Fig. 6.

Vitamin D lowers and increases brain CTFβ and sAβPPα, respectively. A) Vitamin D caused a reduction in brain CTFβ. CTFβ is a proteolytic product of β-secretase cleavage of AβPP and thus these data are also consistent with vitamin D acting through the inhibition of β-secretase activity. B) In contrast, vitamin D mediated an increase in brain sAβPPα levels. These data are consistent with vitamin D mediating the inhibition of β-secretase activity because sAβPPα is derived from AβPPα-secretase cleavage, which competes with β-secretase cleavage of AβPP, and thus the inhibition of β-secretase activity provides more AβPP for β-secretase to produce more sAβPPα (*p < 0.001; ** p > 0.01).