Nuclear recruitment of A1p145 subunit of replication factor C in the early G1 phase of the cell cycle in Faza 567 hepatoma cell line and hepatocyte primary cultures

Cyclic AMP regulates the expression and nuclear translocation of RFC40 in MCF7 cells

We have previously shown that the regulatory subunit of PKA, RIalpha, functions as a nuclear transport protein for the second subunit of the replication factor C complex, RFC40, and that this transport appears to be crucial for cell cycle progression from G1 to S phase. In this study, we found that N(6)-monobutyryl cAMP significantly up-regulates the expression of RFC40 mRNA by 1.8-fold and its endogenous protein by 2.3-fold with a subsequent increase in the RIalpha-RFC40 complex formation by 3.2-fold. Additionally, the nuclear to cytoplasmic ratio of RFC40 increased by 26% followed by a parallel increase in the percentage of S phase cells by 33%. However, there was reduction in the percentage of G1 cells by 16% and G2/M cells by 43% with a concurrent accumulation of cells in S phase. Interestingly, the higher percentage of S phase cells did not correlate with a parallel increase in DNA replication. Moreover, although cAMP did not affect the expression of the other RFC subunits, there was a significant decrease in the RFC40-37 complex formation by 81.3%, substantiating the decrease in DNA replication rate. Taken together, these findings suggest that cAMP functions as an upstream modulator that regulates the expression and nuclear translocation of RFC40.

Functional Interaction of bZIP Proteins and the Large Subunit of Replication Factor C in Liver and Adipose Cells

The transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPalpha) has a vital role in cell growth and differentiation. To delineate further a mechanism for C/EBPalpha-mediated differentiation, we screened C/EBPalpha-interacting proteins through far-Western screening. One of the strongest interactions was with RFC140, the large subunit of the replication factor C complex. C/EBPalpha specifically interacted with RFC140 from rat liver nuclear extract as determined by a combination of affinity chromatography and co-immunoprecipitation. Subsequent far-Western blotting showed that the bZIP domain of C/EBPalpha interacted with the DNA-binding region of RFC140. Overexpression of RFC140 in mammalian cells increased the transactivation activity of C/EBPalpha on both minimal and native promoters. Consistent with the enhanced transactivation, a complex of C/EBPalpha and RFC140 proteins with the cognate DNA element was detected in vitro. The specific interaction between C/EBPalpha and RFC140 was detected in the terminal differentiation of 3T3-L1 preadipocytes to adipocytes. The synergistic transcription effect of these two proteins increased the promoter activity and protein expression of peroxisome proliferator-activated receptor-gamma, which is a main regulator of adipocyte differentiation. Our results demonstrate that the specific transcription factor C/EBPalpha and the general DNA replication factor RFC140 interact functionally and physically. This observation highlights a unique mechanism by which the levels of the general replication factor can strongly modulate the functional activity of the specific transcription factor as a coactivator.

A novel transcription factor is correlated with both glomerular proliferation and sclerosis in the rat renal ablation model

Glomerular accumulation of the extracellular matrix (ECM) with subsequent sclerosis is a common finding in most progressive renal diseases. Recently MSW (Mouse South Western) protein was cloned by its ability to bind the bidirectional promoter of the collagen IV genes. This protein was also reported as the large subunit of the DNA replication complex A1, as well as the promoter binding protein of corticotropin-releasing hormone and the angiotensinogen gene. To investigate the mechanism of accumulation of the ECM as it relates to glomerular cellular events, the expression of MSW protein was studied in the remnant kidney model. Progressive expression of MSW protein was found in the glomerular sclerotic lesion at week 4 and at later time points after renal ablation. The expression of proliferating cell nuclear antigen (PCNA) and type IV collagen was also correlated with the expression of MSW protein by immunofluorescence. RNA dot blot analysis also showed that the expression of MSW mRNA was increased at week 7 in association with the augmented expression of type IV collagen. These results, taken together, suggest that MSW protein plays an important role in the regulation of type IV collagen gene expression in vivo and may contribute to glomerular cell proliferation and the development of glomerulosclerosis.

Expression of laminin gamma 1 cultured hepatocytes involves repeated CTC and GC elements in the LAMC1 promoter

Laminin gamma 1 chain is present in all basement membranes and is expressed at high levels in various diseases, such as hepatic fibrosis. We have identified cis- and trans-acting elements involved in the regulation of this gene in normal rat liver, as well as in hepatocyte primary cultures and hepatoma cell lines. Northern-blot analyses showed that laminin gamma 1 mRNA was barely detectable in freshly isolated hepatocytes and expressed at high levels in hepatocyte primary cultures, as early as 4 h after liver dissociation. Actinomycin D and cycloheximide treatment in vivo and in vitro indicated that laminin gamma 1 overexpression in cultured hepatocytes was under the control of transcriptional mechanisms. Transfection of deletion mutants of the 5' flanking region of murine LAMC1 gene in hepatoma cells that constitutively express laminin gamma 1 indicated that regulatory elements were located between -594 bp and -94 bp. This segment included GC- and CTC-containing motifs. Gel-shift analyses showed that two complexes were resolved with different affinity for the CTC sequence depending on the location of the GC box. The pattern of complex formation with nuclear factors from freshly isolated and cultured hepatocytes was different from that obtained with total liver and similar to that with hepatoma cells. Southwestern analysis indicated that several polypeptides bound the CTC-rich sequence. Affinity chromatography demonstrated that A M(r) 60,000 polypeptide was a major protein binding to the CTC motif. This polypeptide is probably involved in the transcriptional activation of various proto-oncogenes and extracellular matrix genes that are expressed at high levels in both hepatoma cells and early hepatocyte cultures.