrease in the p21CIP/WAF protein whose gene promoter is a known target for KLF6-mediated activation independently of p53; indicating that cells are moving into the S-phase required for viral replication.  Reporter assays showed that KLF6 expression contribute to enhance E1A-mediated activation of the E2A gene transcription that is essential for virus replication.  Moreover, chromatin immnoprecipitation and real-time PCR assays demonstrated that KLF6 interacts transiently with the E2A promoter in Adenovirus infected cells.  The highest DNA binding correlates directly with maximal detection of KLF6 protein level induced at 6 h of infection whereas KLF6 binding was completely abrogated at 12 h and 24 h of infection, in line with lower KLF6 expression. An integrated view of interactions between KLF6 tumor suppressor with c-Jun and E1A oncoproteins supported by these results is as follow: Cells respond inducing KLF6 protein levels as a safeguard signal to the stress generated by Adenovirus infection.  As a consequence KLF6 cooperates to increase p21 CIP/WAF gene promoter transcription in a p53-independent manner.  In turn p21 CIP/WAF contributes to the cell cycle arrest through cdk/cyclin inhibition.  Additionally, increased KLF6 protein level generates a transient reduction/inhibition of c-Jun, alleviating the inhibitory effects on p53 which become free to further induce the p21 CIP/WAF expression.  The final outcome is a transient cell cycle arrest imposed by p21 CIP/WAF that counteracts the earliest steps of infection. Concomitantly, initial KLF6 induction is recruited and subverted for transcription of the Adenovirus E2A promoter along with E1A oncoprotein. Nevertheless, as the course of infection proceeds, KLF6 protein is progressively decreased until undetectable levels.  At later stages the pathways of p21 CIP/WAF induction and inhibition of c-Jun mediated by KLF6 are not longer produced.  Under this condition it is expected a cell cycle progress allowing a cell environment suitable for viral replication. Together, these results are consistent with a critical KLF6 function whose activity can be targeted by cellular and viral oncoproteins as c-Jun and E1A.