* U.S. stocks sank on Wednesday, pushing the S&P 500 to close below 1,900 for the first time since September, on worries about weak energy prices, corporate earnings and the global economy. * JPMorgan shares were up 1.6 percent premarket at $58.25 after the bank’s fourth-quarter profit topped estimates as it kept a tight lid on expenses. * Profit at S&P 500 companies are expected to fall 4.7 percent in the quarter, according to Thomson Reuters data.