Calcipotriol (MC 903), a novel vitamin D3 analogue stimulates terminal differentiation and inhibits proliferation of cultured human keratinocytes

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Abstract
The hormonally active form of vitamin D3, 1,25-dihydroxy vitamin D3 [1,25-(OH)2-D3; calcitriol], regulates the differentiation and proliferation of epidermal keratinocytes in vitro. MC 903 (calcipotriol) is a novel vitamin D3 analogue which is at least 100 times less potent than 1,25-(OH)2-D3 in its effects on calcium homeostasis. The present study compared the effects of MC 903 and 1,25-(OH)2-D3 on terminal differentiation and proliferation of cultured normal human keratinocytes. Keratinocytes were grown in McCoy's 5A medium supplemented with penicillin (50 IU/ml), streptomycin (50 Μg/ml), l-serine (4×10−4 M), and 10% human type AB serum. MC 903, 1,25-(OH)2-D3 or 1α-OH-D3 (10−12 M−10−8 M) was added with each feeding when cultures became preconfluent. After incubation for 24 h with D3 vitamins, cultures were extracted for transglutaminase, and the enzyme activity was indexed against DNA content. The activity of transglutaminase, the enzyme reponsible for cross-linking the proteins of the cornified envelope, was maximally stimulated by 388% with MC 903 (10−8 M), by 328% with 1,25-(OH)2-D3 (10−8 M), and by 27% with 1α-OH-D3 (10−8 M) compared with vehicle. After incubation for 2 weeks the number of keratinocytes with cornified envelopes had increased by 288% with MC 903 (10−8 M), by 360% with 1,25-(OH)2-D3 (10−8 M), and by 149% with 1α-OH-D3 (10−8 M) compared with vehicle. Simultaneously the incorporation of (3H)thymidine into DNA was decreased by 64% with MC 903 (10−8 M), by 71% with 1,25-(OH)2-D3 (10−8 M), and by 10% with 1α-OH-D3 (10−8 M). There was a corresponding decrease in cell number. These results demonstrate that both MC 903 and 1,25-(OH)2-D3 are potent modulators of keratinocytes differentiation and proliferation in vitro. Because MC 903 is much less active than 1,25-(OH)2-D3 in causing hypercalcemia, this compound is a candidate for the treatment of skin diseases characterized by aberrant epidermal differentiation and proliferation.

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