ABSTRACT
Although anorectal disorders such as abscess, fissure, and hemorrhoids are typically outpatient problems, they also occur in the critically ill patient population, where their presentation and management are more difficult. This article will provide a brief review of anorectal anatomy, explain the proper anorectal examination, and discuss the current understanding and treatment concepts with regard to the most common anorectal disorders that the intensive care unit clinician is likely to face.
Subject(s)
Anal Canal/pathology , Critical Care/methods , Critical Illness , Rectal Diseases/diagnosis , Rectal Diseases/therapy , Abscess/diagnosis , Abscess/therapy , Anal Canal/anatomy & histology , Fissure in Ano/diagnosis , Fissure in Ano/therapy , Hemorrhoids/diagnosis , Hemorrhoids/therapy , Humans , Physical Examination , Practice Guidelines as Topic , Rectal Diseases/pathology , Rectal Fistula/diagnosis , Rectal Fistula/therapyABSTRACT
Ribonucleotide reductase (RNR) is an enzyme for the de novo conversion of ribonucleotides to deoxyribonucleotides. The two human RNR small subunits hRRM2 and hp53R2 share 83% sequence homology but show distinct expression patterns and function. Structural analyses of the oxidized form of hRRM2 and hp53R2 indicate that both proteins contain a conserved Gln127-hp53R2/Gln165-hRRM2 close to the dinuclear iron center and the essential tyrosine residue Tyr124-hp53R2/Tyr162-hRRM2 forms hydrogen bonds with the tyrosine and iron ligands, implying a critical role for the glutamine residue in assembling the dityrosyl-diiron radical cofactor. The present work also showed that Tyr221 in hRRM2, which is replaced by Phe183 in hp53R2, forms a hydrogen bond with Tyr162 to extend the hydrogen bond network from Gln165-hRRM2. Mutagenesis and spectroscopic experiments suggested that the tyrosine-to-phenylalanine switch at Phe183-hp53R2/Tyr221-hRRM2 could lead to differences in radical generation or enzymatic activity for hp53R2 and hRRM2. This study correlates the distinct catalytic mechanisms of the small subunits hp53R2 and hRRM2 with a hydrogen-bonding network and provides novel directions for designing and developing subunit-specific therapeutic agents for human RNR enzymes.
Subject(s)
Cell Cycle Proteins/chemistry , Cell Cycle Proteins/metabolism , Free Radicals/metabolism , Iron Compounds/metabolism , Protein Subunits/metabolism , Ribonucleoside Diphosphate Reductase/chemistry , Ribonucleoside Diphosphate Reductase/metabolism , Ribonucleotide Reductases/chemistry , Ribonucleotide Reductases/metabolism , Amino Acid Sequence , Amino Acid Substitution/genetics , Circular Dichroism , Electron Spin Resonance Spectroscopy , Humans , Molecular Sequence Data , Mutagenesis/genetics , Mutant Proteins/chemistry , Mutant Proteins/metabolism , Protein Subunits/chemistry , Sequence AlignmentABSTRACT
BACKGROUND: Ribonucleotide reductase (RR) inhibition by hydroxyurea (HU) causes deoxyribonucleotide (dNTP) depletion, which activates the replication checkpoint, a part of the S-phase checkpoint that responds to DNA damage by inhibiting late origin firing. It also transactivates RR and other genes involved in DNA replication and repair. ICBP90 (overexpressed in breast cancer) is a novel Rb-associating transactivator for the human topoisomerase IIalpha gene and responds to DNA damage-induced checkpoint signaling. MATERIALS AND METHODS: ICBP90 expression was monitored by Western blot. Promoter activity was detected via the luciferase assay and gene silencing via siRNA. Cell death was monitored by the MTT assay. RESULTS: dNTP depletion by HU induced ICBP90, ICBP90 transactivated RR's M2 subunit gene, and ICBP90 induction was necessary for HU-induced M2 accumulation. Blocking the M2 accumulation via anti-ICBP90 siRNA caused greater sensitivity in HU-resistant human cancer. CONCLUSION: A transcriptional intervention strategy is presented through which HU-resistant cancers may be eradicated without dose escalation.
