  As soon as an interaction between holographic dark energy and dark matter is
taken into account, the identification of IR cutoff with Hubble radius
$H^{-1}$, in flat universe, can simultaneously drive accelerated expansion and
solve the coincidence problem. Based on this, we demonstrate that in a non-flat
universe the natural choice for IR cutoff could be the apparent horizon radius,
$\tilde{r}_A={1}/{\sqrt{H^2+k/a^2}}$. We show that any interaction of dark
matter with holographic dark energy, whose infrared cutoff is set by the
apparent horizon radius, implies an accelerated expansion and a constant ratio
of the energy densities of both components thus solving the coincidence
problem. We also verify that for a universe filled with dark energy and dark
matter the Friedmann equation can be written in the form of the modified first
law of thermodynamics, $dE=T_hdS_h+WdV$, at apparent horizon. In addition, the
generalized second law of thermodynamics is fulfilled in a region enclosed by
the apparent horizon. These results hold regardless of the specific form of
dark energy and interaction term. Our study might reveal that in an
accelerating universe with spatial curvature, the apparent horizon is a
physical boundary from the thermodynamical point of view.
