To test if two proteins interact with each other, you need specific antibodies to detect them, for instance mouse antibodies against DMP1, and rabbit antibodies against Fgf23. Then you do two transfections, one with plasmid encoding DMP1 alone, and one with both DMP1 and Fgf23 plasmids. After a suitable time to establish expression in your system, extract total proteins, and then do immuno-precipitation with the anti DMP1 serum and proteinA sepharose. The washed proteinA sepharose pellet will contain DMP1, and potentially some Fgf23 associated with it. You test that by boiling the pellet in a little bit of SDS-PAGE sample buffer, run a denaturing gel and blot on nitrocellulose to do a western blot, using the anti-Fgf23 serum. Using a secondary anti-rabbit HRP conjugate, you will only detect the rabbit antibodies, bound to Fgf23. You should only detect Fgf23 in the sample from the dual transfected cells, not those that contain the DMP1 plasmid alone. When this is successful, you do a control where you use Fgf23 as bait (express Fgf23 alone or with DMP1 in cells, then immunoprecipitate Fgf23 first and detect DMP1 with western afterwards). The control has to work too, then you are sure you have a meaningful protein-protein interaction.

However, I'm not sure if this will help your case. If the presence of DMP1 affects Fgf23 expression, it could be at the level of transcription, translation or protein level. In which cells is DMP1 usually expressed? Is it expressed in the same cell types as those that express Fgf23? Or is DMP1 secreted and affects other cells that produce Fgf23? If it is the latter, DMP1 could be endocytosed by Fgf23 producing cells? In that case you should transfect cells with Fgf23 and incubate them with or without DMP1 in the growth medium. I have merely answered the question in your title, I'm not sure if this is the first course of action in your case. I would need more information about the biological questions you are asking. 

Good luck :)

Although I do miss on the biological background of your system, I am familiar with the prot-prot interaction studies.

You can check for the interactions with the yeast two hybrid screens (split-ubiquitin).

Or use complementation studies (split-YPF).

Dear Jaya, As Jurgen Denecke has mentioned above, I am not sure whether the information which you have provided hints towards FGF23-DMP interaction. First of all, is it already knowh that the DMP-effect on FGF23 expression is at the transcriptional level? I see that DMP is localized extracellularly and in the nucleus, but which fraction contributes to FGF23 regulation needs to be identified.. If you are refering to this publication:

http://www.ncbi.nlm.nih.gov/pubmed/21507898

They state that FGF23 is induced by enhanced activation of FGFR signaling in the DMP mutant mice. This would suggest that there is no real direct interplay between FGF23 and DMP and wharever you are seeing is the effect of transfected nuclear DMP. Instead, I think you should look deep into the literature and find possible modes of how FGF-FGFR signaling is regulated by DMP (most probably in the extracellular matrix). 

Good Luck