IntroductionThe Mer (Mertk, Nyk, c-Eyk) receptor tyrosine kinase is a transmembrane receptor consisting of an extracellular domain with 2 immunoglobulin-like and 2 membrane proximal fibronectin III motifs, a transmembrane region, and an intracellular tyrosine kinase domain. 1,2 These motifs place Mer in the same tyrosine kinase subfamily as Axl 3 and Tyro-3/Sky. 4,5 Mer, Axl, and Tyro-3 share the same ligand, 6,7 Gas6, which has significant homology to the negative coregulator of the blood coagulation pathway protein S. 8 Abnormal expression or activity of the Mer tyrosine kinase may play a role in tumorigenesis. The avian Mer ortholog, eyk, was discovered first as the oncogene in acute avian retrovirus RPL30. The constitutively active tyrosine kinase domain of v-eyk causes fibrosarcomas, endotheliomas, and visceral lymphomatosis in chickens. 9,10 Overexpression of murine Mer tyrosine kinase transforms BaF3 lymphocytes 11 and overexpression of human Mer transforms NIH3T3 cells. 12 Several human cancers overexpress Mer, including mantle cell lymphomas, 13 alveolar rhabdomyosarcomas, 14 gastric cancer, 15 and pituitary adenomas. 16 Mer is also ectopically expressed in pediatric T-cell acute lymphoblastic leukemia, 17 and a Mer transgenic mouse model with ectopic expression of Mer in thymocytes and lymphocytes develops T-cell lymphoblastic leukemia/lymphoma. 18 Axl and Tyro-3 also transform cells in vitro 3,4 and are overexpressed in a spectrum of human cancers.In addition to abnormal function of Mer in cancer, a physiologic role for Mer has recently been described in macrophages. Mer, Axl, and Tyro-3 have been shown to limit the extent of macrophage activation in response to an immune stimulus. 19,20 Mer also plays a significant role in the ability of macrophages to clear apoptotic cells, 21 and Mer deficiency has been linked to the development of autoimmune disorders in mice. Lack of Mer receptor causing defective macrophage apoptotic cell clearance in the Royal College of Surgeons (RCS) rat has also been implicated in the development of retinitis pigmentosa. 22 Interestingly, Mer gene mutations have been defined in a subset of humans with retinitis pigmentosa. 23 Furthermore, a physiologic role for the Mer tyrosine kinase has been described for the normal function of platelets. The interaction of Gas6 with Mer, Axl, and Tyro-3 is important in platelet degranulation and aggregation in response to known agonists. Mice lacking either Gas6 or Mer protein have impaired platelet aggregation in vitro and diminished clot stability in vivo. [24][25][26] A delicate balance of ligand interaction with a tyrosine kinase receptor is necessary to maintain normal tyrosine kinase function without causing overactivation, which could result in human disease. One means of regulating tyrosine kinase activation is through proteolytic cleavage of the membrane-bound protein.Through this process, the total number of membrane-bound receptors is reduced. In addition, the soluble cleavage product may function as a decoy receptor and seques...

The Myc oncoprotein is implicated in transcriptional regulation of a variety of genes pertaining to cell cycle and neoplastic transformation. Examples of both positive and negative regulation have been reported that involve E-box and initiator (Inr) promoter elements, respectively. In both cases, Myc is thought to induce changes in transcription initiation. We have previously shown that overexpression of Myc causes down-regulation of the thrombospondin-1 (tsp-1) gene, an important negative modulator of tumor angiogenesis. In this study, we demonstrate that Myc in combination with Max can bind, albeit with low affinity, to an E-box-like element in the tsp-1 promoter. However, the 2.7 kb DNA segment containing both this non-canonical E-box and an Inr-like sequence does not constitute a Myc-responsive element in a transient expression system. Furthermore, Myc does not significantly affect the rate of initiation or elongation of the tsp-1 mRNA. Thus, in this instance Myc does not act as a canonical transcription factor. Instead, as demonstrated by blocking de novo RNA synthesis, down-regulation of the tsp-1 gene by Myc occurs through increased mRNA turnover. To our knowledge, this is the first example of gene regulation by Myc that involves mRNA destabilization.

Defective clearance of apoptotic cells has been shown in systemic lupus erythematosus (SLE) and is postulated to enhance autoimmune responses by increasing access to intracellular autoantigens. Until now, research has emphasized inherited rather than acquired impairment of apoptotic cell engulfment in the pathogenesis of SLE. Here, we confirm previous results that efficient removal of apoptotic cells (efferocytosis) is bolstered in the presence of wild type mouse serum, through the C3 deposition on the apoptotic cell surface. In contrast, sera from three mouse models of SLE, MerKD, MRLlpr and NZBWF1, did not support and in fact actively inhibited apoptotic cell uptake. IgG autoantibodies were responsible for the inhibition, through the blockade of C3 recognition by macrophages. Consistent with this, IgG removal reversed the inhibitory activity within autoimmune serum and purified autoimmune IgG blocked both the detection of C3 on apoptotic cells and C3-dependent efferocytosis. Sera from SLE patients demonstrated elevated anti-C3b IgG that blocked detection of C3 on apoptotic cells, activity that was not found in healthy controls or patients with rheumatoid arthritis, nor in mice prior to the onset of autoimmunity. We propose that the suppression of apoptotic cell disposal by antibodies against deposited C3 may contribute to increasing severity and/or exacerbations in SLE.