Reply to “Apo-RBP, Holo-RBP, and Insulin Resistance”

REPLY

In their letter, Zhong et al. assert that STRA6 associates not only with holo- but also with apo-RBP (1). In support, they cite reports (references 6 and 7 in their letter) stating that in the presence of a high concentration of an extracellular acceptor, such as apo-RBP, retinol can efflux from preloaded cells. They take these observations to imply that apo-RBP binds to STRA6. We note, however, that neither of the cited reports established that the observed efflux occurred through STRA6 and not by diffusion through the plasma membranes. Moreover, retinol moves into cells down an inwardly directed concentration gradient which is maintained by vitamin A metabolism. It is difficult to imagine any physiological circumstances under which this gradient is reversed, allowing free retinol to undergo efflux. Note as well that, in cells, retinol is bound to a CRBP. As only apo- and not holo-CRBP associates with STRA6 (2), it is not clear how it is envisioned that STRA6 may gain access to intracellular retinol. In any event, the notion that STRA6 can associate with apo-RBP conflicts with direct evidence that demonstrated that only holo-RBP binds the receptor (3).

Zhong et al.'s statement that our observations that STRA6 inhibits insulin signaling are “exactly the opposite” of conclusions of other studies misrepresents the literature. In fact, the cited references did not study STRA6 at all. Notably, we showed that ablation of STRA6 completely protects mice from RBP-induced insulin resistance and that the receptor significantly contributes to insulin resistance brought about by high-fat feeding (4). In agreement, another group recently reported that even partial reduction of STRA6 expression only in adipose tissue is sufficient for increasing insulin responsiveness in mice fed a high-fat diet (5).

The computer model that we used predicts that STRA6 contains 11 transmembrane domains. Whether this model, the model preferred by Zhong et al., or yet another model is correct awaits further investigations.

Our observations showed that STRA6 transfers retinol directly from extracellular holo-RBP to intracellular CRBP-I. Apo-CRBP-I binds to STRA6, accepts retinol, and dissociates from the receptor upon ligation. We identified both the STRA6 residues that mediate its interactions with CRBP-I and the CRBP-I residues that allow it to bind to STRA6 in a ligand-controlled fashion. The retinol-metabolizing enzyme LRAT was also found to be necessary for STRA6 function. This is likely due to the ability of LRAT to unload retinol from CRBP-I, thereby regenerating apo-CRBP-I and enabling it to reassociate with STRA6 (2). The basis for Zhong et al.'s statement that their Fig. 1 contradicts all of these observations is not clear. In any event, in the absence of critical controls, Fig. 1 is uninterpretable. What cells were used? What components of the STRA6-associated machinery do they express? Were different constructs expressed with similar efficiencies? Were ectopic proteins properly localized? Was the single time point used within the initial linear phase of uptake? Etc.