Nginx as a proxy doesn't allow to access backend (Apache) content which is password protected

I have apache as a backend and nginx as a frontend. Some content is password-protected by Apache (HTTP Auth).

When I access it directly - everything works fine, bur when I try it through nginx - seems like nginx doesn't forward or cache the responses from Apache. So inside nginx log I see:

89.21.92.0 - - [17/May/2012:22:13:42 +0200]  GET / HTTP/1.1 | upstream_address x.x.x.x:80 | upstream_status 401 | upstream_response_time 0.026 | msec 1337285622.757 | request_time 0.026
89.21.92.0 - zentavr [17/May/2012:22:13:44 +0200]  GET / HTTP/1.1 | upstream_address - | upstream_status - | upstream_response_time - | msec 1337285624.151 | request_time 0.000

On Apache's side:

y.y.y.y - - [17/May/2012:22:13:42 +0200] "GET / HTTP/1.0" 401 652 "-" "Mozilla/5.0 (Windows NT 6.1; rv:12.0) Gecko/20100101 Firefox/12.0" "89.21.92.0"

What could be the issue?