curl through authenticated proxy and authenticated http resource

I want to do a post (twitter in this case) with a oneliner.

If I don't have proxy

curl -u user:pass -d status="message" http://twitter.com/statuses/update.xml

works perfectly.

But when I am behind a authenticated proxy it doesn't.

I had tryied:

curl -X proxy:port -U proxyUser:proxyPass -u user:pass -d status="message" http://twitter.com/statuses/update.xml

That it jump me with an

proxy do not support basic auth

So do you know what I am doing wrong?

thanks in advance.