WASHINGTON (AP) — No joke: David Letterman yukked it up on late-night TV longer than anyone else. Now his career of comedy has earned him a prestigious award and a celebrity roasting. Letterman was to receive the Mark Twain Prize for American Humor during a star-studded celebration Sunday night at Washington's Kennedy Center. Expected to attend were previous recipients Steve Martin and Bill Murray, late-night host Jimmy Kimmel, musician Eddie Vedder of Pearl Jam and Al Franken, the comedian-turned-senator. The 70-year-old Letterman spent 33 years on late-night TV, hosting long-running shows on NBC and then on CBS.