NEW YORK (MarketWatch) — Major U.S. stock indexes ended lower Monday after extending losses ahead of the closing bell. The Dow Jones Industrial Average (DJI JIA) led the way down, dropping 77.70 points, or 0.5% to 16,008.71, with blue-chip 3M Co. (NYSE:MMM) weighing heavily as it declined 4.4% after shares were downgraded to underweight by Morgan Stanley. The S&P 500 index (SNC:SPX) dropped 4.91 points, or 0.3%, to 1,800.90, while the Nasdaq Composite (NASDAQ:COMP) lost 14.63 points, or 0.4%, to 4,045.26.