Nothing on earth other than humans with guns would threaten a fully grown Grizzly bear.
Don't know about grizzlies specifically, but this Quora thread has some interesting references about lions vs lots of stuff. I was surprised to see that lions almost always get the better of Tigers (which I had thought were bigger), but also that they have the beating of eurasian bears, which they find easier to deal with than tigers.
https://www.quora.com/What-animal-d...-ancient-Rome-all-the-way-through-modern-Rome