SQUAMOUS CARCINOMA CELL-LINES PRODUCE 1,25 DIHYDROXYVITAMIN-D, BUT FAIL TO RESPOND TO ITS PRODIFFERENTIATING EFFECT

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 > NKC > 25 > 12B2 > 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 > NKC > 12B2 > 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.