UnitedHealth Group surged as much as 13% in U.S. pre-market trading Friday after Warren Buffett’s Berkshire Hathaway revealed a newly established stake in the country’s largest health insurer in its quarterly 13F regulatory filing. The purchase, whose size was not immediately disclosed, helped lift Dow Jones Industrial Average futures while the broader S&P 500 futures were little changed. Berkshire’s filing also showed an addition to steelmaker Nucor, sending that stock up about 6%. On the downside, semiconductor-equipment maker Applied Materials slid roughly 14% after issuing weaker-than-expected guidance and pointing to softer demand amid heightened U.S.–China trade tensions. Intel added about 2.5% on separate reports that Washington is discussing taking a stake in the chipmaker. The moves set the tone for a mixed open as investors awaited U.S. retail sales and consumer-sentiment data.