Mortgage brokers may hold the key for companies aiming to succeed in the subprime market. At least, that’s the thinking that is driving the expansion of American Residential Investment Trust. The California-based company, one of the last remaining real estate investment trusts active in the subprime market, recently announced plans to take a controlling interest in LoanCity.com, an online marketplace and technology provider that aims to connect brokers and realtors with lenders.