Mobile live video streaming is expected to become mainstream in the fifth-generation (5G) mobile networks. To boost the Quality of Experience (QoE) of live video streaming services, the integration of Scalable Video Coding (SVC) with Mobile Edge Computing (MEC) becomes a natural candidate due to its scalability, reliability, and the low-latency transmission supports for real-time interactions. However, it still takes efforts to integrate MEC into live video streaming services to exploit its full potentials. We find that the efficiency of the MEC-enabled cellular system can be significantly improved when the requests of users can be redirected to proper MEC servers through optimal user associations. In light of this observation, we jointly address the caching placement, video quality decision, and user association problem in the live video streaming service. Since the proposed nonlinear integer optimization problem is NP-hard, we develop a two-step approach from a Lagrangian optimization under the dual pricing specification. The proposed algorithm has an explicit analytic solution and can be applied from the supply-demand pricing perspective. The simulation results show that the service quality of live video streaming in the MEC-enabled cellular system can be significantly improved through the proposed system.