___Pinnacle Airlines files for bankruptcy protection NEW YORK (AP) â¿¿ Pinnacle Airlines Corp., a regional carrier that flies under contract for Delta, United and US Airways, has filed for bankruptcy protection to deal with its mounting debt. The Memphis, Tenn.-based airline said late Sunday that its current business model isn't sustainable. It had tried for months to combine its operating subsidiaries to save money and recover from lost business as major airlines cutback on flying in response to higher fuel costs. In its filing Sunday, it lists $1.42 billion in debt and $1.54 billion in assets. It ended the third quarter â¿¿ the most recent quarter for which financial results are available â¿¿ with just $81.8 million in cash and cash equivalents. Pinnacle, which mostly flies between major airports in the East, has been hurt by rising costs and problems integrating its airline units â¿¿ Pinnacle, Colgan Air and Mesaba Aviation, which it bought in 2010. It plans to wind down the United Express flights it operates for United Continental Holdings Inc. It will continue Delta Connection flights for Delta Air Lines Inc. It's already in the process of winding down its service with US Airways. ___ Groupon restatement sparks more worries NEW YORK (AP) â¿¿ Groupon's announcement that its revenue and earnings were lower than what it reported in February is sparking fresh worries about the young company's business model. The online deals company said late Friday that it had to revise its previously issued fourth-quarter results to increase the money it set aside for refunds to customers. A big reason is that it's now selling more deals like vacation packages and Botox procedures that are expensive and have higher return rates. Investors responded by selling Groupon's stock on Monday, and its shares fell 17 percent. Some analysts wonder whether Groupon, which began trading publicly less than five months ago, will be able to move past its growing pains.