Role of cyclic GMP signaling in the melanocyte response to gravitational alterations
Scientific background:
Human epidermal melanocytes (located strategically in the basal layer of the skin epidermis) represent a crucial protective barrier against UV irradiation and oxidative stress by generating the radical scavenging pigment melanin. Melanin is also known to act as a photosensitizer that generates active oxygen species upon UV irradiation, which may initiate hypopigmentary disorders (e.g., vitiligo) as well as UV-induced oncogene cell transformation. Finally, melanocytes in vivo are permanently targeted by environmental mechanical stimuli.
Guanosine 3',5'-cyclic monophosphate (cyclic GMP, cGMP) has emerged as a major focus in signal transduction research. Cyclic GMP is synthesized from guanosine 5'-triphosphate (GTP) via activation of the guanylyl cyclase (GC) that exists in a soluble form (sGC), the prime target for signaling activity of nitric oxide (NO), and in membrane-bound (e.g., GCA-F) isoforms, that are ligands for natriuretic peptides as well as enterotoxins and guanylin. Mediating most of the effects of NO and of the natriuretic peptides, cGMP plays an important role in smooth muscle relaxation, inhibition of platelet aggregation, neural communication in the brain, and many (patho)physiological processes by targeting the activities of several downstream effector's families [e.g., cGMP-dependent protein kinases (PKG), ion channels, and phosphodiesterases (PDE)]. The intracellular cGMP levels are determined by a balance between the activities of the GC and the elimination of cGMP by metabolic degradation by PDE, but also by active export into the extracellular space. Two members of the multidrug resistance protein (MRP) family, MRP4 and MRP5, seem to act as export pumps for amphiphilic anion transport like cGMP and cAMP.
Main achievements and current works:
For human melanocytes, it has been shown that the NO-sGC-cGMP-PKG pathway is involved in UVB-induced melanogenesis. Furthermore, different guanylyl cyclase (GC) isoforms are responsible for cGMP synthesis in melanocytic cells. Normal human melanocytes and non-metastatic melanoma cell lines predominantly express sGC, which appears to be associated with melanogenesis, whereas absence of NO-sensitive GC, but upregulated activities of the membrane isoforms GC-A and GC-B were found in highly metastatic phenotypes. NO can further induce perturbation of melanocyte-extracellular matrix interactions, which may contribute to loss of melanocytes or melanoma metastasis. These NO effects appear to be modulated partly by the cGMP-PKG pathway.
In the frame of the current space exploration, we investigated the regulation of cGMP levels in melanocytes at altered gravity (G) conditions. Our studies indicate that cultured melanocytes and non-metastatic melanoma cells respond to long-time exposure to hypergravity (up to 5xg for 24 h) with an elevated cGMP efflux under conditions, where PDE-mediated cGMP hydrolysis is inhibited or cGMP synthesis is induced, e.g., by NO. Cyclic GMP efflux was inhibited in the presence of a selective inhibitor of cGMP-binding PDE (PDE5) and MRP4/5 as selective cGMP exporters (1 µM trequinsin).Transport was further inhibited by probenecid, an inhibitor of endogenous non-selective transporters as well as of MRP4/5, and by cycloheximide as an inhibitor of de novo protein synthesis. Thus, the expression of endogenous non-selective cGMP transporters and/or MRP4/5 is increased under elevated acceleration in human melanocytes and non-metastatic melanoma cells. Similar results were found on mRNA and protein levels. In contrast, elevated acceleration does not affect cGMP efflux in highly metastatic melanoma cells. We propose that altered gravity should be regarded as a possible factor that may induce signaling events in human melanocytes via cGMP that could be important for malignant transformation. Future studies on this aspect in microgravity should clarify whether such mechanisms are also operational under space-flight conditions.
Selected publications:
Ivanova K, Das PK, van den Wijngaard R, Lenz W, Klockenbring T, Malcharzyk V, Drummer C, 2 Gerzer R. (2001). Differential expression of functional guanylyl cyclases in melanocytes: absence of nitric-oxide-sensitive isoform in metastatic cells. J Invest Dermatol, 116:409-416].
Ivanova K, Zadeh NH, Block I, Das PK, Gerzer R. (2004). Stimulation of cyclic GMP efflux in human melanocytes by hypergravity generated by centrifugal acceleration. Pigment Cell Res, 17:471-479;
Ivanova K, Block I, Das PK, Gerzer R (2006). Role of cyclic GMP signaling in the melanocyte response to hypergravity. Signal Transduction, 6:406-413.