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The objective of this review article is to summarize the recent findings about the importance of microRNAs (miRNAs) in regulating lipoprotein metabolism. We highlight the recent findings that uncover the importance of miRNAs in controlling plasma LDL-cholesterol (LDL-C) levels.

Hair follicle stem cells (HF‐SCs) alternate between periods of quiescence and proliferation, to finally differentiate into all the cell types that constitute the hair follicle. Also, they have been recently identified as cells of origin in skin cancer. HF‐SCs localize in a precise region of the hair follicle, the bulge, and molecular markers for this population have been established. Thus, HF‐SCs are good model to study the potential role of oncogenic activations on SC physiology. Expression of a permanently active form of Akt (myrAkt) in basal cells leads to Akt hyperactivation specifically in the CD34+Itga6Hpopulation. This activation causes bulge stem cells to exit from quiescence increasing their response to proliferative stimuli and affecting some functions such as cell migration. HF‐SC identity upon Akt activation is preserved; in this sense, increased proliferation does not result in stem cell exhaustion with age suggesting that Akt activation does not affect self‐renewal an important aspect for normal tissue maintenance and cancer development. Genome‐wide transcriptome analysis of HF‐SC isolated from myrAkt and wild‐type epidermis underscores changes in metabolic pathways characteristic of cancer cells. These differences manifest during a two‐step carcinogenesis protocol in which Akt activation in HF‐SCs results in increased tumor development and malignant transformation. StemCells2014;32:1917–1928

Myeloid-associated differentiation marker (MYADM) protein belongs to the MAL family and regulates raft domains in epithelial cells. Membrane rafts participate in inflammatory responses. This study shows that MYADM is expressed in endothelial cells and controls the endothelial barrier by regulating ICAM-1 expression through ezrin, radixin, and moesin proteins, connectors between plasma membrane domains and actin cytoskeleton.

Rac1 requires compartmentalization into specialized, condensed membranes to mediate cell migration. We show that myeloid-associated differentiation marker (MYADM), a member of the MAL family of proteins with ubiquitous expression, regulates membrane condensation required for Rac1 targeting and, subsequently, cell spreading and migration.

Membrane organization into condensed domains or rafts provides molecular platforms for selective recruitment of proteins. Cell migration is a general process that requires spatiotemporal targeting of Rac1 to membrane rafts. The protein machinery responsible for making rafts competent to recruit Rac1 remains elusive. Some members of the MAL family of proteins are involved in specialized processes dependent on this type of membrane. Because condensed membrane domains are a general feature of the plasma membrane of all mammalian cells, we hypothesized that MAL family members with ubiquitous expression and plasma membrane distribution could be involved in the organization of membranes for cell migration. We show that myeloid-associated differentiation marker (MYADM), a protein with unique features within the MAL family, colocalizes with Rac1 in membrane protrusions at the cell surface and distributes in condensed membranes. MYADM knockdown (KD) cells had altered membrane condensation and showed deficient incorporation of Rac1 to membrane raft fractions and, similar to Rac1 KD cells, exhibited reduced cell spreading and migration. Results of rescue-of-function experiments by expression of MYADM or active Rac1L61 in cells knocked down for Rac1 or MYADM, respectively, are consistent with the idea that MYADM and Rac1 act on parallel pathways that lead to similar functional outcomes.

Plitidepsin (aplidin) is a marine cyclic depsipeptide in phase II clinical development against several neoplasias. Plitidepsin is a potent inducer of apoptosis through the sustained activation of Jun N-terminal kinase (JNK). We have reported that this activation depends on the early induction of oxidative stress, activation of Rac1 small GTPase, and the later down-regulation of MKP-1 phosphatase. Using Scatchard and saturation binding analyses, we have found that 14C-labeled plitidepsin binds to a moderately high-affinity receptor (Kd of 44.8 ± 3.1 and 35.5 ± 4.8 nM, respectively) in MDA-MB-231 breast cancer cells. Two minutes after addition to cells, half of the drug was membrane-bound and was subsequently found in the cytosolic fraction. At 4°C, plitidepsin cellular binding was around 10-fold lower than at 37°C but sufficed to induce cell death, suggesting that this process is triggered from the membrane. Depletion of plasma membrane cholesterol by short treatment with methyl-β-cyclodextrin diminished plitidepsin binding and Rac1 and JNK activation. Rac1 is targeted to the plasma membrane by plitidepsin as shown by subcellular fractioning and immunofluorescence analysis followed by confocal microscopy. Methyl-β-cyclodextrin blocked this effect. A subline of HeLa cells (HeLa-R), partially resistant to plitidepsin, showed similar affinity (Kd of 79.5 ± 2.5 versus 37.7 ± 8.2 nM) but 7.5-fold lower binding capacity than wild-type HeLa cells. Moreover, HeLa-R cells had lower total (71%) and membrane (67%) cholesterol content and membrane-bound Rac1, and showed no Rac1 activation upon plitidepsin treatment. In conclusion, cellular plitidepsin uptake and induction of apoptosis via activation of the Rac1-JNK pathway is membrane-cholesterol dependent.