The use of genetic manipulation of MA-10 Leydig tumor cells to demonstrate the role of mitochondrial proteins in the acute regulation of steroidogenesis.

The true rate-limiting step in steroidogenesis is the delivery of cholesterol to the inner mitochondrial membrane where it is converted to pregnenolone by the cholesterol side-chain cleavage complex. This process is known to require de novo protein synthesis. We have previously described the synthesis of a family of 37, 32, and 30 kilodalton mitochondrial proteins in response to hormone stimulation in MA-10 mouse Leydig tumor cells and have proposed that these proteins are involved in the acute regulation of steroidogenesis. In this study we have used two subclones of MA-10 cells to further demonstrate the correlation between the quantity of these proteins and the production of steroids in response to hormone treatment. One of these, designated MA-10(K3), has been transfected with a mutant gene of the type 1 regulatory subunit of the cAMP-dependent protein kinase under the control of a metallothionein promoter, whereas the other, designated MA-10(P+29), is a constitutive overproducer of a cAMP-phosphodiesterase (PDE). MA-10 parent cells designated (P), produce large amounts of progesterone in response to LH, human CG, and (Bu)2cAMP. The MA-10(K3) cells, on the other hand, whereas significantly higher than controls, produce much less steroid than the parent cells in response to hormone stimulation. Activation of the mutant gene with Zn+2 results in yet a further decrease in the amount of steroid produced. The MA-10(P+29) cells display greatly reduced progesterone production when stimulated with LH, because of the presence of high amounts of PDE, but return to maximally stimulated levels when a PDE inhibitor is present. Quantitation of the synthesis of the mitochondrial proteins described above in MA-10(K3) cells in the presence and absence of Zn+2 and in MA-10(P+29) cells in the presence and absence of PDE inhibitor clearly demonstrate that the amount of the 30 kilodalton mitochondrial proteins present in these cells closely parallels that of progesterone production. The high degree of correlation between the appearance and quantity of these mitochondrial proteins and the production of steroids make them strong candidates for the putative proteins involved in acute regulation of steroidogenesis.