* Stage set for Hollande-Merkel showdown at EU summit
* Papademos clarifies euro remarks but concerns remain
* Dell revenue outlook weighs on sentiment
* Futures down: S&P 7.8 pts, Dow 88 pts, Nasdaq 15 pts
By Angela Moon
NEW YORK, May 22 (Reuters) - U.S. stock index futures fell on Wednesday as caution prevailed ahead of a meeting of European leaders and on mounting concerns over Greece's future in the euro zone.
A weaker-than-expected revenue forecast from computer maker Dell also weighed on investor sentiment.
Leaders of the 27 EU countries are to meet in Brussels later in the day for a summit expected to deal with Europe's economic woes but also ways to prevent debt-mired Greece from skidding into a chaotic bankruptcy. The leaders are to consider ways to generate growth and discuss the idea of regional bonds to be jointly underwritten by all euro zone member states. However, Germany has already reiterated its opposition to such a move.
Dell Inc shares fell 13 percent to $13.10 in premarket trade, a day after the company forecast disappointing second-quarter revenue as U.S. and European corporate tech spending weakens and consumer personal computer sales continue to shrink.
Fears that Greece may have to leave the euro grew after Dow Jones earlier quoted former prime minister Lucas Papademos as saying Greece had no choice but to stick with a painful austerity program or face a damaging exit from the euro zone. His clarification in a television interview later offered little respite.
The "ex-Greek PM was just stating the obvious to anyone paying attention over the past month since the election. He doesn't want Greece leave but it could happen and it would be ugly if it did," said Peter Boockvar, equity strategist at Miller Tabak + Co in New York.
"This said, the markets are on edge and sensitive to every possible out of control scenario coming out of Europe."
Facebook Inc will also be eyed, after sources said that while company officials traveled the country to talk up the company's $16 billion initial public offering, the social networking giant advised analysts for underwriters to reduce revenue and earnings forecasts. The stock was up 0.8 percent to $31.20 in premarket trade.
S&P 500 futures lost 7.8 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 lost 88 points, and Nasdaq 100 futures fell 15 points.
Ariba Inc will also be in the spotlight after top European software company SAP AG said it plans to buy Ariba in a deal valuing the business and commerce network company at $4.3 billion, its latest maneuver against Oracle in the fast-growing Internet-based computing market.
Wells Fargo & Co does not employ the same kind of hedging strategy that has triggered a trading loss of at least $2 billion at rival JPMorgan Chase & Co, the bank's chief risk officer said on Tuesday.
PetSmart Inc posted a better-than-expected quarterly profit and raised its full-year outlook on strong sales across all its product categories, sending its shares up 8 percent in after-market trade.
On the macro front, investors awaited home price index for March, due at 10:00 a.m ET (1400 GMT), as well as new home sales for April, also due at 10:00 a.m. ET.
U.S. stocks closed mostly flat on Tuesday after volatility late in the session, with weakness in materials and energy shares offsetting strength in financials.