Priestley, who played Doherty's twin brother on the hit '90s series, "Beverly Hills, 90210," told People magazine Wednesday that he reached out to her after hearing the heartbreaking news.
"Shannen was a big part of my life," he said. "[Her] and I will always have a bond that is very meaningful. I'm very heartbroken about the news and I wish her and her family the best in this very difficult time."
After keeping her battle with cancer a secret in order to keep her life as normal as possible while working in Hollywood, Doherty announced earlier this week she was fighting stage 4 breast cancer, five years after her initial diagnosis.
"I don't think I've processed it," she told ABC News' Amy Robach in an interview that aired Tuesday on "Good Morning America." "It's a bitter pill to swallow in a lot of ways."
Priestley is one of many of Doherty's costars and friends in the past week who have expressed their support.
"90210" costar Tori Spelling reached out to Doherty in an Instagram comment, "Sending lots of love, support and animal vibes to make you smile xoxo."
Actress Sarah Michelle Gellar, one of Doherty's closest friends, shared a photo of them on Instagram Tuesday and wrote, "Life is tough … but you are tougher."
Actress Selma Blair also commented on one of Doherty's Instagram posts, "You are lighting up the path. You are loved."