** Health insurer UnitedHealth's UNH.N shares rise 3% to $313.30 premarket

** UNH jumped 12% on Friday, biggest single-day jump in 5+ years after Berkshire Hathaway BRKa.N acquired 5 mln share stake (0.55%)

** Triggered retail buying of $43 mln shares, most in 3 months, per Vanda

** Boosted Dow Jones Industrial Average index .DJI briefly to hit record high

** As of Aug 14, short interest at 17.95 mln shares ($4.9 bln) near 3-month high but below April's $6.9 bln YTD peak - S3 Partners

** 19 of 28 analysts rate UNH "buy" or higher; median PT $321 - data compiled by LSEG

** UNH warned in July of billions in additional costs ahead, remains biggest Dow .DJI laggard YTD

** Stock down 40% YTD

(Reporting by Nikhil Sharma)

((Nikhil.Sharma@thomsonreuters.com))