Complex sentence: His exposure to mixed martial arts (MMA) was heightened when he went back to the United States from 2000 to 2003, while making his Hollywood debut, he also took time off to learn the various martial arts forms.
Split into simpler sentences: His exposure to mixed martial arts (MMA) was heightened when he went back to the United States from 200 to 2003. While making his Hollywood debut, he also took time off to learn the various martial arts forms.