Wig-1, a new p53-induced gene encoding a zinc finger protein.

Wild type p53 expressed from a temperature-sensitive (ts p53) construct induces both G1 cell cycle arrest and apoptosis in the p53-negative J3D mouse T lymphoma line (Wang et al., 1995). Using differential display analysis, we have identified one new p53-induced gene, wig-1 (for wild type p53-induced gene 1), whose 7.6 kb and 2.2 kb transcripts are upregulated in ts p53-transfected J3D cells following induction of wild type p53 expression by temperature shift to 32 degrees C. The wig-1 transcripts were also induced in irradiated NIH3T3 and p21-/- fibroblasts but not in irradiated p53-/- fibroblasts. Whole body gamma irradiation caused induction of both wig-1 transcripts in mouse brain, testis, kidney, spleen and lung. A basal wig-1 expression was detected in brain, testis and kidney. The WIG-1 protein contains three zinc finger motifs and a putative nuclear localization signal.

The retinoblastoma protein modulates expression of genes coding for diverse classes of proteins including components of the extracellular matrix.

The product of the retinoblastoma susceptibility gene, pRb, is a negative regulator of cell growth. It functions by regulating the activity of transcription factors. Rb represses some genes by sequestering or inactivating the positive transcription factor E2F and seems to activate some others by interacting with factors like Sp1 or ATF-2. However, there are only a few examples of genes which are positively regulated by pRb. In order to find out if there are common mechanisms for promoter regulation by pRb, we were interested to identify more genes which are either stimulated or repressed by pRb. Using the method of differential display (DDRT-PCR) in combination with nuclear run-on analyses we were able to detect a number of genes which are upregulated by ectopic expression of the Rb gene in Rb-deficient mammary carcinoma cells. We could demonstrate not only stimulation of the endogenous mutant Rb gene but also positive regulation of genes coding for diverse classes of proteins, including the endothelial growth regulator endothelin-1 and the proteoglycans versican and PG40. As a second approach, we investigated gene expression in cell lines established from Rb deficient heterozygous and homozygous knockout mouse embryos and normal mice. We have identified several genes the expression of which correlates positively or negatively with the presence of Rb. These data provide further evidence for pRb being a master regulator of a complex network of gene activities defining the difference between dividing and resting or differentiated cells.

Cyclin D1 expression is regulated by the retinoblastoma protein.

The product of the retinoblastoma susceptibility gene, pRb, acts as a tumor suppressor and loss of its function is involved in the development of various types of cancer. DNA tumor viruses are supposed to disturb the normal regulation of the cell cycle by inactivating pRb. However, a direct function of pRb in regulation of the cell cycle has hitherto not been shown. We demonstrate here that the cell cycle-dependent expression of one of the G1-phase cyclins, cyclin D1, is dependent on the presence of a functional Rb protein. Rb-deficient tumor cell lines as well as cells expressing viral oncoproteins (large tumor antigen of simian virus 40, early region 1A of adenovirus, early region 7 of papillomavirus) have low or barely detectable levels of cyclin D1. Expression of cyclin D1, but not of cyclins A and E, is induced by transfection of the Rb gene into Rb-deficient tumor cells. Cotransfection of a reporter gene under the control of the D1 promoter, together with the Rb gene, into Rb-deficient cell lines demonstrates stimulation of the D1 promoter by Rb, which parallels the stimulation of endogenous cyclin D1 gene expression. Our finding that pRb stimulates expression of a key component of cell cycle control, cyclin D1, suggests the existence of a regulatory loop between pRb and cyclin D1 and extends existing models of tumor suppressor function.

Identification of differentially expressed mRNA species by an improved display technique (DDRT-PCR).

We have significantly improved a method originally developed by Liang and Pardee [Science 257 (1992) 967-971] to display a broad spectrum of expressed genes and to detect differences in expression between different cell types. We have analysed various aspects of the technique and have modified it for both, the application to fast and efficient identification of genes and the use with automatic analysis systems. Based on the mathematical background we have devised the appropriate number of optimal PCR primers. We have also introduced nondenaturating gels for separating double stranded fragments as single bands. By applying the method to regenerating mouse liver, we have identified, out of a total of 38,000 bands, about 70 fragments where the expression of the corresponding genes seems to be differentially regulated at different time points. Application of the method to an automatic DNA sequencer was successfully done. Thus, we have confirmed the usefulness and increased the power of the RNA display technique, which we named differential display reverse transcription PCR (DDRT-PCR), and have extended the range of its application.