The cAMP signaling pathway has opposing effects on Rac and Rho in B16F10 cells: implications for dendrite formation in melanocytic cells.

A hallmark of melanocytic cells is their ability to form dendrites in response to growth factors and to ultraviolet irradiation. It is known that the cyclic adenosine monophosphate (cAMP) second messenger pathway stimulates melanocyte dendrite formation because agents that increase cAMP such as forskolin and dibutyrl cAMP induce dendrite formation in normal human and murine melanocytes and melanoma cell lines. The Rho family of guanosine triphosphate (GTP)-binding proteins regulates cytoskeletal reorganization in all cells tested and Rac and Rho have both been shown to regulate melanocyte dendrite formation. In this report, we analyzed the effect of cAMP on the activation of Rac and Rho and show that elevation of cAMP stimulates Rac and inhibits Rho in B16F10 cells. The Rho GTP-binding proteins have also been shown to either cross-activate or inhibit each other and in this report we show that Rac activates Rho in B16F10 cells. Microinjection of C3 botulinum exoenzyme toxin, an agent that specifically inactivates Rho or microinjection of constitutively active mutant Rac protein-induced dendricity in human melanocytes and in B16F10 and B16F1 murine melanoma cell lines. We conclude that cAMP-mediated dendrite formation in melanocytic cells is mediated through upregulation of Rac activity and downregulation of Rho activity.