Studies on cellular vitamin A metabolism and effects of UV irridation in human keratinocytes and melanocytes

• Reinoids (vitamin A and its derivatives) are modulators of proliferation and differentiation in a variety of cell types, including epidermal cells. Both retinol (ROH) and its metabolite 3,4-didehydroretinol (ddROH) can be converted to the acid form retinoic acid (RA) and 3,4-didehydroretinoic acid (ddRA), ligands for the nuclear receptors inducing gene transcriptions. Using high-performance liquid chromatography we have investigated the endogenous ROH and ddROH concentrations and the metabolism of [3H]ROH in cultures of keratinocytes, melanocytes, Hela cells, melanoma cell and two cell lines of non human origin (CV-1 and F9). Cellular retinoid-binding proteins (CRABP I, CRABP II) were also determined by radioligand binding technique and RT-PCR.Keratinocytes and melanocytes contained high concentrations of ROH and ddROH, Hela- and melanoma cells contained intermediate amounts of retinoids, while in the cell of non human origin only ROH was detected. Analysis of CRABP Il showed a correlation to the cells' ability to accumulate ddROH, suggesting a role for this protein in the 3,4-didehydro metabolic pathway. In melanocytes CRABP I was highly expressed and in melanoma cells CRABP Il dominated. CV-1 and F9 contained low levels of CRABP Il.Incubation with [3H]ROH for 1-24 hours resulted in a rapid appearance of [3H]ddROH in keratinocytes and toa lesser extent in Hela cells,  melanocytes and melanoma cells. At the end of the incubation the amount of [3H]ddROH corresponded to 30% of the total cellular radioactivity in the Hela cells, 10% in the keratinocytes, 4% in melanocytes and only 1 % in the melanoma cells. The cell types of non human origin did not produce [3H]ddROH from [3H]ROH. These findings support the concept that ddROH production is cell and tissue specific.Another purpose of this study was to examine the influence of ultraviolet radiation (UVR) on cellular metabolism of retinoids. A physiological dose of UVR reduced the concentration of ROH, ddROH and [3H]RA in cell cultures by 20-50%. The concentration retumed to control levels in about 1-2 days. The uptake of [3H]ROH which was almost 3-fold higher in melanocytes than in keratinocytes, did not much differ between irradiated and control cells. Also, the formation of [3H]ddROH from [3H]ROH was unaffected by UVR. However, the accumulation of the metabolite all-trans [3H]RA was 60% higher in irradiated than unirradiated keratinocytes and melanocytes. Our data indicate that restoration of cellular vitamin A after UVR is mainly accomplished by a retarded metabolism of RA to less active compounds and indicate that the homeostatic control of RA production isa priority after UVR. Failure of this control may be an important factor in UV-induced carcinogenesis in vivo.

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