1. A method for a User Equipment (UE) transmitting a Medium Access Control Protocol Data Unit (MAC PDU) in a wireless communication system, the method comprising:generating a first buffer status reporting (BSR) MAC Control Element (CE) for a first BSR including an amount of first data available for transmission to a first BS, when the first BSR is triggered by a first MAC entity;
generating a second BSR MAC CE for a second BSR including an amount of second data available for transmission to a second BS, when the second BSR is triggered by a second MAC entity;
generating the MAC PDU including a subheader of the MAC PDU, the first BSR MAC CE and the second BSR MAC CE; and
transmitting the MAC PDU via the second MAC entity to the first BS,
wherein the first MAC entity and the second MAC entity are configured to the UE, and
wherein the subheader of the MAC PDU includes a first indicator indicating the first BS associated with the first BSR and a second indicator indicating the second BS associated with the second BSR.