Mohammad Rubayet Hasan

Cancer Metabolism and Drug Resistance

Metabolic alterations, driven by genetic and epigenetic factors, have long been known to be associated with the etiology of cancer. Furthermore, accumulating evidence suggest that cancer metabolism is intimately linked to drug resistance, which is currently one of the most important challenges in cancer treatment. Altered metabolic pathways help cancer cells to proliferate at a rate higher than normal, adapt to nutrient limited conditions, and develop drug resistance phenotypes. Application of systems biology, boosted by recent advancement of novel high-throughput technologies to obtain cancer-associated, transcriptomic, proteomic and metabolomic data, is expected to make a significant contribution to our understanding of metabolic properties related to malignancy. Indeed, despite being at a very early stage, quantitative data obtained from the omics platforms and through applications of 13C metabolic flux analysis (MFA) in in vitro studies, researchers have already began to gain insight into the complex metabolic mechanisms of cancer, paving the way for selection of molecular targets for therapeutic interventions. In this review, we discuss some of the major findings associated with the metabolic pathways in cancer cells and also discuss new evidences and achievements on specific metabolic enzyme targets and target-directed small molecules that can potentially be used as anti-cancer drugs.

Growth phase-dependent changes in the expression of global regulatory genes and associated metabolic pathways in Escherichia coli

Metabolic regulation in Escherichia coli was studied in terms of the changes in the expression of the global regulatory genes rpoD, rpoS, soxRS, cra, fadR, iclR and arcA at three different growth phases, in batch culture. The expression of rpoS and several rpoS-dependent metabolic pathway genes, such as tktB, talA, fumC, acnA, sucA, acs and sodC, were increased (∼1.5 to 2-fold) as the cells entered the late phase of growth. The changes in the expression of other global regulators and their effects on different metabolic pathway genes were less significant, as compared to rpoS, during the later phases of growth.

Identification of a 250 kDa putative microtubule-associated protein as bovine ferritin. Evidence for a ferritin-microtubule interaction.

We reported previously on the purification and partial characterization of a putative microtubule‐associated protein (MAP) from bovine adrenal cortex with an approximate molecular mass of 250u2003kDa. The protein was expressed ubiquitously in mammalian tissues, and bound to microtubules in vitro and in vivo, but failed to promote tubulin polymerization into microtubules. In the present study, partial amino acid sequencing revealed that the protein shares an identical primary structure with the widely distributed iron storage protein, ferritin. We also found that the putative MAP and ferritin are indistinguishable from each other by electrophoretic mobility, immunological properties and morphological appearance. Moreover, the putative MAP conserves the iron storage and incorporation properties of ferritin, confirming that the two are structurally and functionally the same protein. This fact led us to investigate the interaction of ferritin with microtubules by direct electron microscopic observations. Ferritin was bound to microtubules either singly or in the form of large intermolecular aggregates. We suggest that the formation of intermolecular aggregates contributes to the intracellular stability of ferritin. The interactions between ferritin and microtubules observed in this study, in conjunction with the previous report that the administration of microtubule depolymerizing drugs increases the serum release of ferritin in rats [Ramm GA, Powell LW & Halliday JW (1996) J Gastroenterol Hepatol11, 1072–1078], support the probable role of microtubules in regulating the intracellular concentration and release of ferritin under different physiological circumstances.

Differences in the regulation of microtubule stability by the pro-rich region variants of microtubule-associated protein 4.

We have recently reported a neural variant of microtubule‐associated protein 4 with a short pro‐rich region (MAP4‐SP). Here, we show that the neural MAP4 has reduced microtubule‐stabilizing activity, compared to the ubiquitous MAP4 with a long pro‐rich region (MAP4‐LP), both in vitro and in vivo. Fluorescence recovery after photobleaching analyses revealed that the interaction of MAP4‐SP with the microtubules is very rapid, with a half‐time of fluorescence recovery of 7 ± 2.36 s, compared to 19.5 ± 3.03 s in case of MAP4‐LP. The dynamic interaction of MAP4‐SP with microtubules in neural cells may contribute to the dynamic behaviors of extending neurites.

Microtubule-associated protein 4 binds to actin filaments and modulates their properties

We previously reported that an isoform of microtubule-associated protein 4 (MAP4) is localized to the distal area of developing neurites, where microtubules are relatively scarce, raising the possibility that MAP4 interacts with another major cytoskeletal component, actin filaments. In the present study, we examined the in vitro interaction between MAP4 and actin filaments, using bacterially expressed MAP4 and its truncated fragments. Sedimentation assays revealed that MAP4 and its microtubule-binding domain fragments bind to actin filaments under physiological conditions. The apparent dissociation constant and the binding stoichiometry of the fragments to actin were about 0.1 µm and 1 : 3 (MAP4/actin), respectively. Molecular dissection studies revealed that the actin-binding site on MAP4 is situated at the C-terminal part of the proline-rich region, where the microtubule-binding site is also located. Electron microscopy revealed that the MAP4-bound actin filaments become straighter and longer and that the number of actin bundles increases with greater concentrations of added MAP4 fragment, indicating that MAP4 binding alters the properties of the actin filaments. A multiple sequence alignment of the proline-rich regions of MAP4 and tau revealed two putative actin-binding consensus sequences.