underline vs accentuate

underline is a noun but accentuate is not a noun.

Word	Noun	Adjective	Verb	Adverb
underline	Yes	No	Yes	No
accentuate	No	No	Yes	No

As verbs, accentuate is a hypernym of underline; that is, accentuate is a word with a broader meaning than underline:

Other hypernyms of underline include accent, emphasise, emphasize, punctuate, stress.

underline (noun)	accentuate (noun)
a line drawn underneath (especially under written matter)

underline (verb)	accentuate (verb)
give extra weight to (a communication)	to stress, single out as important
draw a line or lines underneath to call attention to	put stress on; utter with an accent