These results demonstrate for the first time that human keratinocytes under in vivo-like conditions have the capacity of the enzymatic hydroxylation of vitamin D3 to hormonally active calcitriol (1alpha,25(OH)2D3). Supplementation of the culture medium with bovine serum albumin (BSA) up to 1.5%) (w/v) amplifies the conversion of vitamin D3 to 1alpha,25(OH)2D3. The maximum turnover rate of this reaction at 780 nM vitamin D3 in presence of 1.0% (w/v) BSA amounts to approximately 3 pmol 1alpha,25(OH)2D3 per 10(6) cells after 6 h of incubation. The hydroxylation of vitamin D3 to 1alpha,25(OH)2D3 is inhibited by the P-450 oxidase inhibitor ketoconazole. The generation of 1alpha,25(OH)2D3 from vitamin D3 has an apparent Michaelis constant (Km) of 2.3x10(-6) M. The intrinsic conversion of vitamin D3 to biologically active 1alpha,25(OH)2D3 may be of importance for the regulation of proliferation and differentiation of keratinocytes.

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Heat stress (HS) and Zearalenone (ZEN) exposure affect growth, production efficiency, and animal welfare; and, under extreme situations, both can be lethal. Given that both HS and ZEN independently cause oxidative stress, we hypothesized that simultaneous exposure to HS and ZEN would cause greater oxidative stress in porcine skeletal muscle than either condition, alone. To address this hypothesis, crossbred, prepubertal gilts were treated with either vehicle control (cookie dough) or ZEN (40 μg/kg) and exposed to either thermoneutral (TN; 21.0 °C) or 12-h diurnal HS conditions (night: 32.2 °C; day: 35.0 °C) for 7 d. Pigs were euthanized immediately following the environmental challenge and the glycolytic (STW) and oxidative (STR) portions of the semitendinosus muscle were collected for analysis. In STR, malondialdehyde (MDA) concentration, a marker of oxidative stress, tended to increase following ZEN exposure (P = 0.08). HS increased CAT (P = 0.019) and SOD1 (P = 0.049) protein abundance, while ZEN decreased GPX1 protein abundance (P = 0.064) and activity (P = 0.036). In STR, HS did not alter protein expression of HSP27, HSP70, or HSP90. Conversely, in STW, MDA-modified proteins remained similar between all groups. Consistent with STR, ZEN decreased GPX1 (P = 0.046) protein abundance in STW. In STW, ZEN decreased protein abundance of HSP27 (P = 0.032) and pHSP27 (P = 0.0068), while HS increased protein expression of HSP70 (P = 0.04) and HSP90 (P = 0.041). These data suggest a muscle fiber type-specific response to HS or ZEN exposure, potentially rendering STR more susceptible to HS- and/or ZEN-induced oxidative stress, however, the combination of HS and ZEN did not augment oxidative stress.