This paper addresses the problem of streaming packetized media over a lossy packet network through an intermediate proxy server to a client, in a rate-distortion optimized way. The proxy, located at the junction of the backbone network and the last hop to the client, coordinates the communication between the media server and the client using hybrid receiver/senderdriven streaming in a rate-distortion optimization framework. The framework enables the proxy to determine at every instant which packets, if any, it should either request from the media server or retransmit directly to the client, in order to meet a constraint on the average transmission rate on the last hop while minimizing the average end-to-end distortion. Performance gains of up to 2 dB and up to 4 dB are observed over rate-distortion optimized sender-driven systems for the case when the last hop is wireline and wireless, respectively.