This question is a tricky one because you do not want to foster an air of negativity during the interview. If you have heard negative things about the employer, you likely do not yet have the rapport to bring it up let alone help them make a move towards change. Keep your answer light and comfortable for the interviewer to address. The key here is to complete your response on a positive note.
"I read on Glassdoor that growth within your organization can be slow. Could you comment on that for me? I am interested in joining a publication where growth opportunities are present as I prove myself to be a valuable team member. With that said, I have heard many more amazing things about your company, as opposed to negative."
"Negative does not always mean it's true - this is one lesson that I have learned after years of being a journalist. I have heard primarily great things about your publication. Anything negative that I have heard, such as turnover or slow growth, are complaints that nearly all media organizations receive."