A Lowell man in his mid-30s pleaded not guilty Thursday to charges that he tried to extort a Boston celebrity last year and illegally possessed a gun. Felix Paulino entered his plea in federal court in Boston, according to his case docket. Paulino allegedly contacted the unnamed celebrity’s agent in the fall of 2012 and threatened that unless he received a $20,000 payment, he would release a video clip of the married star trying to lure women into a private room at a nightclub. Paulino is also accused of being a felon in possession of a firearm.Travis Andersen can be reached at email@example.com. Follow him on Twitter @TAGlobe.