General Growth Properties Inc., the second-largest U.S. shopping-mall owner, rejected investor Bill Ackman’s call to put itself up for sale and said it plans to remain independent.

“The board has unanimously determined that the best value for all shareholders will be achieved by GGP continuing to execute on its well-conceived business plan,” Sandeep Mathrani, General Growth’s chief executive officer, said in a letter to Ackman, founder of Pershing Square Capital Management LP. The letter was filed with the U.S. Securities and Exchange Commission.