In this paper, the spectrum sharing in both licensed and unlicensed bands under heterogeneous networks (HetNets) is investigated. A frame architecture of licensed-assisted access (LAA) in unlicensed bands is introduced by analyzing its statistical behavior of channel access opportunity. A multi-objective optimization problem is then designed for small cell to enhance energy efficiency by jointly maximizing the achievable sum-rate and minimizing the power consumption. The proposed discontinuous transmission (DTX)-enabled LAA (DLA) mechanism can be employed to find the Pareto optimal solution of resource allocation policy while guaranteeing the quality of service (QoS) of incumbent systems in both licensed and unlicensed bands. Numerical results show that the proposed DLA mechanism can enhance energy efficiency through dynamically allocating radio resources and adjusting system parameters to opportunistic share the licensed and unlicensed resources.