US STOCKS-Wall St lower in choppy trading as Greece summit looms
* Euro zone summit on Greece proposal to start at 1600 GMT
* U.S. May trade deficit widens less than expected
* AMD slumps after cutting quarterly revenue forecast
* Shake Shack falls after Morgan Stanley rating cut
* Indexes down: Dow 0.24 pct, S&P 0.22 pct, Nasdaq 0.45 pct (Updates to open)
By Tanya Agrawal
July 7 (Reuters) - U.S. stocks opened higher before quickly reversing course on Tuesday morning, setting the stage for another volatile session as investors await the outcome of a euro zone summit that will attempt to find a solution to the ongoing debt crisis in Greece.
The summit begins at 12 p.m. ET (1600 GMT) in Brussels and Greek Prime Minister Alexis Tsipras is under pressure to negotiate a deal as the country is running out of cash and the European Central Bank has tightened funding.
Greece's next debt deadline is July 20 when it has to repay $3.9 billion of bonds to the ECB. Finnish finance minister Stubb said the currency bloc was not looking at bridge finance for Greece "at this stage". Continuación...