Expression of Individual Metabolic Enzymes

Owing to the overlapped substrate selectivity of transporters expressed in Caco-2 cells, a particular transporter can be knocked out by using RNAi technology to improve assay selectivity [14]. 

Using putative MDR1 substrates, MDCK cells express substantially higher levels of native efflux transporters than those of LLC-PKi cells. Everything else being equal, the LLC-PKi model is preferred because it forms high-quality monolayers and has low levels of native transporters. 

Catalytic activity of CYP enzymes requires functional coupling with its redox partners, cytochrome P450 NADPH oxidoreductase and cytochrome b5. Measurable levels of these two proteins are natively expressed in most mammalian cell lines. Therefore, introduction of only the CYP cDNA is generally needed for detectable catalytic activity. However, the levels of expression of the redox partner proteins may be low and may not support maximal CYP catalytic activity therefore, enhancement of OR levels may be desirable. This approach has been used successfully with an Adenovirus expression system in LLC-PKi cells [20]. 

It has been reported that the CYP3A4 activity in CYP3A4-transfected Caco-2 cells can be stabilized and improved by adding 5-azacytidine in the culture medium. Cell monolayer normal growth was not affected by the treatment [22]. 

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Figure 5.6. A diagrammatic summary of adjusted hypoxia response systems (the AHRS) proposed as the ancestral physiological phenotype and as a phylogenetic adaptation to hypobaric hypoxia. Summary based largely upon studies of Quechuas and Sherpas. Essentially all of the characteristics summarized here are also expressed in individuals well adapted for endurance performance. In the latter, the main modification involves an upwards regulation of mitochondrial volume densities at the working tissues (altered expression of mitochondrial metabolic enzymes and metabolite transporters above), which is why this is referred to as a high-capacity version of the lower capacity high-altitude phenotype. See text for further details. (Modified from Hochachka et al., 1999.)...

Genetic polymorphisms of xenobiotic-metabolizing enzymes may result in expression of inactive enzymes or enzymes with a reduced or increased metabolic activity (Daly, 1995). For example, the incidence of hydralazine- and procainamide-induced lupus is higher or the disease starts earlier in individuals with the slow acetylator phenotype caused by mutant NAT-2 alleles than in individuals exhibiting the fast-acetylator phenotype (Woosley et al., 1978 von Schmiedeberg et al., 1999). 

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