Upstart Holdings, Inc. (NASDAQ:UPST) shares are trading lower Monday after the company announced a private offering of $300 million convertible senior notes. The Details: Upstart expects to grant the initial purchasers of the notes an option to purchase up to an additional $45 million aggregate principal amount of the notes. The notes will mature on Oct. 1, 2029, unless earlier converted, repurchased or redeemed and will be convertible into cash, shares of Upstart’s common stock, or a combination thereof, at Upstart’s election. Upstart said the interest rate, initial conversion rate, and other…