Borthwick's new deal was confirmed earlier on Wednesday, and it has now been revealed that Sarries have secured the future of two more key players.
Scotland international Brown, 29, joined from Glasgow in 2010 while De Kock has spent the last five years at the club.
The South African said: "I am preparing to start my sixth season at the club, and I can honestly say I've never been happier.
"Last season was fantastic, but everybody understands we will have to work twice as hard to retain the Premiership title, everybody is completely committed and everybody is moving in the same direction."
Sarries also revealed the signing of 19-year-old scrum-half Ben Spencer from Cambridge after the youngster impressed during pre-season.
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