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.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.