DETROIT - General Motors Co. terminated three U.S. marketing executives as part of this week's management overhaul, Automotive News reported. Cadillac's Steve Shannon and John Howell were dismissed Monday, said eight sources familiar with the moves. Jay Spenchian, an executive director who worked on Cadillac and other brands, was also let go, the sources said. The departures show GM's upheaval ran deeper than the 18 changes cited in the company's press release Tuesday. One of the biggest moves shifted responsibility for U.S. sales to North American President Mark Reuss from Susan Docherty, who remains U.S. marketing chief. Reuss said he made the changes to flatten GM's North American organization, which he said was not moving fast enough to boost sales. Cadillac's 32 percent sales decline in the United States last year was the steepest of any surviving GM brand. Through February, Cadillac's 14 percent increase in 2010 sales lags the gains of Buick, GMC and Chevrolet. On Tuesday, GM also moved Bryan Nesbitt back to design after seven months as Cadillac's chief. Instead of one boss, each of GM's surviving U.S. brands -- Cadillac, Chevrolet and Buick-GMC -- now has one leader for sales and service and another for marketing. Ed Peper, Cadillac's general sales manager since July, will report to Kurt McNeil, named Tuesday as Cadillac's head of sales and service. “The titles are not fully determined for the new structure,” said GM spokesman Chris Preuss. “Ed will have a similar role that he had before.” Shannon, one of the three Cadillac managers dismissed Monday, was executive director of marketing. Chevrolet's general director of advertising, Kim Kosak, will replace Shannon but may have a different title, three of the sources said. She now will report to Don Butler, who picked up responsibility for Cadillac marketing Tuesday. He rejoins GM from Inrix, a telematics company. Shannon had worked for GM since 1982. In 2007, he was tapped to run Saab. In October 2008, Shannon became executive director of product and marketing for the "premium" Cadillac, Saab and Hummer lines. He was named Cadillac's executive director of marketing about two months later. Spenchian, 51, directed cross-brand marketing. Howell was Cadillac's product director. Shannon, Howell and Spenchian were part of the team that picked Publicis Groupe-backed BBH (Bartle Bogle Hegarty) in January as its new ad agency. The brand had been looking for a new shop since last fall. Incumbent agency Modernista opted not to defend the account.