Research Groups
Mammalian Biology: Virology
Research Interests and Description
Staff Research Scientist: Vijay Kumar, PhD
Group Leader: Shahid Jameel
Research Interests
HBx, oncogenic cooperation, cell cycle regulation, nucleolar dynamics under stress, anticancer strategies.
Description of Research
Cell cycle deregulation by HBx
The role of HBx in cell cycle deregulation is now well recognized. HBx expression leads to early and sustained levels of cyclin E and CDK2, increased CDK2 kinase activity and S phase progression. While intracellular level of cyclin E was stabilized, a rapid proteasomal degradation of CDK inhibitor p27Kip1 was observed. p27Kip1 downregulation was also seen in the liver of HBx transgenic mice. Interestingly, the p27Kip1 gene was transcriptionally downregulated by AP-1 complexes. Thus, the HBx protein seems to override normal cell cycle restraints by directly interacting with key cell cycle regulators.
Regulation of cyclin E gene during cell cycle
The G1-specific cyclin E expression periodically oscillates in every cycle of proliferating cells while its catalytic subunit CDK2 is constitutively expressed. Working on the cell cycle regulatory activity of some proto-oncogenes, we observed that c-ETS1 transcriptionally up-regulated both cyclin E and CDK2 genes. The process was mediated by kinetic coherence of c-ETS1 expression and its recruitment to both promoters during G1/S-phase transition. Further, enforced expression of c-ETS1 helped G0-arrested cells to progress to G1/S-phases apparently due to co-expression of cyclin E/CDK2 genes. NF-kB family members are known to play a pivotal role in many cellular and organismal functions. We studied involvement of NF-kB in G1/S phase regulation following its induction by TNFa. NF-kB, which appeared to down-regulate cyclin E gene transcription following its recruitment on the cyclin E promoter. There was an accumulation of free CDK2 and arrest of cells in G1/S-phase. Thus, NF-kB seems to be a negative regulator of cell cycle.
Regulation of ubiquitin-proteasome system by viral HBx
c-Myc is associated with cell proliferation and tumor development. Further, myc genes are reported to be amplified in many HCC cases. We studied the oncogenic cooperation between viral HBx and Myc in an HBx microenvironment. Co-expression of HBx and c-Myc resulted in a significant increase in the intracellular stability of c-Myc by inhibiting its ubiquitination. Further, we showed that HBx bound ubiquitin ligase Skp2 that prevented Myc ubiquitination. We suggest that the sustained presence of c-Myc, combined with mitogenic activity inherent to HBx, may be associated with cell cycle deregulation and transformation.
Epigenetic control of origin licensing
Recent genome-wide mapping of human replication origins suggest that transcriptional regulatory elements are crucial for origin selection. To unveil the molecular underpinnings, we analyzed the human lamin B2 origin that spatially overlaps with the TIMM 13 promoter. We noted that early G1 occupancy of c-Myc is not associated with transactivation of the TIMM 13 gene and, rather, facilitated the loading of MCM4 and acetylase HBO1 leading to nucleosome remodeling. Moreover, Myc binding to DNA was dependent on transient demethylation of E-box during early G1. Thus, epigenetic control of E-box could be a mechanism for specifying the licensing of early replicating origins.
Model depicting the molecular underpinnings of LB2 ORI licensing
Cell cycle regulated epigenetic modification of the E-box element at LB2 ORI signals the occupancy of c-Myc to demethylated E-box during early G1. The subsequent Myc-dependent histone modification during mid G1 phase favors the remodeling of nucleosomes at LB2 ORI that specifies licensing by facilitating the loading of MCMs.
Recent Publications
Singh, A.K. Swarnalatha, M., Kumar, V. 2011. c-Ets-1 facilitates G1/S-phase transition by upregulating cyclin E and cdk2 genes and cooperates with hepatitis B virus X protein for their deregulation. J. Biol. Chem. 286, 21961-21970 PubMed link
Bui-Nguyen, T.M., Pakala, S.B., Sirigiri1, R.D., Xia, W., Hung, M.C., Sarin, S.K., Kumar, V., Slagle, B.L., Kumar, R. 2010. NF-kappaB signaling mediates the induction of MTA1 by hepatitis B virus transactivator protein HBx. Oncogene 29, 1179-1189 PubMed link
Janbandhu, V.C., Singh, A.K., Mukherji, A., Kumar, V. 2010. p65 negatively regulates transcription of cyclin E gene. J. Biol Chem. 285, 17453-17464 PubMed link
Khattar, E., Kumar, V. 2010. Mitogenic regulation of p27Kip1 gene is mediated by AP-1 transcription factors. J. Biol. Chem. 285, 4554-4561 PubMed link
Mukherji, A., Janbandhu, V.C., Kumar, V. 2008. GSK-3beta-dependent destabilization of cyclin D1 mediates replicational stress-induced arrest of cell cycle. FEBS Lett. 582, 1111-1116 Pubmed link
Mukherji, A., Janbandhu, V.C., Kumar, V. 2007. HBx-dependent cell cycle deregulation involves interaction with cyclin E/A-cdk2 complex and destabilization of p27Kip1. Biochem. J. 401, 247-256 Pubmed link
