LKB1 is recruited to the p21/WAF1 promoter by p53 to mediate transcriptional activation.

The tumor suppressor LKB1 is an evolutionarily conserved serine/threonine kinase. In humans, LKB1 can be inactivated either by germ-line mutations resulting in Peutz-Jeghers syndrome or by somatic mutations causing predisposition to multiple sporadic cancers. LKB1 has wide-ranging functions involved in tumor suppression and cell homeostasis, including establishing cell polarity, setting energy metabolic balance (via phosphorylation of AMP-dependent kinase), regulating the cell cycle, and promoting apoptosis. LKB1 function was previously linked to the tumor suppressor p53 and shown to activate the p53 target gene p21/WAF1. In this study, we further investigated LKB1 activation of the p21/WAF1 gene and addressed whether LKB1 is directly involved at the gene promoter. We find that, consistent with previous studies, LKB1 stabilizes p53 in vivo, correlating with activation of p21/WAF1. We show that LKB1 physically associates with p53 in the nucleus and directly or indirectly phosphorylates p53 Ser15 (previously shown to be phosphorylated by AMP-dependent kinase) and p53 Ser392. Further, these two p53 residues are required for LKB1-dependent cell cycle G(1) arrest. Chromatin immunoprecipitation analyses show that LKB1 is recruited directly to the p21/WAF1 promoter, as well as to other p53 activated promoters, in a p53-dependent fashion. Finally, a genetic fusion of LKB1 to defective p53, deleted for its activation domains, promotes activation of p21/WAF1. These results indicate that LKB1 has a direct role in activation of p21/WAF1 gene.

Role for RhoB and PRK in the suppression of epithelial cell transformation by farnesyltransferase inhibitors.

Recent genetic investigations have established that RhoB gain-of-function is sufficient to mediate the antitransforming effects of farnesyltransferase inhibitors (FTIs) in H-Ras-transformed fibroblast systems. In this study, we addressed the breadth and mechanism of RhoB action in epithelial cells transformed by oncoproteins which are themselves insensitive to FTI inactivation. Rat intestinal epithelial (RIE) cells transformed by activated K-Ras or Rac1 were highly sensitive to FTI-induced actin reorganization and growth inhibition, despite the inability of FTI to block prenylation of either K-Ras or Rac1. Ectopic expression of the geranylgeranylated RhoB isoform elicited in cells by FTI treatment phenocopied these effects. Analysis of RhoB effector domain mutants pointed to a role for PRK, a Rho effector kinase implicated in the physiological function of RhoB in intracellular receptor trafficking, and these findings were supported further by experiments in a fibroblast system. We propose that FTIs recruit the antioncogenic RhoB protein in the guise of RhoB-GG to interfere with signaling by pro-oncogenic Rho proteins, possibly by sequestering common exchange factors or effectors such as PRK that are important for cell transformation.

Signaling kinase AMPK activates stress-promoted transcription via histone H2B phosphorylation.

The mammalian adenosine monophosphate-activated protein kinase (AMPK) is a serine-threonine kinase protein complex that is a central regulator of cellular energy homeostasis. However, the mechanisms by which AMPK mediates cellular responses to metabolic stress remain unclear. We found that AMPK activates transcription through direct association with chromatin and phosphorylation of histone H2B at serine 36. AMPK recruitment and H2B Ser36 phosphorylation colocalized within genes activated by AMPK-dependent pathways, both in promoters and in transcribed regions. Ectopic expression of H2B in which Ser36 was substituted by alanine reduced transcription and RNA polymerase II association to AMPK-dependent genes, and lowered cell survival in response to stress. Our results place AMPK-dependent H2B Ser36 phosphorylation in a direct transcriptional and chromatin regulatory pathway leading to cellular adaptation to stress.

Trimethylation of histone H3 lysine 4 by Set1 in the lytic infection of human herpes simplex virus 1.

Human herpes simplex virus 1 (HSV-1) is a double-stranded DNA virus that causes facial, ocular, and encephalitic disease in humans. Previous work showed that the genome of HSV-1 is associated with acetylated and methylated histones during lytic infection. However, the physiological role of histone modifications in lytic infection of HSV-1 is unclear. We examined the role of protein methylation in lytic infection of HSV-1 using a protein methylation inhibitor, 5'-deoxy-5'-methylthioadenosine (MTA). We found that MTA strongly reduces the transcription and replication of HSV-1. Moreover, MTA treatment decreases the level of trimethylation of lysine 4 in histone H3 (H3K4me3) on the HSV-1 genome. These results suggest that protein methylation, and in particular, histone methylation, is involved in the lytic infection of HSV-1. To delineate the underlying mechanism, we investigated the role of two H3K4 methyltransferases, Set1 and Set7/9, in the lytic infection of HSV-1. Using small interference RNA, we found that the reduction of Set1, but not Set7/9, reduces the transcription and replication of HSV-1 and specifically decreases H3K4me3 on the virus genome. These results indicate that H3K4me3 mediated by Set1 is required for optimal gene expression and replication of HSV-1 during lytic infection and suggest that this pathway could be a potential point of pharmacological intervention during HSV-1 infection.