US STOCKS-Wall St opens lower on Fed worry after 8-week run up

* Apple (NasdaqGS: AAPL - news) up after Topsy buy, UBS (Xetra: UB0BL6 - news) upgrade

* Cyber Monday sales appear strong

* Yum Brands (Munich: TGR.MU - news) ' November China sales flat

* Indexes down: Dow 0.42 pct, S&P 0.31 pct, Nasdaq 0.19 pct

By Chuck Mikolajczak

NEW YORK, Dec 3 (Reuters) - U.S. stocks opened lower on Tuesday, putting the S&P 500 on track for a third consecutive decline, on concerns the Federal Reserve may begin to trim its stimulus earlier than some had anticipated.

Stronger-than-expected data on manufacturing and construction spending on Monday prompted speculation the Fed may lean towards scaling back its stimulus of $85 billion in monthly bond purchases, helping to send major indexes lower.

While virtually all market participants accept the central bank will begin to trim the stimulus at some point, the timing remains in question, with many analysts expecting the announcement in March.

"It's not a question of if, it's a question of when and you always have the question of how much of it is real and how much of it is emotional in nature?" said Gordon Charlop, a managing director at Rosenblatt Securities in New York.

After an eight-week run of gains for the S&P 500 that saw the benchmark index hit a series of record highs, market participants may also be engaged in some minor profit-taking.

"The near term trend seems to be to the downside, but that being said I am not going to sit here and say we are sensing the beginning of some sort of three- to five-percent correction or something more significant than that," Charlop said.

The Fed has maintained it would begin to slow the program when certain economic measures meet its targets. The economic calendar is packed this week with data that may provide some insight, culminating with Friday's November payrolls report.

The Dow Jones industrial average fell 67.17 points or 0.42 percent, to 15,941.6, the S&P 500 lost 5.54 points or 0.31 percent, to 1,795.36 and the Nasdaq Composite dropped 7.881 points or 0.19 percent, to 4,037.379.

After a disappointing to start to the holiday shopping season, U.S. online sales are expected to hit $2 billion on "Cyber Monday," the highest since the data firm comScore began tracking such information. Amazon.com Inc shares slipped 1.2 percent to $387.78 while the S&P retail index shed 0.5 percent.

Apple Inc has acquired social media search and analytics startup Topsy, an unusual purchase for a hardware-focused company that has made few forays into social networking. UBS upgraded the iPhone maker to a "buy" rating. Apple shares were up 1.8 percent to $560.90 as the biggest boost to both the S&P 500 and Nasdaq 100 indexes.

Yum Brands Inc lost 3.2 percent to $75.20, the worst performer on the S&P 500, after it said November sales at established KFC restaurants in China, its top market, failed to grow despite a successful half-priced chicken promotion, and it forecast a return to earnings per share growth in 2014.

OncoMed Pharmaceuticals Inc (NasdaqGS: OMED - news) surged 77 percent to $24.76 after the company said Celgene Corp (Munich: CG3.MU - news) would develop and market six of its anti-cancer stem cell experimental drugs for an upfront payment of $155 million. Celgene shares dipped 0.8 percent to $161.82.