EXPRESSION OF ALTERNATIVE FORMS OF RAS EXCHANGE FACTORS GRF AND SOS1 IN DIFFERENT HUMAN TISSUES AND CELL-LINES

DNA probes and antibodies specific for different coding regions of hum an SOS1 and GRF genes were used to screen expression of these genes in a variety of adult and fetal human tissues and cell lines. Despite pr evious reports of the exclusive expression of hGRF RNA in brain, we al so observed expression of this gene in various other tissues including lung and pancreas, as web as several tumor cell lines. At least three different hGRF mRNA transcripts were observed depending on the probe used, with the larger transcripts being detected by probes correspondi ng to the 5' end of the gene while smaller transcripts were detected b y probes corresponding to the 3' end. Expression of hSOS1-related tran scripts was more ubiquitous and homogeneous than with hGRF, with simil ar levels of specific transcripts being detected in most tissues and c ell lines tested. Three to five different transcripts were detected in human tissues when using probes for the 5' end and middle regions of this gene, whereas only two were detected with probes corresponding to the 3' end. Screening of multiple human tumor cell lines showed ubiqu itous expression of three specific transcripts, although the level and ratio of each of these transcripts varied widely among individual cel l lines. Consistent with the variety of transcripts detected, several protein forms were also identified in Western immunoblots with antiser a raised against specific domains of hSOS1 and human Ras-GRF gene prod ucts. Fluorescence in situ chromosomal hybridization suggested that, i n both cases, the multiple forms arise from single chromosomal loci. T he heterogeneity of hGRF and hSOS1 gene products detected (which appea r to retain in most cases a functional catalytic domain), suggests tha t differentially expressed, alternatively spliced hSOS1 and hGRF forms may contribute to fine regulation of Ras activation in different tiss ues or at different stages of development.