Ja MacLaughlin

Aging decreases the capacity of human skin to produce vitamin D3.

An evaluation of surgically obtained skin (age range, 8-92 yr) revealed that there is an age-dependent decrease in the epidermal concentrations of provitamin D3 (7-dehydrocholesterol). To ascertain that aging indeed decreased the capacity of human skin to produce vitamin D3, some of the skin samples were exposed to ultraviolet radiation and the content of previtamin D3 was determined in the epidermis and dermis. The epidermis in the young and older subjects was the major site for the formation of previtamin D3, accounting for greater than 80% of the total previtamin D3 that was produced in the skin. A comparison of the amount of previtamin D3 produced in the skin from the 8- and 18-yr-old subjects with the amount produced in the skin from the 77- and 82-yr-old subjects revealed that aging can decrease by greater than twofold the capacity of the skin to produce previtamin D3. Recognition of this difference may be extremely important for the elderly, who infrequently expose a small area of skin to sunlight and who depend on this exposure for their vitamin D nutritional needs.

1,25(OH)2D3 increases calcium and phosphatidylinositol metabolism in differentiating cultured human keratinocytes.

The effect of 1,25(OH)(2)D(3) on the intracellular calcium, (Ca(+2))i, in both cultured human keratinocytes and in cultured human dermal fibroblasts was investigated. When the intracellular calcium (Ca(+2))i in cultured human keratinocytes, grown in a serum-free medium containing 1.8 mM calcium, was measured by the fluorescent calcium-indicator, Furu-2, the (Ca(+2)i increased 154%, 202%, and 409% over the control value after incubation with 1,25(OH)(2)D(3) at 10(-10) m, 10(-8) m, and 10(-6) m, respectively. This response was immediate (15 seconds), specific (no effect with either 25(OH)D(3) at 10(-8) m or vitamin D(3) at 10(-8) m), and occurred with or without EGTA in the medium. In contrast, 1,25(OH)(2)D(3) did not increase the (Ca(2+))i in either cultured human keratinocytes that were grown in low calcium (0.05 mm), serum-free medium or in cultured human dermal fibroblasts that were grown in medium containing 0.05 mm calcium and 1% serum. The effect of 1,25(OH)(2)D(3) on the the turnover of phosphatidylinositol was investigated as a possible cause for the observed increase in (Ca(+2)i. Cultured human keratinocytes that were incubated with (3)H-inositol demonstrated a 50 % +/- 10% increase in the triphosphated, plasma membrane-bound metabolite of phosphatidylinositol, PIP(2), by 15 seconds, followed by a rapid decrease at 30 seconds, then a return toward basal levels by 1 minute. Lysophosphatidylinositol, which results from the sn-2 deacylation of phosphatidylinositol by phospholipase A(2), decreased 20% +/- 8% within 30 seconds, then increased to 200% +/- 10% of the control value by 5 minutes. The accumulation of IP(3) was increased 50% to 100% above the control value within 30 seconds and this increase was substained during the 5-minute incubation period. Stimulation of phosphatidylinositol turnover by 1,25(OH)(2)D(3) was not detected in either cultured human keratinocytes that were grown in serum-free, low calcium medium or in cultured human dermal fibroblasts that were grown in 1% serum.

The photoproduction of 1 alpha ,25-dihydroxyvitamin D3 in skin. An approach to the therapy of vitamin-D-resistant syndromes.

Cutaneous 7-dehydrocholesterol, exposed to ultraviolet radiation, converts to previtamin D3, which in turn converts in skin to vitamin D3 and is carried into the circulation. We investigated the feasibility of the photochemical conversion in skin of hydroxylated derivatives of 7-dehydrocholesterol - such as 1 alpha, 25-dihydroxy-7-dehydrocholesterol (1 alpha, 25-(OH)2-7-DHC) - to the corresponding hydroxylated previtamin as an alternative method of delivery of 1 alpha, 25-dihydroxyvitamin D3 (1 alpha, 25-(OH)2-D3) to subjects who are deficient in the endogenous metabolite. In human volunteers and in vitamin-D-deficient rats [24-3H] 1 alpha, 25-(OH)2-D3 appeared in blood after [24-3H] 1 alpha, 25-(OH)2-7-DHC was applied to the skin and exposed to ultraviolet radiation. In anephric rats, intestinal calcium absorption and serum calcium levels were elevated after a topical dose of 1 alpha, 25-(OH)2-7-DHC combined with ultraviolet phototherapy. Delivery of equivalent doses of 1 alpha, 25-(OH)2-D3 through the skin and orally showed that there was more prolonged stimulation in intestinal calcium absorption and serum calcium elevation after cutaneous administration. The photochemical conversion of precursors may be useful in the treatment of patients with impaired vitamin-D metabolism.

Cultured human keratinocytes cannot metabolize vitamin D3 to 25-hydroxyvitamin D3

The metabolism of [3H] vitamin D3 was studied in cultured human keratinocytes (CHK). Intact CHK were incubated for 1, 6, 12, 24 and 48 h with [3H] vitamin D3 and the lipid soluble fractions from the media and cells were extracted by high‐performance liquid chromatography (HPLC). Vitamin D3 and its metabolites, 25‐OH‐D3, 24,25(OH)2D3 were added to the extracts, as markers, prior to HPLC, HPLC analysis of the lipid extracts did not reveal any monohydroxylated metabolites. CHK incubated for one hour with [3H]25‐OH‐D3 showed a 10±4% conversion to [3H]1,25,(OH)2D3 whereas no conversion to [3H]1,25(OH)2D3 was observed in control CHKs that were boiled prior to incubation with [3H]25‐OH‐D3. These findings suggest that cultured neonatal keratinocytes are incapable of metabolizing vitamin D3 to 25‐OH‐D3.

Studies of metabolism in embryonic development, IV. Protein synthesis in mitochondria and ribosomes of unfertilized and fertilized trout eggs

Abstract 1. 1. Parallel studies of protein synthesis in mitocbondria and ribosomes of fertilized and unfertilized eggs were made up to the tenth day of age. The levels and the rates of increase with age of mitochondrial protein synthesis in unfertilized and fertilized eggs were identical; the activity of the ribosomes increased more rapidly in fertilized than in unfertilized eggs. 2. 2. During early stages the protein synthetic capacity of the mitochondria of unfertilized and fertilized eggs greatly exceeded that of the ribosomes. Near hatching the mitochondria and the ribosomes made approximately equal contributions to the protein synthetic capacity of the embryonated egg.

An Effect of Sunscreens on Cutaneous Vitamin D3 Synthesis

Sunlight triggers a variety of photochemical reactions in the skin. Acute exposure to the wavelengths 290 to 315 nm (ultraviolet light B, UVB) causes erythema, whereas chronic exposure produces increased incidence of skin cancer. Solar irradiation also mediates the synthesis of vitamin D,. The adverse effects of UVB can be prevented by topical sunscreens such as para-aminobenzoic acid (PABA). The present study determined: 1) the effect of PABA on the production of vitamin D precursors in human skin specimens in vitro, 2) the effect of PABA on vitamin D synthesis in vivo, and 3) the long-term effect of chronic PABA use on vitamin D stores.