Liver-specific silencing of the human gene encoding succinyl-CoA: 3-ketoacid CoA transferase.

The human succinyl-CoA: 3-ketoacid CoA transferase (SCOT) gene encodes the ketolytic enzyme that functions in the mitochondrial matrix. The activation of acetoacetate to acetoacetyl-CoA by SCOT is essential for the use of ketone bodies as an energy source. The ketolytic capacity of tissues is proportional to their level of SCOT activity. Normal hepatocytes, the site of ketone body synthesis, have no detectable SCOT protein. The absence of SCOT in hepatocytes is an important element in energy metabolism, suppressing ketolysis in the liver. To study the tissue-specific silencing of SCOT expression, we analyzed the promoter function of SCOT gene in three different human cell lines. Immunoblot analysis showed that SCOT protein was detectable in HeLa cervical cancer cells and Chang liver cells. However, SCOT protein was not detected in HepG2 hepatoma cells and liver tissues, indicating that HepG2 hepatoma cells maintain the characteristics of liver cells in the ketone body metabolism. Luciferase reporter assays in HeLa and Chang liver cells showed that the 361-bp proximal region of the SCOT gene was responsible for the basal promoter activity and contained two GC boxes, each of which was bound in vitro by Sp1, a ubiquitously expressed transcription factor. These results suggest that these GC boxes may be important for SCOT gene expression. Moreover, the region between -2168 and -361 appeared to inhibit the SCOT promoter activity in HepG2 cells. Thus, liver-specific silencing of the SCOT gene expression may be mediated in part by its 5'-flanking sequence.

Involvement of rhp23, a Schizosaccharomyces pombe homolog of the human HHR23A and Saccharomyces cerevisiae RAD23 nucleotide excision repair genes, in cell cycle control and protein ubiquitination.

A functional homolog (rhp23) of human HHR23A and Saccharomyces cerevisiae RAD23 was cloned from the fission yeast Schizosaccharomyces pombe and characterized. Consistent with the role of Rad23 homologs in nucleotide excision repair, rhp23 mutant cells are moderately sensitive to UV light but demonstrate wild-type resistance to gamma-rays and hydroxyurea. Expression of the rhp23, RAD23 or HHR23A cDNA restores UV resistance to the mutant, indicating that rhp23 is a functional homolog of the human and S.cerevisiae genes. The rhp23::ura4 mutation also causes a delay in the G2 phase of the cell cycle which is corrected when rhp23, RAD23 or HHR23A cDNA is expressed. Rhp23 is present throughout the cell but is located predominantly in the nucleus, and the nuclear levels of Rhp23 decrease around the time of S phase in the cell cycle. Rhp23 is ubiquitinated at low levels, but overexpression of the rhp23 cDNA induces a large increase in ubiquitination of other proteins. Consistent with a role in protein ubiquitination, Rhp23 binds ubiquitin, as determined by two-hybrid analysis. Thus, the rhp23 gene plays a role not only in nucleotide excision repair but also in cell cycle regulation and the ubiquitination pathways.

Functional conservation of HIV-1 Vpr and variability in a mother-child pair of long-term non-progressors.

Increasing evidence suggests that HIV-1 Vpr is required in vivo for viral pathogenesis. Since Vpr displays multiple activities, little is known about which Vpr-specific activities are conserved in naturally occurring viruses or how natural mutations in Vpr might modulate viral pathogenesis in HIV-infected individuals. The goals of this study were to evaluate the functional variability of Vpr in naturally occurring viruses. The Vpr-specific activities of nuclear localization, induction of cell cycle G2 arrest and cell death were compared between viruses isolated from the fast progressing AIDS patients and a mother-child pair of long-term non-progressors (LTNPs). Wild-type Vpr activities were found in all of the viruses that were isolated from the fast progressing AIDS patients except for the truncated Vpr(IIIB) which lacked these activities. In contrast, defective Vpr were readily detected in viral populations isolated, over an 11-year period, from the mother-child pair. Sequence analyses indicated that these Vpr carried unique amino acid substitutions that frequently interrupted a highly conserved domain containing an N-terminal alpha-helix-turn-alpha-helix. Thus, Vpr activities are generally conserved in naturally occurring viruses. The functionally defective Vpr identified in the mother-child pair of LTNPs are likely to be unique and may possibly contribute to the slow disease progression.