do you have the blocking peptide for your antibody?  If you use this and one band goes away, that is your protein band of interest.  Also, do you know if your protein goes through multiple post-translational modifications or dimerization?  Do the other bands follow the same pattern, ie, all seem to increase or decrease?  Just because you have multiple bands does not always mean that the antibody is completely non-specific.

Thanks for your reply.

Unfortunately I have not spesific blocking peptide but I decided to use inhibitor of Plk1( wortmannin) and I will load my samples with and without inhibitor into the gel.

There are many isoforms of PI kinases-1, they might be different enough from each other to give differend molweight bands in PAAG, especially if you are working with malignant cells/tissues biomaterial. When companies make antibodies they try to produce "universal" ones, not for the whole protein but for small peptide sequence from it. "Universal" mAbs are more required ones, this might be the answer for the question

Thanks Elena for your reply, 

First, I am sorry for I didn't write full name of my protein. Plk1 is polo-like kinase 1. It also has  isoform. In addition to this I am working with malignant cells, PC3.