Inhibitors of ADP-ribosylation suppress phenotypic modulation and proliferation of smooth muscle cells cultured from rat aorta.

The effects of hexamethylenebisacetamide (HMBA), an inhibitor of poly-ADP-ribosylation, and meta-iodobenzylguanidine (MIBG), an inhibitor of mono-ADP-ribosylation, on the phenotypic properties and proliferation of cultured rat aortic smooth muscle cells were studied using a combination of structural and chemical methods. The results show that HMBA and MIBG both slowed down the transition of the cells from a contractile to a synthetic phenotype in primary culture. While the control cells rapidly lost most of their myofilaments and built up an extensive endoplasmic reticulum and Golgi complex, a conspicuous fraction of the drug-treated cells retained a characteristic smooth muscle morphology for at least 6 days. Moreover, most of the treated cells remained positive for smooth muscle alpha-actin and desmin throughout this period. In contrast, the drugs lacked distinct effects on cell morphology and cytoskeletal organization in secondary cultures. Nevertheless, they strongly inhibited serum-stimulated cell growth both in primary and secondary cultures. The ability of serum-starved cells to synthesize DNA after exposure to platelet-derived growth factor or serum was also restrained. Notably, the drugs could be added several hours after the mitogens without loss of effect, suggesting that they did not prevent the entrance into but rather the progression through the cell cycle. Accordingly, the expression of early response genes like c-fos, c-jun and c-myc was not blocked by the drugs. On the other hand, HMBA reduced the expression of transcripts for smooth muscle alpha-actin, type IV collagenase, collagen type I, and osteopontin both in primary and secondary cultures. Weaker and more variable effects were obtained with MIBG. Taken together, the findings support the notion that poly- and mono-ADP-ribosylation of proteins are involved in the control of smooth muscle cell differentiation and growth.