U.S. stock index futures were lower on Friday as investors braced for key monthly jobs data for clues on the direction of the economy and the sustainability of a recent run-up.

* The government's all-important non-farm payrolls report, to be released at 8:30 a.m. EDT (1230 GMT), will show the number of jobs lost in July.

* Economists surveyed by Reuters forecast the Labor Department report will show 320,000 workers lost their jobs in July, the least for any month since September last year.

* Wall Street's second day of declines Thursday came on the heels of a four-day rally that pushed U.S. indexes to close at their highest levels since last fall. Investors trimmed long positions in risky assets, including stocks before the non-farm payrolls report.

* Despite the pullback, the broad S&P 500 is up 47.4 percent from its 12-year closing low in early March, driven by a string of economic data suggesting a recovery and an earnings season with most S&P 500 companies beating expectations.

* S&P 500 index futures were down 2.50 points, and 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 fell 22 points, while Nasdaq 100 futures shed 2.75 points.

* Energy-related stocks looked set to weigh on the market as U.S. front-month crude oil prices retreated about 1 percent.

* In good news for the struggling auto sector, the U.S. Senate approved a $2 billion extension of the cash for clunkers incentive program late Thursday and sent the measure to the White House.

(Reporting by Angela Moon, Editing by Jeffrey Benkoe)