It is not clear when you say "Trasnfecting tumor cells with Protein A?" Do you Transfect cells with DNA coding for Protein A or you treat tumor cells with Protein A. 

Clarification is required.

1} If you do transfection with the help of "a" gene then you may replace the "c" and do transfection. Then evaluate presence of Protein B in the transfected cells?

2) If you can create a functional mutant of protein A(deletion/site directed mutagenesis) and then check the presence of Protein B.

----If you know about protein A and B then,

1) If you know the structure of protein A then you may check its potential to bind DNA. This will answer whether it act as transcription factor. { various in silico tools freely available}

2) Gene location of b will also be helpful in predicting the protein A role.

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It is important to consider that "Protein B expression may be induced by Protein A but not necessary that it is transcription factor/co-activator. It may be the response of cells upon protein A treatment/expression".

if upon expression of protein A,protein B expression is going up, this can means several things.

First, figure out if increase in expression B is at the protein stability or transcription.

To check this, compare the half life of protein B in the presence or absence or protein A

if half life is similar, look the transcript levels (mRNA). If transcript of protein B goes up, 

protein A could be the transcription factor for B,

or

protein A regulates the transcription regulator for B, means indirect regulation. 

If you go step by step you will know the answer

good luck