Is it possible to have zero or negative attack damage?

Like, if say a level 1 Karthus (45 AD) is the victim of a maxed Mocking Shout from an enemy Tryndamere (-80 AD), his AD would theoretically go below zero. Does this stop at 0, or 1? Also, if the enemy Trundle uses a maxed Rabid Bite (-20 AD) on this level 1 Karthus while under the effects of Mocking Shout, what would happen? Would it have no effect due to Karthus' AD being the minimum possible value? I think it'd be rather interesting to see a Karthus doing -55 damage and healing his target per hit for more than he would have hit before...