* Bank shares tumble after JPMorgan reveals loss * Sector was already down on Europe, growth fears * Nvidia rallies in premarket after results * Investors look to producer price, sentiment data * Futures down: Dow 56 pts, S&P 7.1 pts, Nasdaq 8.75 pts By Ryan Vlastelica NEW YORK, May 11 (Reuters) - U.S. stock index futures fell on Friday and were on track to extend the week's losses after JPMorgan Chase & Co revealed a trading loss of at least $2 billion from a failed hedging strategy that weighed on bank shares. The news sent shares of the Dow component down 8.1 percent to $37.42 in premarket trading, and is the latest hurdle for a sector already besieged by the sovereign debt crisis in Europe and fears of slowing growth globally. While other gains partially offset the trading loss, JPMorgan Chase estimates the business unit with the portfolio will lose $800 million in the current quarter, excluding private equity results and litigation expenses. The bank had previously expected the unit to earn a profit of about $200 million. Jamie Dimon, the chief executive of the biggest U.S. bank by assets, cautioned that losses could grow by another $1 billion. "While this is an isolated incident that has nothing to do with how the economy will recover, it is very painful to hear," said Tim Speiss, head of personal wealth advisors at EisnerAmper in New York. "Investors need to look at what this might do to JPMorgan's equity value, and whether it is something that could trouble the whole sector." Bank of America Corp fell 3 percent to $7.47 before the bell while Citigroup Inc lost 3.8 percent to $29.50 and the Financial Select Sector SPDR was off 1.9 percent to $14.70. The S&P financial sector will likely extend its losses of almost 3 percent so far this month. Financial stocks have been among the most volatile in recent months as investors question what the growth outlook for the U.S. and the debt crisis of Europe will mean for the group's profits. JPMorgan has fallen 11.4 percent since the end of March. The CBOE VIX Volatility Index is up almost 10 percent this month in a sign of growing caution. S&P 500 futures fell 7.1 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures sank 56 points and Nasdaq 100 futures lost 8.75 points. For the week, the S&P is down 0.8 percent, the Dow is off 1.4 percent and the Nasdaq is down 0.8 percent. All three are on track for their second straight week of losses. Investors are looking ahead to the April Producer Price Index, due at 8:30 a.m. (1230 GMT), as well as the Thomson Reuters/University of Michigan's preliminary May consumer sentiment index. Economists in a Reuters survey expect a reading of 76.2 compared with 76.4 in the final April report, with producer prices flat. Software maker CA Inc late Thursday continued its run of estimate-beating profit, as demand rose at its North American business. Nvidia Corp shares rose 8.1 percent to $13.43 before the bell after reporting adjusted first-quarter earnings that beat expectations. With 449 of the S&P 500 companies reporting results through Thursday morning, 66.4 percent exceeded estimates, according to Thomson Reuters data, compared with more than 80 percent at the start of earnings season. The Dow rose modestly to break a six-day losing streak on Thursday, though a weak outlook from Cisco Systems Inc capped advances. The S&P 500 could not hold enough gains to close above its April low. Still, the index has rebounded after falling to a two-month low near 1,340 on Wednesday.