RACK1 cooperates with NRASQ61K to promote melanoma in vivo.

Melanoma is the deadliest skin cancer. RACK1 (Receptor for activated protein kinase C) protein was proposed as a biological marker of melanoma in human and domestic animal species harboring spontaneous melanomas. As a scaffold protein, RACK1 is able to coordinate the interaction of key signaling molecules implicated in both physiological cellular functions and tumorigenesis. A role for RACK1 in rewiring ERK and JNK signaling pathways in melanoma cell lines had been proposed. Here, we used a genetic approach to test this hypothesis in vivo in the mouse. We show that Rack1 knock-down in the mouse melanoma cell line B16 reduces invasiveness and induces cell differentiation. We have developed the first mouse model for RACK1 gain of function, Tyr::Rack1-HA transgenic mice, targeting RACK1 to melanocytes in vivo. RACK1 overexpression was not sufficient to initiate melanomas despite activated ERK and AKT. However, in a context of melanoma predisposition, RACK1 overexpression reduced latency and increased incidence and metastatic rate. In primary melanoma cells from Tyr::Rack1-HA, Tyr::NRasQ61K mice, activated JNK (c-Jun N-terminal kinase) and activated STAT3 (signal transducer and activator of transcription 3) acted as RACK1 oncogenic partners in tumoral progression. A sequential and coordinated activation of ERK, JNK and STAT3 with RACK1 is shown to accelerate aggressive melanoma development in vivo.

Canine melanoma diagnosis: RACK1 as a potential biological marker.

Melanoma diagnosis in dogs can be challenging due to the variety of histological appearances of canine melanocytic neoplasms. Markers of malignancy are needed. Receptor for activated C-kinase 1 (RACK1) was found to characterize melanomas in other mammals. We investigated the value of RACK1 detection in the classification of 19 cutaneous and 5 mucosal melanocytic neoplasms in dogs. These tumors were categorized as melanocytomas or benign and melanomas or malignant after evaluation of their morphology, mitotic index, and Ki-67 growth fraction. Using immunofluorescence, we confirmed microphthalmia-associated transcription factor (MITF) as a marker of normal and transformed melanocytic cells in dog tissues. All control (n = 10) and tumoral (n = 24) samples stained positively for MITF (34/34, 100%). Whereas RACK1 was not detected in healthy skin melanocytes, melanocytic lesions were all positive for RACK1 signal (24/24, 100%). RACK1 cytoplasmic staining appeared with 2 distinct distribution patterns: strong, diffuse, and homogeneous or granular and heterogeneous. All melanoma samples (13/13, 100%) stained homogeneously for RACK1. All melanocytomas (11/11, 100%) stained heterogeneously for RACK1. Immunohistochemistry was less consistent than immunofluorescence for all labelings in melanocytic lesions, which were often very pigmented. Thus, the fluorescent RACK1-MITF labeling pattern helped to distinguish melanomas from melanocytomas. Furthermore, RACK1 labeling correlated with 2 of 11 morphological features linked to malignancy: cell and nuclear size. These results suggest that RACK1 may be used as a marker in dog melanomas.

The patchwork mouse phenotype: implication for melanocyte replacement in the hair follicle.

Mice homozygous for the recessive patchwork (pwk) mutation are characterized by a variegated pigment pattern with a mixture of unpigmented and normally pigmented hairs. The pigmented hair bulbs contain functional melanocytes. By contrast, the unpigmented hair bulbs contain no melanocytes. This lack results from the death of melanoblasts in the hair follicle at the end of embryogenesis. Here, we report that melanoblasts and melanocytes are found in the epidermis of pwk/pwk mice. Furthermore, these epidermal pigment cells are able to colonize new hair follicles after skin wounding. Despite the presence of epidermal pigment cells with a colonization potential, a follicle that had produced an unpigmented hair produces a new unpigmented hair during the successive hair growth cycles. This hair color continuity is also true for the pigmented hair follicles. Thus, in normal conditions, the hair acts as an independent functional unit as regards its pigment cells population.

Survival of patchwork melanoblasts is dependent upon their number in the hair follicle at the end of embryogenesis.

The recessive patchwork (pwk) mutation in mice is associated with a unique hair follicle phenotype. Mice homozygous for patchwork exhibit a variegated coat containing a mixture of white and fully pigmented hairs, but no partially pigmented hairs. We have investigated the etiology of this mutation. We report here that the white hairs result from the lack of melanocytes in the follicle. As indicated by the coat color pattern of patchwork<-->albino chimeras, the target cell for the patchwork mutation is the melanocyte and/or its precursor. Examination of these chimeras also suggested that patchwork does not act in a cell-autonomous manner. The colonization of the skin by melanoblasts in patchwork embryos was studied using a lacZ transgene. Melanoblasts die by apoptosis in hair follicles from homozygous pwk/pwk fetuses starting at embryonic day 18.5, indicating that patchwork acts from this stage. The combination of pwk and KitW-ei, a mutation responsible for a reduced number of melanoblasts in the hair follicle, suggested that pwk gene product is necessary for low numbers of melanoblasts to survive and differentiate in the hair follicle from embryonic day 18.5 onward. We conclude that the pigmented hairs on the coat of pwk/pwk mice may be attributed to a community effect among melanoblasts in the hair follicle at the end of embryogenesis.