The active metabolite of vitamin D3, 1,25 dihydroxyvitamin D3 [1,25(OH)2D], is produced
by normal human keratinocytes (NKC) and regulates their differentiation. Squamous
carcinoma cell (SCC) lines lack the ability to differentiate in vitro, which might
involve defective 1,25(OH)2D synthesis or response. To address this possibility we
obtained four SCC lines (12F2, 12B2, 25, and A431) and first determined whether they
could produce 1,25(OH)2D from its substrate 25 hydroxyvitamin D3 (250HD). All could
(12F2 greater than NKC greater than 25 greater than 12B2 greater than A431). Furthermore,
exogenously added 1,25(OH)2D inhibited 1,25(OH)2D production and stimulated 24,25
dihydroxyvitamin D3 [24,25(OH)2D] production in all cell lines but with different
potency (25 = A431 greater than NKC greater than 12B2 greater than 12F2). Cellular
binding studies suggested that the high-affinity binding site for 1,25(OH)2D in NKC
is not found in 12F2 and 12B2. When the effect of 1,25(OH)2D on differentiation was
determined, only NKC responded with an increase in cornified envelope formation, although
some of the cell lines responded to the proliferative [at low 1,25(OH)2D concentration]
or antiproliferative [at high 1,25(OH)2D concentration] effect of 1,25(OH)2D. Thus,
although SCC lines synthesize 1,25(OH)2D and respond to exogenous 1,25(OH)2D with
respect to appropriate regulation of endogenous 250HD metabolism, these cell lines
fail to respond to the differentiating influence of this vitamin D metabolite.