**4. Conclusions**

The human SHBG gene is expressed at the mRNA and protein levels in prostate cells. Considering its sex hormone binding properties, we examined its ability to affect the androgenome of prostate cells. Using a functional microarray approach, we have obtained evidence that indeed, SHBG does affect the expression of DHT-responsive genes. In LNCaP cells, SHBG overexpression exerts global effects that include genes involved in prostate cancer (eg. c-myc and TIMP2), hormonal signaling (eg. GPR30), and the expression of AR co-regulators (eg. FKBP5), among others.

The scope of SHBG's influence on the androgenome appears to be broad and complex, involving many aspects of AR activation. Two possible mechanisms include the binding and sequestering of intracellular androgen, and the indirect modulation of AR co-regulator expression. It remains to be determined whether signaling through RSHBG is also involved.

Given the ability of prostate cells to greatly ramp up their expression of endogenously synthesized SHBG (we have overexpressed SHBG in LNCaP, PC3, and DU145 cells), this raises the question of whether they regulate their androgenome by modulating intracellular SHBG levels. It is likely that a decrease in intracellular SHBG levels results in an equilibrium shift towards increased free intracellular testosterone and free intracellular DHT. We speculate that in those prostate cancer cells which undergo deletions of the SHBG/p53 locus, SHBG expression will be reduced. Deletions of the SHBG/p53 locus could thus provide a genetic means by which prostate cancer patients placed on current androgen ablation therapies can progress- enabling cells to survive under conditions of diminished androgen due to the relative increase in free intracellular androgen available to activate AR-mediated signaling. If this speculation is confirmed, it will be of interest to see how such patients respond to newer therapies that target testosterone and DHT biosynthesis, such as abiraterone.

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It is intriguing to also speculate that, in addition to the prostate, locally expressed SHBG plays a functional role in the hormone response of other tissues. We have preliminary evidence that endogenously expressed SHBG can modulate the estrogen response of human breast cells (Kahn et al, 2008). And, if plasma SHBG levels provide a clue into how altered SHBG expression may contribute to the disease state at a cellular level, it will be of great interest to investigate whether there is a connection between tissue specific SHBG expression and Type 2 diabetes. This does not detract in any way from the importance of plasma SHBG levels on androgen and estrogen responsiveness in humans. Instead, it serves to broaden the scope of SHBG influence on the response to sex steroids to the individual tissue and cellular level.
