Billionaire industrialist, donor, activist and conservative icon David Koch, 78, will step down from his roles at Koch Industries and other Koch-affiliated organizations due to health reasons, according to an internal memo obtained by CNBC Tuesday.

He will also be leaving his roles as executive positions at Koch Chemical Technology group as well as his role as chairman of the board for the Americans for Prosperity Foundation - the charity associated with the Koch brothers' primary political organization. He will become director emeritus at Koch industries.

David's brother Charles Koch broke the news Koch Industries employees in the memo, saying that his brother's health had continued to deteriorate and that he is "deeply saddened" by his brother's retirement.

"David has always been a fighter and is dealing with this challenge in the same way," reads the memo. "His guidance and loyalty, especially in our most troubled times, has been unwavering."

David Koch also received well wishes from a top executive within their political sphere:

"Due to health reasons, David Koch will be resigning from the board of Americans for Prosperity Foundation," Mark Holden, co-chairman of the Koch-backed Seminar Network, said. "We greatly appreciate his vital role on the board and all that he has done to help us build a strong foundation for our future success. We wish him and his family well." Koch was diagnosed with prostate cancer more than two decades ago. Over the years, he and his family foundation, along with other recipients of Koch philanthropic support, have donated millions to battling cancer and several other causes. He is a board member of the Prostate Cancer Foundation. -CNBC

Koch was diagnosed with prostate cancer over two decades ago, while he and his family foundation have donated millions to battling cancer and other causes. David is a board member of the Prostate Cancer Foundation.

The announcement comes a day after the Koch-backed political network unveiled a multimillion-dollar, multi-year campaign against President Donald Trump's tariffs. The campaign will include media buys, activist education, grass-roots mobilization, lobbying and policy analysis. The network has also recently pivoted from focusing their efforts on supporting candidates to legislative initiatives, including protecting recipients of the Deferred Action Childhood Arrivals program, also known as DACA, and cutting back on government spending. -CNBC

The Koch political network has raised eyebrows of late for reaching out to Democrats on various issues. Last week, American for Prosperity took out a digital ad thanking Sen. Heidi Heitkamp (D-ND) for co-sponsoring a bill to deregulate mid-sized banks.

Read Charles Koch's memo to employees here: