Insulin and insulin-like growth factor-I and -II modulate human granulosa-lutein cell steroidogenesis: enhancement of steroidogenic acute regulatory protein (StAR) expression.

Insulin and insulin-like growth factors (IGF)-I and -II stimulate granulosa cell steroidogenesis. Since steroidogenic acute regulatory protein (StAR) regulates the rate-limiting step in steroid hormone biosynthesis, the ability of insulin and IGF to modulate StAR protein and mRNA expression was examined in two human granulosa cell culture systems: (i) proliferating granulosa-lutein cells and (ii) luteinized-granulosa cells derived during in-vitro fertilization (IVF). In proliferating granulosa-lutein cells, IGF-I and IGF-II increased StAR protein approximately 4-5-fold, while insulin and 8-bromoadenosine 3',5'-cAMP (8-Br-cAMP) increased amounts of StAR protein 2.5- and 6-fold respectively. A combination of IGFs/insulin and 8-Br-cAMP increased StAR 7-9-fold. Luteinized granulosa cells also had increased StAR expression after treatment with IGF-I (2. 8-fold), IGF-II (3-fold), insulin (2.5-fold) and 8-Br-cAMP (4. 5-fold). Progesterone production generally followed a pattern similar to StAR protein in both cell systems. In proliferating granulosa-lutein cells, both IGF-I and insulin increased StAR mRNA (3-fold) and 8-Br-cAMP increased StAR mRNA 4-fold, whereas a combination of IGF-I and 8-Br-cAMP had an additive effect on StAR mRNA expression (7-fold). Transient transfection of proliferating granulosa-lutein cells with StAR promoter-luciferase reporter constructs demonstrated that IGF-I, IGF-II, and insulin had no effect on the StAR promoter activity, while 8-Br-cAMP stimulated StAR promoter function. The results indicate that: (i) IGFs and insulin stimulate StAR mRNA and protein expression in human granulosa-lutein cells; (ii) IGF-I and 8-Br-cAMP have an additive effect on StAR gene expression; and (iii) IGF-I increases StAR mRNA and protein by a mechanism that does not involve activation of the proximal StAR gene promoter.