When you framed it, how did you do it?  Did you 
do it professionally?  From what I've heard, if 
it's done professionally, it's "sealed" from air 
and is put behind a UV-protectant glass, making 
it less prone to light-fade.

To answer your original question, though, yes, 
direct light does cause fade, as does heat.  :(

Toei, at least from what I've seen of Dragonball 
and Sailor Moon cels, usually DO fade no matter 
what you do.  There ARE some that have only a 
teeeny tiny bit, but I know that yellow tones are 
the worst with their paint.

Hope this helps!
Kristina