By Tanya Agrawal(Reuters) - U.S. stock index futures were lower on Tuesday, following three straight days of losses on rising speculation that the Federal Reserve will raise rates as soon as September.* The blue-chip Dow index on Monday slipped into negative territory for 2015, while the S&P 500 and the Nasdaq touched their lowest levels in over a month.* Data on Monday showed U.S. small business confidence rose to a five-month high in May, with owners expecting a solid improvement in profits, which bodes well for the economy's prospects in the months ahead.* Investors will also keep an eye out for data on job openings and labor turnover in April. Job openings are expected to have increased from the previous month. The data is due at 10:00 a.m. ET.* The Fed has said it will raise rates only when it sees the economy improving. Economic indicators have been mixed, but the labor market has strengthened, with the May employment report coming in above expectations.* The sell-off in U.S. markets on Monday spread to the global market, with European shares following Asian stocks lower. Concerns about a lack of progress in talks between Greece and its creditors also continue to weigh.* Lululemon Athletica shares rose 3.2 percent to $63.49 in premarket trading after the Canadian yogawear retailer raised its full-year revenue and earnings forecast.* Sage jumped 13.1 percent to $85 after the drug developer's experimental injectable drug was found effective in treating postpartum depression.* Dow component Apple was down marginally at $127.65, a day after the launch of its streaming music service, Apple Music.Futures snapshot at 7:30 a.m. ET:* S&P 500 e-minis were down 1.75 points, or 0.08 percent, with 191,393 contracts traded.* Nasdaq 100 e-minis were down 9.75 points, or 0.22 percent, on volume of 32,452 contracts.* Dow e-minis were down 17 points, or 0.1 percent, with 33,726 contracts changing hands.(Reporting by Tanya Agrawal; Editing by Savio D'Souza)Join the conversation about this story »