Stocks Open Lower, Alcoa Weighs  NEW YORK (Reuters) - U.S. stocks opened lower on Friday,  weighed down by profit warnings from companies such as Alcoa  Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AA.N target=/stocks/quickinfo/fullquote"&gt;AA.N&lt;/A&gt;, the world's No. 1 aluminum maker, which raised  concerns about U.S. corporate earnings strength.