The efficiency of gene transfer is many times higher in viral than in liposomal transfection. One reason for this is an insufficient intracellular transport of the exogenous DNA into the nucleus in lipofection. Using liposomal transfection techniques for gene therapy is safer than viral approaches, so it would be of great importance to find solutions for their enhancement. We found a 4.51-fold increase in liposomal transfection of T-47D breast cancer cells by the addition of progesterone and a 2.81-fold increase by the addition of cholesterol. The transfection efficiency was measured as the activity of the delivered reporter gene luciferase. The addition of progesterone and cholesterol in combination led to a further enhancement of the transfection efficiency up to 13.72-fold. This additive effect could also be seen when we combined cholesterol with other steroids, but not by the combination of different steroids. All of these steroids alone had also the potential to increase liposomal transfection. Therefore we suggest that steroids and cholesterol enhance liposomal transfection by different mechanisms. Both substances have been shown to shift the exogenous DNA from the cytosol to the nucleus. Steroids normally act through intracellular steroid receptors, which migrate into the nucleus upon activation. The transfected DNA might be co-transported to the nucleus together with the migration of the activated steroid receptors. Even if cholesterol causes also an intracellular shift of DNA to the nucleus, its impact on the fluidity of cellular membranes or on the stability of lipoplexes in serum containing media could be mainly responsible for its effect. If the steroid enhanced liposomal transfection is dependent on the presence of steroid receptors, a specifically-enhanced gene delivery for a subgroup of gynecological tumours expressing high levels of steroid receptors could be possible.

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