AceShowbiz -

Valerie Harper revealed on Wednesday, March 6 that she was diagnosed with terminal brain cancer in January. Her best friend, Mary Tyler Moore, whom with she starred together on "The Mary Tyler Moore Show", reacted to the sad news. "I'm absolutely devastated by this news," Moore told PEOPLE.

"Valerie has given so much joy, laughter and love to the world," Moore, who has been friends with Harper for four decades, added. "I join her fans and send much love and positive thoughts to her and her family during this difficult time."

One day before going public about the diagnosis, Harper called Moore. Harper recalled that after some small talk, "She said, 'What's up?' I said, 'It's incurable cancer.' She said, 'Val, I'm so sorry.' " Before ending their phone call, "Mary said, 'One day at a time,' " Harper continued. "That's how we've got to live our lives. That's the best we can do."

The Rhoda Morgenstern of "MTM Show" was diagnosed with leptomeningeal carcinomatosis, a rare and swiftly lethal condition that occurs when cancer cells spread into the fluid-filled membrane surrounding the brain. Doctors estimated she had about three months to live. "I don't think of dying. I think of being here now," the 73-year-old actress said.

Moore wasn't the only friend who sent words of support to Harper. Cloris Leachman, who is also an "MTM Show" alum, reached out to Harper on Twitter. "My longtime friendship w/ @ValerieHarper has been 1 of the gr8est gifts of my life. I'm here 4 any/everything she may need one day at a time," he tweeted.

Larry King also tweeted, "I've known Valerie Harper for years- enjoyed intv'ing her & being in her company-wishing her strength & good thoughts."