Blue Cross Blue Shield of Michigan on Monday said it has asked a Detroit federal judge to dismiss an antitrust lawsuit by insurance giant Aetna Inc. Hartford, Conn.-based Aetna filed the lawsuit last month against the Blues, claiming the state's largest health insurer has prevented Aetna from expanding in Michigan. The lawsuit is a piggyback on the fall 2010 lawsuit by the U.S. Department of Justice and former Michigan Attorney General Mike Cox that accuses the Blues of stifling competition and driving up rates for consumers through the use of certain requirements in hospital contracts that are called most-favored nation clauses.