Roles of microfilaments and intermediate filaments in adrenal steroidogenesis

The problem for the steroidogenic cell if it is to accelerate steroid synthesis in response to trophic stimulation, consists in moving cholesterol from the sites of synthesis and storage to mitochondria at an accelerated rate. The most intensely studied situation is that in which the sterol is stored as ester in lipid droplets. Cholesterol ester must be de-esterified and transported to mitochondria where steroid synthesis begins. Since droplets and mitochondria are now known to be attached to intermediate filaments and since these structures are not contractile, it appears to be necessary to invoke the actions of other cytoskeletal elements. Actin microfilaments are involved in cholesterol transport so that it is tempting to propose that the contractile properties of actomyosin are used in this process. It is known that an energy-dependent contractile process involving actin is capable of disrupting intermediate filaments. Since the intermediate filaments appear to act by keeping lipid droplets and mitochondria apart, disruption of the filaments accompanied by a contractile process would be expected to allow these two structures to come together. This would open the way for the transfer of cholesterol to the steroidogenic pathway. This should be regarded as a first step. The events necessary for entry of cholesterol from droplets into the mitochondria remain to be clarified. In addition, the transport process for newly synthesized cholesterol that is not stored in droplets, is still not understood. At least four protein kinase enzymes have been identified in the cytoskeletons of adrenal cells, namely, Ca2+/calmodulin-dependent kinase, protein kinase (Ca2+ and phospholipid-dependent), myosin light chain kinase, and protein kinase A (cyclic AMP-dependent). The Ca2+/calmodulin kinase promotes transport of cholesterol to mitochondria and does so under conditions in which phosphorylation of vimentin and myosin light chain occurs. Phosphorylation of vimentin results in disruption of intermediate filaments while phosphorylation of light chain promotes contraction of the actomyosin ring. It now appears that intermediate filaments are cross-linked by actin filaments so that such contraction would be expected to produce significant structural changes in the cytoskeleton and the attached organelles. Although the details of the changes taking place in the organ in vivo are not known, the potential for interaction between droplets and mitochondria as the result of these changes in intermediate filaments and actomyosin, is clear. Protein kinase C is activated by ACTH and cyclic AMP, although this activation does not appear to be directly involved in the regulation of steroid synthesis. Nevertheless, vimentin is a substrate for this enzyme, and changes in the organisation of vimentin filaments and the attached organelles under the influence of protein kinase C have been reported in other cells. Presumably these changes represent part of the response to ACTH because when protein kinase C is activated by phorbol ester, the cytoskeletal changes necessary for rounding up take place but such changes are not accompanied by increased steroid synthesis. Protein kinase A causes rounding of adrenal cells. and cytoskeletons. This kinase also causes increased cholesterol transport and, hence, stimulation of steroid synthesis. The enzyme also causes phosphorylation of vimentin but with a different cytoskeletal reorganisation from that seen with the other three kinase enzymes. Clearly phosphorylation plays a major role in these responses. Phosphorylation alters the morphology and the functions of the cytoskeleton and this, in turn, is associated with accelerated cholesterol transport. It is now necessary to define the details of the specific phosphorylation reactions that occur during the response to ACTH, that is, which amino acids are phosphorylated and to what extent by each of the kinase enzymes.