Inter-cell interference limits the performance of heterogeneous cellular networks. This paper investigates cooperative distributed beamforming techniques for interference mitigation. We consider a network with macrocells overlaid with dense small cells. A low-complexity distributed algorithm is proposed for coordinated beamforming across base stations. The beamforming design jointly optimizes useful signal power while minimizing interference leakage to other cells. We derive the optimality conditions and present a decentralized iterative optimization method. Simulation results illustrate that our proposed scheme provides substantial gains for edge user throughput and cell-edge spectral efficiency.