WASHINGTON — A U.S. appeals court on Tuesday threw out a federal regulation that implements key subsidies that are vital to President Barack Obama’s health care overhaul.The U.S. Court of Appeals for the District of Columbia Circuit accepted one of the main legal challenges to the policy by conservatives opposed to an expansion of the federal government.A three-judge panel returned the case to a lower court with instructions to rule in favor of appellants who had sued to overturn the rule.The ruling has the potential to cripple the implementation of the law by making health insurance unaffordable for many people.