DocuSignDOCU.O added 3.1% as the e-Signature product provider beat second-quarter results estimates and raised its annual revenue forecast. By Shristi Achar A and Amruta Khandekar Sept 8 (Reuters) - U.S. stock index futures were subdued on Friday as investors awaited a fresh inflation reading next week after recent economic data stoked worries interest rates could remain higher for longer. "While our base case is for no further hikes in this cycle, we expect economic uncertainty to keep equity markets volatile and range-bound in the coming months," said Mark Haefele, chief investment officer at UBS Global Wealth Management.