Relationship between protein levels and gene expression of dihydropyrimidine dehydrogenase in human tumor cells during growth in culture and in nude mice

Protein levels and gene expression of dihydropyrimidine dehydrogenase (DPD) , the rate-limiting enzyme for degradation of 5-fluorouracil, were studied in two human tumor cell lines (fibrosarcoma MT-1080 and pancreatic carcinom a MIAPaCa-2) in various growth phases of the cultured cells and of tumor xe nografts implanted into nude mice. DPD catalytic activity and DPD protein c ontent in cytosolic preparations mere determined by means of radioenzymatic assay and western blot analysis, respectively. Relative DPD mRNA expressio n was determined by using a semi-quantitative reverse transcription-polymer ase chain reaction in which glyceraldehyde-3-phosphate dehydrogenase mRNA w as used as an internal standard. DPD activity and protein content in cultur es of both cell lines increased in proportion to cell density. (DPD activit ies ranged from undetectable to 84 pmol/min/mg protein in the HT-1080 cells and from undetectable to 335 pmol/min/mg protein in the MIAPaCa-2 cells). DPD mRYA levels, on the other hand, tended to decrease slightly during cell growth. DPD activity and protein content in HT-1080 tumor xenografts incre ased during growth in proportion to tumor weight (DPD activities ranged fro m 7 to 131 pmol/min/mg protein), but DPD mRNA levels did not correlate with tumor weight. DPD activity and protein content in MIAPaCa-2 tumor xenograf ts did not change much, and seemed to have already plateaued, since the tum ors were small (weighing about 30 mg). These findings suggest that DPD prot ein expression during tumor growth is controlled at the post-transcriptiona l level.