Mitochondrial heat shock protein participates in placental steroidogenesis

The human placenta, which does not express the StAR protein, synthesizes large amounts of progesterone. The rate-limiting step for steroidogenesis is the transport of cholesterol which is divided into two steps: 1) cholesterol flux from cytoplasm to outer membrane mitochondria, and 2) cholesterol transport from outer to inner mitochondrial membrane. The proteins mediating placental cholesterol influx have not been clearly identified. We investigated the proteins involved in the transport of cholesterol in syncytiotrophoblast mitochondria from human placenta. Two proteins, one of 30 kDa, and another of 60 kDa, were identified using anti-MLN64 antibodies. The 30 kDa protein corresponds to a fragment of MLN64, and the 60 kDa protein was identified as a heat shock protein. During steroidogenesis, mitochondria released MLN64 protein to supernatant. When this supernatant was added to fresh isolated mitochondria, progesterone synthesis increased; a similar result was obtained with the addition of the recombinant MLN64-START protein. In the presence of flurescein-5-maleimide or N-ethyl-maleimide, the mitochondrial synthesis of progesterone was inhibited in a dose-dependent fashion without changes in mitochondrial respiration. 2D-electrophoretic pattern showed that flurescein-5-maleimide- fluorescence was associated with HSP60. Both MLN64 and HSP60 were identified in mitochondrial contact sites. The results suggest that HSP60 is involved in the steroidogenic metabolism of human placenta. A tight association between MLN64 and HSP60 is suggested for cholesterol transport in the human placenta.