Futures down as Greece weighs; GDP data awaited

* The Commerce Department is expected to report U.S. economy likely shrank in the first quarter instead of growing, as was estimated last month. * While several economists have cast doubts on the accuracy of GDP estimates for the quarter, investors will peruse the data for clues to narrow down the timing of an increase in interest rates. * The GDP report is also expected to show a second quarterly drop in corporate profits because of the strong dollar and oil prices.