Research Interests:Vitamins D and A are now recognized to be precursors to the hormones, calcitriol (1,25-dihydroxyvitamin D3) and retinoic acid, respectively. Both hormones, acting through specific transcription factors are potent cell-differentiating agents and in addition, calcitriol regulates the plasma level of calcium. My laboratory utilizes unique in vitro cultured cell or transfected cell models to study the activation or breakdown of calcitriol or retinoic acid. Among the biological actions of calcitriol and retinoic acid is the ability to induce their own catabolic enzymes. The long-term aim of our research is to establish the structure and function of the cytochrome P-450 enzymes involved in the complex metabolic pathways of calcitriol or retinoic acid and thus the role of these enzymes in the regulation of extracellular calcium homeostasis and cell differentiation at the whole organism level.
Recently, we have successfully elucidated the complex pattern of catabolism of calcitriol to calcitroic acid and retinoic acid to 4-hydroxy retinoic acid occurring inside target cells. Currently, utilising collaborations with partners from the pharmaceutical industry, we are applying this basic knowledge to the study of the metabolism of several new vitamin D analogs (see Figure 1), under development around the world for the treatment of osteoporosis, psoriasis and cancer. In the course of this work, we are using vitamin D analogs as probes in structure/activity studies to explore the specificity of the substrate-binding pocket of the cytochrome P-450 isoforms involved in vitamin D metabolism. We have cloned the cDNA's and genes for two of these cytochromes, and are presently using mammalian and bacterial expression systems containing the cDNA's encoding these P-450's in studies aimed at pinpointing the location of the vitamin D-binding, ferridoxin-binding and heme-binding domains. Molecular modelling (Figure 2) and site-directed mutagenesis studies of the cytochrome P-450's are also underway. It is expected that these studies will not only reveal useful basic information about the structure and mechanism of catalysis of cytochrome P-450's but will also lead to rational vitamin D and retinoid drug design in the future.