Effects of 1,25‐dihydroxyvitamin D3 and its 20‐epi analogues (MC 1288, MC 1301, KH 1060), on clonal keratinocyte growth: evidence for differentiation of keratinocyte stem cells and analysis of the modulatory effects of cytokines

Keratinocytes are functionally divided into stem cells, transit amplifying cells and terminally differentiated cells. In a hyperproliferative skin disease, psoriasis, increased mitotic activity of the stem cells is chiefly responsible for epidermal hyperplasia. The effects of 1,25dihydroxyvitamin D3 (1,25(OH)2D3) and potent vitamin D3 analogues (MC 1288: 20-epi-1,25(OH)2D3, MC 1301: 20-epi-24a-homo-26,27-dimethyl-1,25(OH)2D3, KH 1060: 20-epi-22-oxa-24a-homo-26,27-dimethyl-1,25(OH)2D3) on the stem cells were investigated. 2. Stem cells were identified retrospectively as those giving rise to large keratinocyte colonies in culture (holoclones). 1,25(OH)2D3 (10(-8)-10(-6) M) suppressed formation of holoclones by stimulating the progenitor cell differentiation into the phenotype expressing differentiation markers (keratins K1/K10 and involucrin). 3. 20-Epi vitamin D3 analogues were more potent than 1,25(OH)2D3 in inhibiting the clonal keratinocyte growth. This activity correlated with the ability to induce cell differentiation (KH 1060 > MC 1301 > MC 1288 > 1,25(OH)2D3). 4. Cytokines modulated the effects of 1,25(OH)2D3 on clonal growth. One of the following cytokines (epidermal growth factor, transforming growth factor alpha, interleukin-1 alpha, interleukin-1 beta, interleukin-6, interleukin-8) was required for 1,25(OH)2D3 to suppress clonal growth and induce cell differentiation. In contrast, keratinocyte growth factor and insulin-like growth factor I attenuated the effects of 1,25(OH)2D3. 5. In conclusion, 1,25(OH)2D3 and 20-epi vitamin D3 analogues suppress clonal growth by directly inducing the differentiation of progenitor cells. It is conceivable that stimulation of stem cells differentiation is a major mechanism of action of vitamin D3 compounds in psoriasis. Balance between different types of cytokines in psoriatic epidermis may be an important factor determining the clinical effect of vitamin D-based therapy.