De Niro and Heche appeared in the 1997 political satire 'Wag the Dog.'
In a statement to ET, the two-time Oscar-winning actor said, "I’m very sad to hear of the tragic passing of Anne Heche. She was a wonderful actress and I enjoyed working with her tremendously in 'Wag the Dog.' Sad!Sad!Sad!"
De Niro, 78, and Heche appeared in the 1997 political satire, in which De Niro starred as the calculating spin master Conrad Brean and Heche as Winifred Ames, an aide to the president who hilariously tried to divert the media's attention from a brewing White House scandal. Dustin Hoffman, Denis Leary, William H. Macy and Woody Harrelson also appear.
While the actress' heart is still beating, she is brain dead, which is the definition of death according to California law. Her rep told ET in a statement that Heche has not been taken off life support in order to allow OneLegacy Foundation enough time to find organ recipients who will be a match.
"Today we lost a bright light, a kind and most joyful soul, a loving mother, and a loyal friend," the statement reads in part. "Anne will be deeply missed but she lives on through her beautiful sons, her iconic body of work, and her passionate advocacy. Her bravery for always standing in her truth, spreading her message of love and acceptance, will continue to have a lasting impact."
Ed Helms, James Gunn and Rosanna Arquette were just some of those in Hollywood who also paid tribute to the actress following the tragic news.